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2545d?avr?07/04 features high performance, low power avr ? 8-bit microcontroller advanced risc architecture ? 131 powerful instructions ? most single clock cycle execution ? 32 x 8 general purpose working registers ? fully static operation ? up to 20 mips throughput at 20 mhz ? on-chip 2-cycle multiplier non-volatile progra m and data memories ? 4/8/16k bytes of in-system sel f-programmable flash (atmega48/88/168) endurance: 10,000 write/erase cycles ? optional boot code section with independent lock bits in-system programming by on-chip boot program true read-while-w rite operation ? 256/512/512 bytes eeprom (atmega48/88/168) endurance: 100,000 write/erase cycles ? 512/1k/1k byte internal sram (atmega48/88/168) ? programming lock for software security peripheral features ? two 8-bit timer/counters with se parate prescaler and compare mode ? one 16-bit timer/counter with separate prescaler, compare mode, and capture mode ? real time counter with separate oscillator ? six pwm channels ? 8-channel 10-bit adc in tqfp and mlf package ? 6-channel 10-bit adc in pdip package ? programmable serial usart ? master/slave spi serial interface ? byte-oriented 2-wire serial interface ? programmable watchdog timer with separate on-chip oscillator ? on-chip analog comparator ? interrupt and wake-up on pin change special microcontroller features ? power-on reset and programmable brown-out detection ? internal calibrated oscillator ? external and internal interrupt sources ? five sleep modes: idle, adc noise re duction, power-save, power-down, and standby i/o and packages ? 23 programmable i/o lines ? 28-pin pdip, 32-lead tqfp and 32-pad mlf operating voltage: ? 1.8 - 5.5v for atmega48v/88v/168v ? 2.7 - 5.5v for atmega48/88/168 temperature range: ?-40 c to 85 c speed grade: ? atmega48v/88v/168v: 0 - 4 mhz @ 1.8 - 5.5v, 0 - 10 mhz @ 2.7 - 5.5v ? atmega48/88/168: 0 - 10 mhz @ 2.7 - 5.5v, 0 - 20 mhz @ 4.5 - 5.5v low power consumption ? active mode: 1 mhz, 1.8v: 240a 32 khz, 1.8v: 15a (including oscillator) ? power-down mode: 0.1a at 1.8v 8-bit microcontroller with 8k bytes in-system programmable flash atmega48/v atmega88/v atmega168/v preliminary rev. 2545d?avr?07/04
2 atmega48/88/168 2545d?avr?07/04 pin configurations figure 1. pinout atmega48/88/168 disclaimer typical values contained in this datasheet are based on simulations and characteriza- tion of other avr microcontrollers manufactured on the same process technology. min and max values will be available afte r the device is characterized. 1 2 3 4 5 6 7 8 24 23 22 21 20 19 18 17 (pcint19/oc2b/int1) pd3 (pcint20/xck/t0) pd4 gnd vcc gnd vcc (pcint6/xtal1/tosc1) pb6 (pcint7/xtal2/tosc2) pb7 pc1 (adc1/pcint9) pc0 (adc0/pcint8) adc7 gnd aref adc6 avcc pb5 (sck/pcint5) 32 31 30 29 28 27 26 25 9 10 11 12 13 14 15 16 (pcint21/oc0b/t1) pd5 (pcint22/oc0a/ain0) pd6 (pcint23/ain1) pd7 (pcint0/clko/icp1) pb0 (pcint1/oc1a) pb1 (pcint2/ss/oc1b) pb2 (pcint3/oc2a/mosi) pb3 (pcint4/miso) pb4 pd2 (int0/pcint18) pd1 (txd/pcint17) pd0 (rxd/pcint16) pc6 (reset/pcint14) pc5 (adc5/scl/pcint13) pc4 (adc4/sda/pcint12) pc3 (adc3/pcint11) pc2 (adc2/pcint10) tqfp top view 1 2 3 4 5 6 7 8 9 10 11 12 13 14 28 27 26 25 24 23 22 21 20 19 18 17 16 15 (pcint14/reset) pc6 (pcint16/rxd) pd0 (pcint17/txd) pd1 (pcint18/int0) pd2 (pcint19/oc2b/int1) pd3 (pcint20/xck/t0) pd4 vcc gnd (pcint6/xtal1/tosc1) pb6 (pcint7/xtal2/tosc2) pb7 (pcint21/oc0b/t1) pd5 (pcint22/oc0a/ain0) pd6 (pcint23/ain1) pd7 (pcint0/clko/icp1) pb0 pc5 (adc5/scl/pcint13) pc4 (adc4/sda/pcint12) pc3 (adc3/pcint11) pc2 (adc2/pcint10) pc1 (adc1/pcint9) pc0 (adc0/pcint8) gnd aref avcc pb5 (sck/pcint5) pb4 (miso/pcint4) pb3 (mosi/oc2a/pcint3) pb2 (ss/oc1b/pcint2) pb1 (oc1a/pcint1) pdip 1 2 3 4 5 6 7 8 24 23 22 21 20 19 18 17 32 31 30 29 28 27 26 25 9 10 11 12 13 14 15 16 mlf top view (pcint19/oc2b/int1) pd3 (pcint20/xck/t0) pd4 gnd vcc gnd vcc (pcint6/xtal1/tosc1) pb6 (pcint7/xtal2/tosc2) pb7 pc1 (adc1/pcint9) pc0 (adc0/pcint8) adc7 gnd aref adc6 avcc pb5 (sck/pcint5) (pcint21/oc0b/t1) pd5 (pcint22/oc0a/ain0) pd6 (pcint23/ain1) pd7 (pcint0/clko/icp1) pb0 (pcint1/oc1a) pb1 (pcint2/ss/oc1b) pb2 (pcint3/oc2a/mosi) pb3 (pcint4/miso) pb4 pd2 (int0/pcint18) pd1 (txd/pcint17) pd0 (rxd/pcint16) pc6 (reset/pcint14) pc5 (adc5/scl/pcint13) pc4 (adc4/sda/pcint12) pc3 (adc3/pcint11) pc2 (adc2/pcint10) note: bottom pad should be soldered to ground.
3 atmega48/88/168 2545d?avr?07/04 overview the atmega48/88/168 is a low-power cmos 8-bit microcontroller based on the avr enhanced risc architecture. by executing powe rful instructions in a single clock cycle, the atmega48/88/168 achieves throughputs approaching 1 mips per mhz allowing the system designer to optimize power consumption versus processing speed. block diagram figure 2. block diagram port c (7) port b (8) port d (8) usart 0 8bit t/c 2 16bit t/c 1 8bit t/c 0 a/d conv. internal bandgap analog comp. spi twi sram flash eeprom watchdog oscillator watchdog timer oscillator circuits / clock generation power supervision por / bod & reset vcc gnd program logic debugwire 2 gnd aref avcc data b u s adc[6..7] pc[0..6] pb[0..7] pd[0..7] 6 reset xtal[1..2] cpu
4 atmega48/88/168 2545d?avr?07/04 the avr core combines a rich instruction se t with 32 general purpose working registers. all the 32 registers are directly connected to the arithmetic logic unit (alu), allowing two independent registers to be accessed in one single instruction executed in one clock cycle. the resulting architecture is more code efficient while achieving throughputs up to ten times faster than conventional cisc microcontrollers. the atmega48/88/168 provides the following features: 4k/8k/16k bytes of in-system programmable flash with r ead-while-write capabilities, 256/512/512 bytes eeprom, 512/1k/1k bytes sram, 23 general purpose i/o lines, 32 general purpose working reg- isters, three flexible timer/counters with compare modes, internal and external interrupts, a serial programmable usart, a byte-oriented 2-wire serial interface, an spi serial port, a 6-channel 10-bit adc (8 channels in tqfp and mlf packages), a pro- grammable watchdog timer with internal oscillator, and five software selectable power saving modes. the idle mode stops the cpu while allowing the sram, timer/counters, usart, 2-wire serial interface, spi port, and interrupt system to continue functioning. the power-down mode saves the register contents but freeze s the oscillato r, disabling all other chip functions until the next interrupt or hardware reset. in power-save mode, the asynchronous timer continues to run, al lowing the user to maintain a timer base while the rest of the device is sleeping. the adc noise reductio n mode stops the cpu and all i/o modules except asynchronous time r and adc, to minimize switching noise during adc conversions. in standby mode, the crystal/resonator oscillator is running while the rest of the device is sleeping. this allows very fast star t-up combined with low power consumption. the device is manufactured using atmel?s high density non-volatile memory technology. the on-chip isp flash allows the program memory to be reprogrammed in-system through an spi serial interface, by a conventional non-volatile memory programmer, or by an on-chip boot program running on the avr core. the boot program can use any interface to download the application program in the application flash memory. soft- ware in the boot flas h section will continue to run while the application flash section is updated, providing true read-while-write operation. by combining an 8-bit risc cpu with in-system self-programmable fl ash on a monolithic chip, the atmel atmega48/88/168 is a powerful microcontroller that provides a highly flexible and cost effective solution to many embedded control applications. the atmega48/88/16 8 avr is supported with a full su ite of program and system devel- opment tools including: c compilers, macro assemblers, program debugger/simulators, in-circuit emulators, and evaluation kits. comparison between atmega48, atmega88, and atmega168 the atmega48, atmega88 and atmega168 differ only in memory sizes, boot loader support, and interrupt vector sizes. table 1 summarizes the different memory and inter- rupt vector sizes for the three devices. atmega88 and atmega168 support a real read-while-write self-programming mech- anism. there is a separate boot loader se ction, and the spm instruction can only execute from there. in atmega48, there is no read-while-write support and no sepa- rate boot loader section. the spm instruction can execute from the entire flash. table 1. memory size summary device flash eeprom ram in terrupt vector size atmega48 4k bytes 256 bytes 512 bytes 1 instruction word/vector atmega88 8k bytes 512 bytes 1k bytes 1 instruction word/vector atmega168 16k bytes 512 bytes 1k bytes 2 instruction words/vector
5 atmega48/88/168 2545d?avr?07/04 pin descriptions vcc digital supply voltage. gnd ground. port b (pb7..0) xtal1/ xtal2/tosc1/tosc2 port b is an 8-bit bi-directional i/o port with internal pull-up resistors (selected for each bit). the port b output buffers have symmetrical drive characteristics with both high sink and source capability. as inputs, port b pi ns that are externally pulled low will source current if the pull-up resistors are activated. the port b pins are tri-stated when a reset condition becomes active, even if the clock is not running. depending on the clock selection fuse settings, pb6 can be used as input to the invert- ing oscillator amplifier a nd input to the internal clock operating circuit. depending on the clock selection fuse settings, pb7 can be used as output from the inverting oscillator amplifier. if the internal calibrated rc oscillator is used as chip clock source, pb7..6 is used as tosc2..1 input for the asyn chronous timer/counter2 if the as2 bit in assr is set. the various special features of port b are elaborated in ?alternate functions of port b? on page 69 and ?system clock and clock options? on page 24. port c (pc5..0) port c is a 7-bit bi-directional i/o port with internal pull-up resistors (selected for each bit). the pc5..0 output buffers have symmetrical drive characteristics with both high sink and source capability. as inputs, port c pins that are externally pulled low will source current if the pull-up resistors are activated. the port c pins are tri-stated when a reset condition becomes active, even if the clock is not running. pc6/reset if the rstdisbl fuse is programmed, pc6 is used as an i/o pin. note that the electri- cal characteristics of pc6 differ from those of the other pins of port c. if the rstdisbl fuse is unprogrammed, pc6 is used as a reset input. a low level on this pin for longer than the minimum pulse le ngth will generate a re set, even if the clock is not running. the minimum pulse length is given in table 20 on page 41. shorter pulses are not guaranteed to generate a reset. the various special features of port c are elaborated in ?alternate functions of port c? on page 73. port d (pd7..0) port d is an 8-bit bi-directional i/o port with internal pull-up resistors (selected for each bit). the port d output buffers have symmetrical drive characteristics with both high sink and source capability. as inputs, port d pi ns that are externally pulled low will source current if the pull-up resistors are activated. the port d pins are tri-stated when a reset condition becomes active, even if the clock is not running. the various special features of port d are elaborated in ?alternate functions of port d? on page 75. av cc av cc is the supply voltage pin for the a/d converter, pc3..0, and adc7..6. it should be externally connected to v cc , even if the adc is not used. if the adc is used, it should be connected to v cc through a low-pass filter. note that pc6..4 use digital supply voltage, v cc . aref aref is the analog reference pin for the a/d converter.
6 atmega48/88/168 2545d?avr?07/04 adc7..6 (tqfp and mlf package only) in the tqfp and mlf package, adc7..6 serv e as analog inputs to the a/d converter. these pins are powered from the analog supply and serve as 10-bit adc channels. about code examples this documentation contains simple code examples that briefly show how to use various parts of the device. these code examples assume that the part specific header file is included before compilation. be aware that not all c compiler vendors include bit defini- tions in the header files and interrupt handling in c is compiler dependent. please confirm with the c compiler documentation for more details.
7 atmega48/88/168 2545d?avr?07/04 avr cpu core introduction this section discusses the avr core archit ecture in general. the main function of the cpu core is to ensure correct program execution. the cpu must therefore be able to access memories, perform calculations, control peripherals, and handle interrupts. architectural overview figure 3. block diagram of the avr architecture in order to maximize performance and para llelism, the avr uses a harvard architecture ? with separate memories and buses for program and data. instructions in the program memory are executed with a single level pipel ining. while one instruction is being exe- cuted, the next instruction is pre-fetched from the program memory. this concept enables instructions to be executed in every clock cycle. the program memory is in- system reprogrammable flash memory. the fast-access register file contains 32 x 8-bit general purpose working registers with a single clock cycle access time. this allows single-cycle arithmetic logic unit (alu) operation. in a typical alu operation, two operands are output from the register file, flash program memory instruction register instruction decoder program counter control lines 32 x 8 general purpose registrers alu status and control i/o lines eeprom data bus 8-bit data sram direct addressing indirect addressing interrupt unit spi unit watchdog timer analog comparator i/o module 2 i/o module1 i/o module n
8 atmega48/88/168 2545d?avr?07/04 the operation is executed, and the result is stored back in the register file ? in one clock cycle. six of the 32 registers can be used as three 16-bit indirect address register pointers for data space addressing ? enabling efficient address calculations. one of the these address pointers can also be used as an addr ess pointer for look up tables in flash pro- gram memory. these added function registers are the 16-bit x-, y-, and z-register, described later in this section. the alu supports arithmetic and logic operations between registers or between a con- stant and a register. single register operations can also be executed in the alu. after an arithmetic operation, the status register is updated to reflect information about the result of the operation. program flow is provided by conditional and unconditional jump and call instructions, able to directly address the whole address s pace. most avr instructions have a single 16-bit word format. every program memory address contains a 16- or 32-bit instruction. program flash memory space is divided in two sections, the boot program section and the application program section. both sections have dedicated lock bits for write and read/write protection. the spm instruction that writes into the application flash memory section must reside in the boot program section. during interrupts and subroutine calls, the return address program counter (pc) is stored on the stack. the stack is effectively allocated in the general data sram, and consequently the stack size is only limited by the total sram size and the usage of the sram. all user programs must initialize the sp in the reset routine (before subroutines or interrupts are executed). the stack pointer (sp) is read/write accessible in the i/o space. the data sram can easily be accessed through the five different addressing modes supported in the avr architecture. the memory spaces in the avr architecture are all linear and regular memory maps. a flexible interrupt module has its control registers in the i/o space with an additional global interrupt enable bit in the status register. all interrupts have a separate interrupt vector in the interrupt vector table. the interrupts have priority in accordance with their interrupt vector position. the lower the interrupt vector address, the higher the priority. the i/o memory space contains 64 addresse s for cpu peripheral functions as control registers, spi, and other i/o functions. the i/o memory can be accessed directly, or as the data space locations following those of the register file, 0x20 - 0x5f. in addition, the atmega48/88/168 has extended i/o space from 0x60 - 0xff in sram where only the st/sts/std and ld/lds/ldd instructions can be used. alu ? arithmetic logic unit the high-performance avr alu operates in direct connection with all the 32 general purpose working registers. within a single clock cycle, arithmetic operations between general purpose registers or between a register and an immediate are executed. the alu operations are divided into three main categories ? arithmetic, logical, and bit-func- tions. some implementations of the architecture also provide a powerful multiplier supporting both signed/unsigned multiplication and fractional format. see the ?instruc- tion set? section for a detailed description.
9 atmega48/88/168 2545d?avr?07/04 status register the status register contains information about the result of the most recently executed arithmetic instruction. this information can be used for altering program flow in order to perform conditional operations. note that the status register is updated after all alu operations, as specified in the instructio n set reference. this will in many cases remove the need for using the dedicated compare instructions, resulting in faster and more compact code. the status register is not automatically stored when entering an interrupt routine and restored when returning from an interrupt. this must be handled by software. the avr status register ? sreg ? is defined as: bit 7 ? i: global interrupt enable the global interrupt enable bit must be set for the interrupts to be enabled. the individ- ual interrupt enable control is then performed in separate control registers. if the global interrupt enable register is cleared, n one of the interrupts are enabled independent of the individual interrupt enable settings. the i-bit is cleared by hardware after an interrupt has occurred, and is set by the reti instruction to enable subsequent interrupts. the i- bit can also be set and cleared by the application with the sei and cli instructions, as described in the instruction set reference. bit 6 ? t: bit copy storage the bit copy instructions bld (bit load) and bst (bit store) use the t-bit as source or destination for the operated bit. a bit from a register in the register file can be copied into t by the bst instruction, and a bit in t can be copied into a bit in a register in the register file by the bld instruction. bit 5 ? h: half carry flag the half carry flag h indicates a half carry in some arithmetic operations. half carry is useful in bcd arithmetic. see the ?instruction set description? for detailed information. bit 4 ? s: sign bit, s = n v the s-bit is always an exclusive or between the negative flag n and the two?s comple- ment overflow flag v. see the ?instruction set description? for detailed information. bit 3 ? v: two?s complement overflow flag the two?s complement overflow flag v supports two?s complement arithmetics. see the ?instruction set description? for detailed information. bit 2 ? n: negative flag the negative flag n indicates a negative result in an arithmetic or logic operation. see the ?instruction set description? for detailed information. bit 1 ? z: zero flag the zero flag z indicates a zero result in an arithmetic or logic operation. see the ?instruction set description? for detailed information. bit 0 ? c: carry flag the carry flag c indicates a carry in an arit hmetic or logic operation. see the ?instruc- tion set description? for detailed information. bit 76543210 ithsvnzcsreg read/write r/w r/w r/w r/w r/w r/w r/w r/w initial value 0 0 0 0 0 0 0 0
10 atmega48/88/168 2545d?avr?07/04 general purpose register file the register file is optimized for the avr enhanced risc instruction set. in order to achieve the required performance and flexib ility, the following inpu t/output schemes are supported by the register file: one 8-bit output operand and one 8-bit result input two 8-bit output operands and one 8-bit result input two 8-bit output operands and one 16-bit result input one 16-bit output operand and one 16-bit result input figure 4 shows the structure of the 32 general purpose working registers in the cpu. figure 4. avr cpu general purpose working registers most of the instructions operating on the register file have direct access to all registers, and most of them are single cycle instructions. as shown in figure 4, each register is also assigned a data memory address, mapping them directly into the first 32 locations of the user data space. although not being phys- ically implemented as sram locations, this memory organization provides great flexibility in access of the registers, as t he x-, y- and z-pointer registers can be set to index any register in the file. 7 0 addr. r0 0x00 r1 0x01 r2 0x02 ? r13 0x0d general r14 0x0e purpose r15 0x0f working r16 0x10 registers r17 0x11 ? r26 0x1a x-register low byte r27 0x1b x-register high byte r28 0x1c y-register low byte r29 0x1d y-register high byte r30 0x1e z-register low byte r31 0x1f z-register high byte
11 atmega48/88/168 2545d?avr?07/04 the x-register, y-register, and z-register the registers r26..r31 have some added f unctions to their general purpose usage. these registers are 16-bit address pointers for indirect addressing of the data space. the three indirect address registers x, y, and z are defined as described in figure 5. figure 5. the x-, y-, and z-registers in the different addressing modes these addres s registers have functions as fixed dis- placement, automatic increment, and automatic decrement (see the instruction set reference for details). stack pointer the stack is mainly used for storing temporary data, for storing local variables and for storing return addresses after interrupts and subroutine calls. the stack pointer regis- ter always points to the top of the stack. note that the stack is implemented as growing from higher memory locations to lower memory locations. this implies that a stack push command decreases the stack pointer. the stack pointer points to the data sram stack area where the subroutine and inter- rupt stacks are located. this stack space in the data sram must be defined by the program before any subroutine calls are executed or interrupts are enabled. the stack pointer must be set to point above 0x0100, preferably ramend. the stack pointer is decremented by one when data is pushed onto the stack with the push instruction, and it is decremented by two when the return address is pushed onto the stack with subrou- tine call or interrupt. the stack pointer is incremented by one when data is popped from the stack with the pop instruction, and it is incremented by two when data is popped from the stack with return from subroutine ret or return from interrupt reti. the avr stack pointer is implemented as two 8-bit registers in the i/o space. the num- ber of bits actually used is implementation dependent. note that the data space in some implementations of the avr architecture is so small that only spl is needed. in this case, the sph register will not be present. 15 xh xl 0 x-register 707 0 r27 (0x1b) r26 (0x1a) 15 yh yl 0 y-register 707 0 r29 (0x1d) r28 (0x1c) 15 zh zl 0 z-register 70 7 0 r31 (0x1f) r30 (0x1e) bit 151413121110 9 8 sp15 sp14 sp13 sp12 sp11 sp10 sp9 sp8 sph sp7 sp6 sp5 sp4 sp3 sp2 sp1 sp0 spl 76543210 read/write r/w r/w r/w r/w r/w r/w r/w r/w r/w r/w r/w r/w r/w r/w r/w r/w initial value ramend ramend ramend ramend ramend ramend ramend ramend ramend ramend ramend ramend ramend ramend ramend ramend
12 atmega48/88/168 2545d?avr?07/04 instruction execution timing this section describes the general access timing concepts for instruction execution. the avr cpu is driven by the cpu clock clk cpu , directly generated from the selected clock source for the chip. no intern al clock division is used. figure 6 shows the parallel instruction fetches and instruction executions enabled by the harvard architecture and the fast-access register file concept. this is the basic pipelin- ing concept to obtain up to 1 mips per mhz with the corresponding unique results for functions per cost, functions per clocks, and functions per power-unit. figure 6. the parallel instruction fetches and instruction executions figure 7 shows the internal timing concept fo r the register file. in a single clock cycle an alu operation using two register operands is executed, and the result is stored back to the destination register. figure 7. single cycle alu operation reset and interrupt handling the avr provides several different interrupt sources. these interrupts and the separate reset vector each have a separate program vector in the program memory space. all interrupts are assigned individual enable bits which must be written logic one together with the global interrupt enable bit in the status register in order to enable the interrupt. depending on the program counter value, in terrupts may be automatically disabled when boot lock bits blb02 or blb12 are programmed. this feature improves software security. see the section ?memory programming? on page 270 for details. the lowest addresses in the program memory space are by default defined as the reset and interrupt vectors. the complete list of vectors is shown in ?interrupts? on page 51. the list also determines the priority levels of the different interrupts. the lower the address the higher is the priori ty level. reset has the high est priority, and next is int0 ? the external interrupt request 0. the interrupt vectors can be moved to the start of the boot flash section by setting the ivsel bit in the mcu control register (mcucr). refer to ?interrupts? on page 51 for more information. the reset vector can also be clk 1st instruction fetch 1st instruction execute 2nd instruction fetch 2nd instruction execute 3rd instruction fetch 3rd instruction execute 4th instruction fetch t1 t2 t3 t4 cpu total execution time register operands fetch alu operation execute result write back t1 t2 t3 t4 clk cpu
13 atmega48/88/168 2545d?avr?07/04 moved to the start of the boot flash section by programming the bootrst fuse, see ?boot loader support ? read-while-write self-programming, atmega88 and atmega168? on page 255. when an interrupt occurs, the global interrupt enable i-bit is cleared and all interrupts are disabled. the user software can write logic one to the i-bit to enable nested inter- rupts. all enabled interrupts can then interrupt the current interrupt routine. the i-bit is automatically set when a return from interrupt instruction ? reti ? is executed. there are basically two types of interrupts. the first type is triggered by an event that sets the interrupt flag. for these interrupts, the program counter is vectored to the actual interrupt vector in order to execute the interrupt handling routine, and hardware clears the corresponding interrupt flag. interr upt flags can also be cleared by writing a logic one to the flag bit position(s) to be cleared. if an interrupt condition occurs while the corresponding interrupt enable bit is cleared, the interrupt flag will be set and remem- bered until the interrupt is enabled, or the flag is cleared by software. similarly, if one or more interrupt conditions occur while the global interrupt enable bit is cleared, the cor- responding interrupt flag(s) will be set and remembered until the global interrupt enable bit is set, and will then be executed by order of priority. the second type of interrupts will trigger as long as the interrupt condition is present. these interrupts do not necessarily have interrupt flags. if the interrupt condition disap- pears before the interrupt is enabled , the interrupt will not be triggered. when the avr exits from an in terrupt, it will always return to the main pr ogram and exe- cute one more instruction before any pending interrupt is served. note that the status register is not automatically stored when entering an interrupt rou- tine, nor restored when returning from an interrupt routine. this must be handled by software. when using the cli instruction to disable in terrupts, the interrupts will be immediately disabled. no interrupt will be exec uted after the cli instruction, even if it occurs simulta- neously with the cli instruction. the following example shows how this can be used to avoid interrupts during the timed eeprom write sequence. assembly code example in r16, sreg ; store sreg value cli ; disable interrupts during timed sequence sbi eecr, eempe ; start eeprom write sbi eecr, eepe out sreg, r16 ; restore sreg value (i-bit) c code example char csreg; csreg = sreg; /* store sreg value */ /* disable interrupts during timed sequence */ _cli(); eecr |= (1< 14 atmega48/88/168 2545d?avr?07/04 when using the sei instructio n to enable interrupts, the in struction following sei will be executed before any pending interrupts, as shown in this example. interrupt response time the interrupt execution response for all the enabled avr interrupts is four clock cycles minimum. after four clock cycles the program vector address for the actual interrupt handling routine is executed. during this four clock cycle period, the program counter is pushed onto the stack. the vector is normally a jump to the interrupt routine, and this jump takes three clock cycles. if an interr upt occurs during execution of a multi-cycle instruction, this instruction is completed before the interrupt is served. if an interrupt occurs when the mcu is in sleep mode, the interrupt execution response time is increased by four clock cycles. this increase comes in addition to the start-up time from the selected sleep mode. a return from an interrupt handling routine ta kes four clock cycles. during these four clock cycles, the program counter (two bytes) is popped back from the stack, the stack pointer is incremented by two, and the i-bit in sreg is set. assembly code example sei ; set global interrupt enable sleep ; enter sleep, waiting for interrupt ; note: will enter sleep before any pending interrupt(s) c code example __enable_interrupt(); /* set global interrupt enable */ __sleep(); /* enter sleep, waiting for interrupt */ /* note: will enter sleep before any pending interrupt(s) */
15 atmega48/88/168 2545d?avr?07/04 avr atmega48/88/168 memories this section describes the different memories in the atmega48/88/168. the avr archi- tecture has two main memory spaces, the data memory and the program memory space. in addition, the atme ga48/88/168 features an eepr om memory for data stor- age. all three memory spaces are linear and regular. in-system reprogrammable flash program memory the atmega48/88/168 contains 4/8/16k bytes on-chip in-system reprogrammable flash memory for program storage. since all avr instructions are 16 or 32 bits wide, the flash is organized as 2/4/8k x 16. for software security, the flash program memory space is divided into two sections, boot loader section and application program sec- tion in atmega88 and atmega168. atmega48 does not have separate boot loader and application program sections, and the spm instruction can be executed from the entire flash. see selfprgen description in section ?store program memory control and status register ? spmcsr? on page 250 and page 260for more details. the flash memory has an endurance of at least 10,000 write/erase cycles. the atmega48/88/168 program counter (pc) is 11/12/13 bits wide, thus addressing the 2/4/8k program memory locations. the operation of boot program section and associ- ated boot lock bits for software protection are described in detail in ?self-programming the flash, atmega48? on page 248 and ?boot loader support ? read-while-write self- programming, atmega88 and atmega168? on page 255. ?memory programming? on page 270 contains a detailed description on flash programming in spi- or parallel pro- gramming mode. constant tables can be allocated within th e entire program memory address space (see the lpm ? load program memory instruction description). timing diagrams for instruction fetch and exec ution are presented in ?instruction execu- tion timing? on page 12. figure 8. program memory map, atmega48 0x0000 0x7ff program memory application flash section
16 atmega48/88/168 2545d?avr?07/04 figure 9. program memory map, atmega88 and atmega168 sram data memory figure 10 shows how the atmega48/88/168 sram memory is organized. the atmega48/88/168 is a complex microcontroller with more peripheral units than can be supported within the 64 locations reserved in the opcode for the in and out instruc- tions. for the extended i/o space from 0x60 - 0xff in sram, only the st/sts/std and ld/lds/ldd instructions can be used. the lower 768/1280/1280 data memory locations address both the register file, the i/o memory, extended i/o memory, and the internal data sram. the first 32 locations address the register file, the next 64 location the standard i/o memory, then 160 loca- tions of extended i/o memory, and the next 512/1024/1024 locations address the internal data sram. the five different addressing modes for the data memory cover: direct, indirect with dis- placement, indirect, indirect with pre-decrement, and indirect with post-increment. in the register file, registers r26 to r31 feature the indirect addressing pointer registers. the direct addressing reaches the entire data space. the indirect with displacement mode reaches 63 address locations from the base address given by the y- or z-register. when using register indirect addressing modes with automatic pre-decrement and post- increment, the address registers x, y, and z are decremented or incremented. the 32 general purpose working registers, 64 i/o registers, 160 extended i/o regis- ters, and the 512/1024/1024 bytes of internal data sram in the atmega48/88/168 are all accessible through all these addressing modes. the register file is described in ?general purpose register file? on page 10. 0x0000 0x0fff/0x1fff program memory application flash section boot flash section
17 atmega48/88/168 2545d?avr?07/04 figure 10. data memory map data memory access times this section describes the general access ti ming concepts for internal memory access. the internal data sram access is performed in two clk cpu cycles as described in figure 11. figure 11. on-chip data sram access cycles eeprom data memory the atmega48/88/168 contai ns 256/512/512 by tes of data eeprom memory. it is organized as a separate data space, in which single bytes can be read and written. the eeprom has an endurance of at least 100,000 write/erase cycles. the access between the eeprom and the cpu is descri bed in the following, specifying the eeprom address registers, the eeprom data register, and the eeprom control register. ?memory programming? on page 270 contains a detailed description on eeprom pro- gramming in spi or parallel programming mode. eeprom read/write access the eeprom access registers are accessible in the i/o space. the write access time for the eeprom is given in table 3. a self-timing function, how- ever, lets the user software detect when t he next byte can be written. if the user code contains instructions that write the eepr om, some precautions must be taken. in heavily filtered power supplies, v cc is likely to rise or fall slowly on power-up/down. this causes the device for some period of time to run at a voltage lower than specified as 32 registers 64 i/o registers internal sram (512/1024/1024 x 8) 0x0000 - 0x001f 0x0020 - 0x005f 0x02ff/0x04ff/0x04f f 0x0060 - 0x00ff data memory 160 ext i/o reg. 0x0100 clk wr rd data data address address valid t1 t2 t3 compute address read write cpu memory access instruction next instruction
18 atmega48/88/168 2545d?avr?07/04 minimum for the clock frequency used. see ?preventing eeprom corruption? on page 22 for details on how to avoid problems in these situations. in order to prevent unintentional eeprom writes, a specific write procedure must be fol- lowed. refer to the descripti on of the eeprom control regi ster for details on this. when the eeprom is read, the cpu is halt ed for four clock cycles before the next instruction is executed. when the eeprom is written, the cpu is halted for two clock cycles before the next instruction is executed. the eeprom address register ? eearh and eearl bits 15..9 ? res: reserved bits these bits are reserved bits in the atme ga48/88/168 and will al ways read as zero. bits 8..0 ? eear8..0: eeprom address the eeprom address registers ? eearh and eearl specify t he eeprom address in the 256/512/ 512 bytes eeprom space. the eeprom data byte s are addr essed lin- early between 0 and 255/511/511. the initial value of eear is undefined. a proper value must be written before the eeprom may be accessed. eear8 is an unused bit in atmega48 and must a lways be written to zero. the eeprom data register ? eedr bits 7..0 ? eedr7.0: eeprom data for the eeprom write operation, the eedr register contains the data to be written to the eeprom in the address gi ven by the eear register. for the eeprom read oper- ation, the eedr contains the data read out from the eeprom at the address given by eear. the eeprom control register ? eecr bits 7..6 ? res: reserved bits these bits are reserved bits in the atme ga48/88/168 and will al ways read as zero. bits 5, 4 ? eepm1 and eepm0: eeprom programming mode bits the eeprom programming mode bit setting defines which pr ogramming action that will be triggered when writing eepe. it is possi ble to program data in one atomic operation (erase the old value and program the new val ue) or to split the erase and write opera- bit 151413121110 9 8 ???????eear8eearh eear7 eear6 eear5 eear4 eear3 eear2 eear1 eear0 eearl 76543210 read/writerrrrrrrr/w r/w r/w r/w r/w r/w r/w r/w r/w initial value0000000x xxxxxxxx bit 76543210 msb lsb eedr read/write r/w r/w r/w r/w r/w r/w r/w r/w initial value00000000 bit 76543210 ? ? eepm1 eepm0 eerie eempe eepe eere eecr read/write r r r/w r/w r/w r/w r/w r/w initial value 0 0 x x 0 0 x 0
19 atmega48/88/168 2545d?avr?07/04 tions in two different operations. the programming times for the different modes are shown in table 2. while eepe is set, any write to eepmn will be ignored. during reset, the eepmn bits will be reset to 0b00 un less the eeprom is busy programming. bit 3 ? eerie: eeprom ready interrupt enable writing eerie to one enables the eeprom ready interrupt if the i bit in sreg is set. writing eerie to zero disables the interrupt. the eeprom ready interrupt generates a constant interrupt when eepe is cleared. bit 2 ? eempe: eeprom master write enable the eempe bit determines whether setti ng eepe to one causes the eeprom to be written. when eempe is set, setting eepe wit hin four clock cycles will write data to the eeprom at the selected address if eempe is zero, setting eepe will have no effect. when eempe has been written to one by software, hardware clears the bit to zero after four clock cycles. see the description of the eepe bit for an eeprom write procedure. bit 1 ? eepe: eeprom write enable the eeprom write enable signal eepe is the write strobe to the eeprom. when address and data are correctly set up, the eepe bit must be written to one to write the value into the eeprom. the eempe bit must be written to one before a logical one is written to eepe, otherwise no eeprom wr ite takes place. the following procedure should be followed when writing the eeprom (the order of steps 3 and 4 is not essential): 1. wait until eepe becomes zero. 2. wait until selfprgen in spmcsr becomes zero. 3. write new eeprom addr ess to eear (optional). 4. write new eeprom data to eedr (optional). 5. write a logical one to the eempe bit while writing a zero to eepe in eecr. 6. within four clock cycles after settin g eempe, write a logical one to eepe. the eeprom can not be programmed during a cpu write to the flash memory. the software must check that the flash programming is completed before initiating a new eeprom write. step 2 is only relevant if the software contains a boot loader allowing the cpu to program the flash. if the flash is never being updated by the cpu, step 2 can be omitted. see ?boot loader support ? read-while-write self-programming, atmega88 and atmega168? on page 255 for details about boot programming. caution: an interrupt be tween step 5 and step 6 will make the write cycle fail, since the eeprom master write enable will time-out. if an interrupt routine accessing the eeprom is interrupting an other eeprom access , the eear or eedr register will be modified, causing the interrupted eeprom ac cess to fail. it is recommended to have the global interrupt flag cleared during all the steps to avoid these problems. table 2. eeprom mode bits eepm1 eepm0 programming time operation 0 0 3.4 ms erase and write in one operation (atomic operation) 0 1 1.8 ms erase only 1 0 1.8 ms write only 1 1 ? reserved for future use
20 atmega48/88/168 2545d?avr?07/04 when the write access time has elapsed, the eepe bit is cleared by hardware. the user software can poll this bit and wait for a zero before writing the next byte. when eepe has been set, the cpu is halted for two cycles before the next instruction is executed. bit 0 ? eere: eeprom read enable the eeprom read enable signal eere is the read st robe to the eeprom. when the correct address is set up in the eear regist er, the eere bit must be written to a logic one to trigger the eeprom re ad. the eeprom read access takes one instruction, and the requested data is availabl e immediately. when the ee prom is read, the cpu is halted for four cycles before the next instruction is executed. the user should poll the eepe bi t before starting the read operation. if a write operation is in progress, it is neither possible to read the eeprom, nor to change the eear register. the calibrated oscillator is us ed to time the eeprom access es. table 3 lists the typical programming time for eeprom access from the cpu. the following code examples show one assembly and one c function for writing to the eeprom. the examples assume that interrupts are controlled (e.g. by disabling inter- rupts globally) so that no inte rrupts will occur during execut ion of these functions. the examples also assume that no flash boot loader is present in the software. if such code is present, the eeprom write function must also wait for any ongoing spm com- mand to finish. table 3. eeprom programming time symbol number of calibrated rc oscillator cycles typ programming time eeprom write (from cpu) 26,368 3.3 ms
21 atmega48/88/168 2545d?avr?07/04 assembly code example eeprom_write: ; wait for completion of previous write sbic eecr,eepe rjmp eeprom_write ; set up address (r18:r17) in address register out eearh, r18 out eearl, r17 ; write data (r16) to data register out eedr,r16 ; write logical one to eempe sbi eecr,eempe ; start eeprom write by setting eepe sbi eecr,eepe ret c code example void eeprom_write( unsigned int uiaddress, unsigned char ucdata) { /* wait for completion of previous write */ while(eecr & (1< 22 atmega48/88/168 2545d?avr?07/04 the next code examples show assembly an d c functions for reading the eeprom. the examples assume that interr upts are controlled so that no interrupts will occur during execution of these functions. preventing eeprom corruption during periods of low v cc, the eeprom data can be corrupt ed because the supply volt- age is too low for the cpu and the eeprom to operate prope rly. these issues are the same as for board level sys tems using eeprom, and the sa me design solutions should be applied. an eeprom data corruption can be caused by two situations when the voltage is too low. first, a regular write sequence to the eeprom requires a minimum voltage to operate correctly. secondly, the cpu itself ca n execute instructions incorrectly, if the supply voltage is too low. eeprom data corruption can easily be avoided by following this design recommendation: keep the avr reset active (low) during pe riods of insufficient power supply voltage. this can be done by enabling the internal brown-out detector (bod). if the detection level of the internal bod does not match the needed detection level, an external low v cc reset protection circuit can be used. if a reset occurs while a write operation is in progress, the write op eration will be complete d provided that the power supply voltage is sufficient. assembly code example eeprom_read: ; wait for completion of previous write sbic eecr,eepe rjmp eeprom_read ; set up address (r18:r17) in address register out eearh, r18 out eearl, r17 ; start eeprom read by writing eere sbi eecr,eere ; read data from data register in r16,eedr ret c code example unsigned char eeprom_read( unsigned int uiaddress) { /* wait for completion of previous write */ while(eecr & (1< 23 atmega48/88/168 2545d?avr?07/04 i/o memory the i/o space definition of the atmega48/88/168 is shown in ?register summary? on page 325. all atmega48/88/168 i/os and peripherals are placed in the i/o space. all i/o locations may be accessed by the ld/lds/ldd and st/sts/std instructions, transferring data between the 32 general purpose working registers and the i/o space. i/o registers within the address range 0x00 - 0x1f are directly bit-accessible using the sbi and cbi instructions. in these registers, the value of single bits can be checked by using the sbis and sbic instructions. refer to the in struction set section for more details. when using the i/o specific commands in and out, the i/o addresses 0x00 - 0x3f must be used. when addressing i/o registers as data space using ld and st instructions, 0x20 must be added to these addresses. the atmega48/88/168 is a complex microcontroller with more peripheral units than can be supported within the 64 location reserved in opcode for the in and out instructions. for the extended i/o space from 0x60 - 0xff in sram, only the st/sts/std and ld/lds/ldd instructions can be used. for compatibility with futu re devices, reserved bits should be written to ze ro if accessed. reserved i/o memory addresses should never be written. some of the status flags are cleared by writing a logical one to them. note that, unlike most other avrs, the cbi and sbi instructions will only oper ate on the specified bit, and can therefore be used on registers containing such status flags. the cbi and sbi instructions work with registers 0x00 to 0x1f only. the i/o and peripherals control registers are explained in later sections. general purpose i/o registers the atmega48/88/168 contains three general purpose i/o registers. these registers can be used for storing any information, and they are particularly useful for storing glo- bal variables and status flags. general purpose i/o registers within the address range 0x00 - 0x1f are directly bit-accessible us ing the sbi, cbi, sbis, and sbic instructions. general purpose i/o register 2 ? gpior2 general purpose i/o register 1 ? gpior1 general purpose i/o register 0 ? gpior0 bit 76543210 msb lsb gpior2 read/write r/w r/w r/w r/w r/w r/w r/w r/w initial value00000000 bit 76543210 msb lsb gpior1 read/write r/w r/w r/w r/w r/w r/w r/w r/w initial value00000000 bit 76543210 msb lsb gpior0 read/write r/w r/w r/w r/w r/w r/w r/w r/w initial value00000000
24 atmega48/88/168 2545d?avr?07/04 system clock and clock options clock systems and their distribution figure 12 presents the principal clock system s in the avr and their distribution. all of the clocks need not be active at a given time. in order to reduce power consumption, the clocks to modules not being used can be halted by using differ ent sleep modes, as described in ?power management and sleep modes? on page 35. the clock systems are detailed below. figure 12. clock distribution cpu clock ? clk cpu the cpu clock is routed to parts of the system concerned with operation of the avr core. examples of such modules are the general purpose register file, the status reg- ister and the data memory holding the stack pointer. halting the cpu clock inhibits the core from performing general operations and calculations. i/o clock ? clk i/o the i/o clock is used by the majority of t he i/o modules, like timer/counters, spi, and usart. the i/o clock is also used by the external interrupt module, but note that some external interrupts are detected by asynchronous logic, allowing such interrupts to be detected even if the i/o clock is halted. also note that start condition detection in the usi module is carried out asynchronously when clk i/o is halted, twi address recognition in all sleep modes. flash clock ? clk flash the flash clock controls operation of the fl ash interface. the flash clock is usually active simultaneously with the cpu clock. general i/o modules asynchronous timer/counter cpu core ram clk i/o clk asy avr clock control unit clk cpu flash and eeprom clk flash source clock watchdog timer watchdog oscillator reset logic clock multiplexer watchdog clock calibrated rc oscillator timer/counter oscillator crystal oscillator low-frequency crystal oscillator external clock adc clk adc system clock prescaler
25 atmega48/88/168 2545d?avr?07/04 asynchronous timer clock ? clk asy the asynchronous timer clock allows the asynchronous timer/counter to be clocked directly from an external clock or an exter nal 32 khz clock crysta l. the dedicated clock domain allows using this timer/counter as a real-time counter even when the device is in sleep mode. adc clock ? clk adc the adc is provided with a dedicated clock domain. this allows halting the cpu and i/o clocks in order to reduce noise generated by digital circuitry. this gives more accu- rate adc conversion results. clock sources the device has the following clock source options, selectable by flash fuse bits as shown below. the clock from the selected source is input to the avr clock generator, and routed to the appropriate modules. note: 1. for all fuses ?1? means unprogrammed while ?0? means programmed. default clock source the device is shipped with internal rc oscillator at 8.0mhz and with the fuse ckdiv8 programmed, resulting in 1.0mhz system clock. the startup time is set to maximum and time-out period enabled. (cksel = "0010", sut = "10", ckdi v8 = "0"). the default set- ting ensures that all users can make t heir desired clock source setting using any available programming interface. clock startup sequence any clock source needs a sufficient v cc to start oscillating and a minimum number of oscillating cycles before it can be considered stable. to ensure sufficient v cc , the device issues an internal reset with a time-out delay (t tout ) after the device reset is released by all other reset sources. ?system control and reset? on page 40 describes the start conditions for the internal reset. the delay (t tout ) is timed from the watchdog oscillator and the number of cycles in the delay is set by the sutx and ckselx fuse bits. the selectable delays are shown in table 5. the fre- quency of the watchdog oscillator is voltage dependent as shown in ?atmega48/88/168 typical characteristics ? preliminary data? on page 298. main purpose of the delay is to keep the avr in reset until it is supplied with minimum v cc . the delay will not monitor the actual voltage and it will be required to select a delay table 4. device clocking options select (1) device clocking option cksel3..0 low power crystal oscillator 1111 - 1000 full swing crystal oscillator 0111 - 0110 low frequency crystal oscillator 0101 - 0100 internal 128 khz rc oscillator 0011 calibrated internal rc oscillator 0010 external clock 0000 reserved 0001 table 5. number of watchdog oscillator cycles typ time-out (v cc = 5.0v) typ time-out (v cc = 3.0v) number of cycles 0 ms 0 ms 0 4.1 ms 4.3 ms 4k (4,096) 65 ms 69 ms 8k (8,192)
26 atmega48/88/168 2545d?avr?07/04 longer than the v cc rise time. if this is not possible, an internal or external brown-out detection circuit should be used. a bo d circuit will ensu re sufficient v cc before it releases the reset, and the time-out delay can be disabled. disabling the time-out delay without utilizing a brown-out detect ion circuit is not recommended. the oscillator is required to oscillate for a minimum number of cycles before the clock is considered stable. an internal ripple counte r monitors the oscilla tor output clock, and keeps the internal reset active for a give n number of clock cycles. the reset is then released and the device will start to execut e. the recommended osc illator start-up time is dependent on the clock type, and varies fr om 6 cycles for an externally applied clock to 32k cycles for a low frequency crystal. the start-up sequence for the clock includes both the time-out delay and the start-up time when the device starts up from reset. when starting up from power-save or power- down mode, v cc is assumed to be at a sufficient level and only the start-up time is included. low power crystal oscillator pins xtal1 and xtal2 are input and output, respectively, of an inverting amplifier which can be configured for use as an on-c hip oscillator, as shown in figure 13. either a quartz crystal or a ceramic resonator may be used. this crystal oscillator is a low power oscillator, with reduced voltage swing on the xtal2 output. it gives the lowest power consumption, but is not capable of driving other clock inputs, and may be more susceptible to noise in noisy environments. in these cases, refer to the ?f ull swing crystal osc illator? on page 28. c1 and c2 should always be equal for both crystals and resonators. the optimal value of the capacitors depends on the crystal or resonator in use, the amount of stray capac- itance, and the electromagnetic noise of the environment. some initial guidelines for choosing capacitors for use with crystals are given in table 6. for ceramic resonators, the capacitor values given by the manufacturer should be used. figure 13. crystal oscillator connections the low power oscillator can operate in three different modes, each optimized for a specific frequency range. the operating mode is selected by the fuses cksel3..1 as shown in table 6 on page 27. xtal2 xtal1 gnd c2 c1
27 atmega48/88/168 2545d?avr?07/04 notes: 1. the frequency ranges are preliminary values. actual values are tbd. 2. this option should not be used with crystals, only with ceramic resonators. 3. if 8 mhz frequency exceeds the specification of the device (depends on v cc ), the ckdiv8 fuse can be programmed in order to divide the internal frequency by 8. it must be ensured that the resulting divided clock meets the freq uency specification of the device. the cksel0 fuse together with the sut1..0 fuses select the start-up times as shown in table 7. notes: 1. these options should only be used wh en not operating close to the maximum fre- quency of the device, and only if frequency stabili ty at start-up is not important for the application. these options ar e not suitable for crystals. 2. these options are intended for use with ceramic resonators and will ensure fre- quency stability at start-up. they can also be used with crystals when not operating close to the maximum frequency of the device, and if frequency stability at start-up is not important for the application. table 6. low power crystal osc illator operating modes (3) frequency range (1) (mhz) cksel3..1 recommended range for capacitors c1 and c2 (pf) 0.4 - 0.9 100 (2) ? 0.9 - 3.0 101 12 - 22 3.0 - 8.0 110 12 - 22 8.0 - 16.0 111 12 - 22 table 7. start-up times for the low power cr ystal oscillator clock selection oscillator source / power conditions start-up time from power-down and power-save additional delay from reset (v cc = 5.0v) cksel0 sut1..0 ceramic resonator, fast rising power 258 ck 14ck + 4.1 ms (1) 000 ceramic resonator, slowly rising power 258 ck 14ck + 65 ms (1) 001 ceramic resonator, bod enabled 1k ck 14ck (2) 010 ceramic resonator, fast rising power 1k ck 14ck + 4.1 ms (2) 011 ceramic resonator, slowly rising power 1k ck 14ck + 65 ms (2) 100 crystal oscillator, bod enabled 16k ck 14ck 1 01 crystal oscillator, fast rising power 16k ck 14ck + 4.1 ms 1 10 crystal oscillator, slowly rising power 16k ck 14ck + 65 ms 1 11
28 atmega48/88/168 2545d?avr?07/04 full swing crystal oscillator pins xtal1 and xtal2 are input and output, respectively, of an inverting amplifier which can be configured for use as an on-c hip oscillator, as shown in figure 13. either a quartz crystal or a ceramic resonator may be used. this crystal oscillator is a full swing osc illator, with rail-to-rail swing on the xtal2 out- put. this is useful for driving other clock inputs and in noisy environments. the current consumption is higher than the ?low power cryst al oscillator? on page 26. note that the full swing crystal oscillator will only operate for v cc = 2.7 - 5.5 volts. c1 and c2 should always be equal for both crystals and resonators. the optimal value of the capacitors depends on the crystal or resonator in use, the amount of stray capac- itance, and the electromagnetic noise of the environment. some initial guidelines for choosing capacitors for use with crystals are given in table 9. for ceramic resonators, the capacitor values given by the manufacturer should be used. the operating mode is selected by th e fuses cksel3..1 as shown in table 8. notes: 1. the frequency ranges are preliminary values. actual values are tbd. 2. if 8 mhz frequency exceeds the specification of the device (depends on v cc ), the ckdiv8 fuse can be programmed in order to divide the internal frequency by 8. it must be ensured that the resulting divided clock meets the freq uency specification of the device. figure 14. crystal oscillator connections table 8. full swing crystal osc illator operating modes (2) frequency range (1) (mhz) cksel3..1 recommended range for capacitors c1 and c2 (pf) 0.4 - 20 011 12 - 22 xtal2 xtal1 gnd c2 c1
29 atmega48/88/168 2545d?avr?07/04 notes: 1. these options should only be used wh en not operating close to the maximum fre- quency of the device, and only if frequency stabili ty at start-up is not important for the application. these options ar e not suitable for crystals. 2. these options are intended for use with ceramic resonators and will ensure fre- quency stability at start-up. they can also be used with crystals when not operating close to the maximum frequency of the device, and if frequency stability at start-up is not important for the application. low frequency crystal oscillator the device can utilize a 32.768 khz watch cr ystal as clock source by a dedicated low frequency crystal oscillator. the crystal s hould be connected as shown in figure 13. when this oscillator is selected, start-up times are determined by the sut fuses and cksel0 as shown in table 10. note: 1. these options should only be used if frequ ency stability at start-up is not important for the application. table 9. start-up times for the full swing crystal oscillato r clock selection oscillator source / power conditions start-up time from power-down and power-save additional delay from reset (v cc = 5.0v) cksel0 sut1..0 ceramic resonator, fast rising power 258 ck 14ck + 4.1 ms (1) 000 ceramic resonator, slowly rising power 258 ck 14ck + 65 ms (1) 001 ceramic resonator, bod enabled 1k ck 14ck (2) 010 ceramic resonator, fast rising power 1k ck 14ck + 4.1 ms (2) 011 ceramic resonator, slowly rising power 1k ck 14ck + 65 ms (2) 100 crystal oscillator, bod enabled 16k ck 14ck 1 01 crystal oscillator, fast rising power 16k ck 14ck + 4.1 ms 1 10 crystal oscillator, slowly rising power 16k ck 14ck + 65 ms 1 11 table 10. start-up times for the low frequenc y crystal oscillator clock selection power conditions start-up time from power-down and power-save additional delay from reset (v cc = 5.0v) cksel0 sut1..0 bod enabled 1k ck 14ck (1) 000 fast rising power 1k ck 14ck + 4.1 ms (1) 001 slowly rising power 1k ck 14ck + 65 ms (1) 010 reserved 0 11 bod enabled 32k ck 14ck 1 00 fast rising power 32k ck 14ck + 4.1 ms 1 01 slowly rising power 32k ck 14ck + 65 ms 1 10 reserved 1 11
30 atmega48/88/168 2545d?avr?07/04 calibrated internal rc oscillator the calibrated inte rnal rc oscillator by default provid es a 8.0 mhz clock. the frequency is nominal value at 3v and 25 c. the device is shipped with the ckdiv8 fuse pro- grammed. see ?system clock prescaler? on page 33 for more details. this clock may be selected as the system clock by program ming the cksel fuses as shown in table 11. if selected, it will operate with no external components. during reset, hardware loads the calibration byte into the osccal regist er and thereby automatically calibrates the rc oscillator. at 3v and 25 c, this calibration gives a frequency of 8 mhz 1%. the oscillator can be calibrated to any frequency in the range 7.3 - 8.1 mhz within 1% accuracy, by changing the osccal register. when this oscillator is used as the chip clock, the watchdog oscillator will still be used for the watchdog timer and for the reset time-out. for more information on the pre-programmed calibration value, see the section ?calibration byte? on page 274. notes: 1. the device is shipped with this option selected. 2. the frequency ranges are preliminary values. actual values are tbd. 3. if 8 mhz frequency exceeds the specification of the device (depends on v cc ), the ckdiv8 fuse can be programmed in order to divide the internal frequency by 8. when this oscillator is selected, start-up times are determined by the sut fuses as shown in table 12 on page 30. note: 1. if the rstdisbl fuse is programmed , this start-up time will be increased to 14ck + 4.1 ms to ensure programming mode can be entered. 2. the device is shipped wit h this option selected. oscillator calibration register ? osccal bits 7..0 ? cal7..0: oscillator calibration value the oscillator calibration register is used to trim the calibrated internal rc oscillator to remove process variations from the oscillato r frequency. the factory-calibrated value is automatically written to this register during chip reset, giving an oscillator frequency of 8.0 mhz at 25c. the applicatio n software can write this regi ster to change the oscillator frequency. the oscillator can be calibrated to any frequency in the range 7.3 - 8.1 mhz within 1% accuracy. calibration outside that range is not guaranteed. table 11. internal calibrated rc os cillator oper ating modes (1)(3) frequency range (2) (mhz) cksel3..0 7.3 - 8.1 0010 table 12. start-up times for the internal calib rated rc oscillator clock selection power conditions start-up time from power- down and power-save additional delay from reset (v cc = 5.0v) sut1..0 bod enabled 6 ck 14ck (1) 00 fast rising power 6 ck 14ck + 4.1 ms 01 slowly rising power 6 ck 14ck + 65 ms (2) 10 reserved 11 bit 76543210 cal7 cal6 cal5 cal4 cal3 cal2 cal1 cal0 osccal read/write r/w r/w r/w r/w r/w r/w r/w r/w initial value device spec ific calibration value
31 atmega48/88/168 2545d?avr?07/04 note that this oscillator is used to time eepr om and flash write accesses, and these write times will be affect ed accordingly. if the eeprom or flash are written, do not cali- brate to more than 8.8 mhz. otherwise, the eeprom or flash write may fail. the cal7 bit determines the range of o peration for the oscillator. setting this bit to 0 gives the lowest frequency range, setting this bit to 1 gives the highest frequency range. the two frequency ranges are overlapping, in other words a setting of osccal = 0x7f gives a higher frequency than osccal = 0x80. the cal6..0 bits are used to tune the frequency within the selected range. a setting of 0x00 gives the lowest frequency in that range, and a setting of 0x7f gives the highest frequency in the range. incrementing cal6..0 by 1 will give a frequency increment of less than 2% in the frequency range 7.3 - 8.1 mhz. 128 khz internal oscillator the 128 khz internal oscillato r is a low power oscillator pr oviding a clock of 128 khz. the frequency is nominal at 3v and 25 c. this clock may be select as the system clock by programming the c ksel fuses to ?11? as shown in table 13. note: 1. the frequency is preliminary value. actual value is tbd. when this clock source is selected, start- up times are determined by the sut fuses as shown in table 14. note: 1. if the rstdisbl fuse is programmed , this start-up time will be increased to 14ck + 4.1 ms to ensure programming mode can be entered. external clock the device can utilize a external clock source as shown in figure 15. to run the device on an external clock, the cksel fuses must be programmed as shown in table 15. notes: 1. the frequency ranges are preliminary values. actual values are tbd. 2. if 8 mhz frequency exceeds the specification of the device (depends on v cc ), the ckdiv8 fuse can be programmed in order to divide the internal frequency by 8. it must be ensured that the resulting divided clock meets the freq uency specification of the device. table 13. 128 khz internal oscillator operating modes nominal frequency cksel3..0 128 khz 0011 table 14. start-up times for the 1 28 khz internal oscillator power conditions start-up time from power- down and power-save additional delay from reset sut1..0 bod enabled 6 ck 14ck (1) 00 fast rising power 6 ck 14ck + 4 ms 01 slowly rising power 6 ck 14ck + 64 ms 10 reserved 11 table 15. full swing crystal osc illator oper ating modes (2) frequency range (1) (mhz) cksel3..0 recommended range for capacitors c1 and c2 (pf) 0 - 100 0000 12 - 22
32 atmega48/88/168 2545d?avr?07/04 figure 15. external clock drive configuration when this clock source is selected, start- up times are determined by the sut fuses as shown in table 16. when applying an external clock, it is required to avoid sudden changes in the applied clock frequency to ensure stable operation of the mcu. a variation in frequency of more than 2% from one clock cycle to the next can lead to unpredictable behavior. if changes of more than 2% is required, ensure that the mcu is kept in reset during the changes. note that the system clock prescaler can be used to implement run-time changes of the internal clock frequency while still ensuring stable operat ion. refer to ?system clock prescaler? on page 33 for details. clock output buffer the device can output the system clock on the clko pin. to enable the output, the ckout fuse has to be programmed. this mode is suitable when the chip clock is used to drive other circuits on the system. the cl ock also will be output du ring reset, and the normal operation of i/o pin w ill be overridden when the fuse is programmed. any clock source, including the internal rc oscillator, can be selected when the clock is output on clko. if the system clock prescaler is us ed, it is the divided system clock that is output. timer/counter oscillator the device can operate its timer/counter2 from an external 32.768 khz watch crystal or a external clock source. the timer/counte r oscillator pins (tosc1 and tosc2) are shared with xtal1 and xtal2. this means t hat the timer/counter oscillator can only be used when an internal rc oscillator is selected as system clock source. see figure 13 on page 26 for crystal connection. applying an external clock source to tosc1 requires extclk in the assr register written to logic one. see ?asynchronous operation of the timer/counter? on page 149 for further description on selecting external clock as input instead of a 32 khz crystal. table 16. start-up times for the external clock selection power conditions start-up time from power- down and power-save additional delay from reset (v cc = 5.0v) sut1..0 bod enabled 6 ck 14ck 00 fast rising power 6 ck 14ck + 4.1 ms 01 slowly rising power 6 ck 14ck + 65 ms 10 reserved 11 nc external clock signal xtal2 xtal1 gnd
33 atmega48/88/168 2545d?avr?07/04 system clock prescaler the atmega48/88/168 has a system clock prescaler, and the system clock can be divided by setting the ?clock prescale register ? clkpr? on page 337. this feature can be used to decrease the system clock fre quency and the power consumption when the requirement for processing power is low. this can be used with all clock source options, and it will affect the clock frequency of the cp u and all synchronous peripherals. clk i/o , clk adc , clk cpu , and clk flash are divided by a factor as shown in table 20 on page 41. when switching between prescaler settings, the system clock prescaler ensures that no glitches occurs in the clock system. it also ensures that no intermediate frequency is higher than neither the clock frequency corresponding to the previous setting, nor the clock frequency corresponding to the new setting. the ripple counter that implements the prescaler runs at the frequency of the un divided clock, which may be faster than the cpu's clock frequency. hence, it is not possible to determine the state of the prescaler - even if it were readable, and the exact time it takes to switch from one clock division to the other cannot be exactly predicted. from the time the clkps values are written, it takes between t1 + t2 and t1 + 2 * t2 before the new clock frequency is active. in this interval, 2 active clock edges are produced. here, t1 is the previous clock period, and t2 is the period corresponding to the new prescaler setting. to avoid unintentional changes of clock frequency, a special write procedure must befollowed to change the clkps bits: 1. write the clock prescaler change enable (clkpce) bit to one and all other bitsin clkpr to zero. 2. within four cycles, write the desire d value to clkps while writing a zero to clkpce. interrupts must be disabled when changing prescaler setting to make sure the write pro- cedure is not interrupted. clock prescale register ? clkpr bit 7 ? clkpce: clock prescaler change enable the clkpce bit must be written to logic one to enable change of the clkps bits. the clkpce bit is only updated when the other bi ts in clkpr are simu ltaneously written to zero. clkpce is cleared by hardware four cycl es after it is written or when clkps bits are written. rewriting the clkpce bit within th is time-out period does neither extend the time-out period, nor clear the clkpce bit. bits 3..0 ? clkps3..0: clock prescaler select bits 3 - 0 these bits define the division factor between the selected clock source and the internal system clock. these bits can be written run- time to vary the clock frequency to suit the application requirements. as t he divider divides the master clock input to the mcu, the speed of all synchronous peripherals is reduce d when a division factor is used. the divi- sion factors are given in table 17. the ckdiv8 fuse determines the initial value of the clkps bits. if ckdiv8 is unpro- grammed, the clkps bits will be reset to ?0000?. if ckdiv8 is programmed, clkps bits are reset to ?0011?, giving a division factor of 8 at start up. this feature should be used if the selected clock source has a higher frequency than the maximum frequency of the device at the present operating conditions. note that any value can be written to the clkps bits regardless of the ckdiv8 fuse setting. the application software must ensure that a sufficient division factor is ch osen if the selected clock source has a higher bit 76543210 clkpce ? ? ? clkps3 clkps2 clkps1 clkps0 clkpr read/write r/w r r r r/w r/w r/w r/w initial value 0 0 0 0 see bit description
34 atmega48/88/168 2545d?avr?07/04 frequency than the maximum frequency of th e device at the present operating condi- tions. the device is shipped with the ckdiv8 fuse programmed. table 17. clock prescaler select clkps3 clkps2 clkps1 clkps0 clock division factor 0000 1 0001 2 0010 4 0011 8 0100 16 0101 32 0110 64 0111 128 1000 256 1001 reserved 1010 reserved 1011 reserved 1100 reserved 1101 reserved 1110 reserved 1111 reserved
35 atmega48/88/168 2545d?avr?07/04 power management and sleep modes sleep modes enable the application to shut down unused modules in the mcu, thereby saving power. the avr provides various sleep modes allowing the user to tailor the power consumption to the application?s requirements. to enter any of the five sleep modes, the se bit in smcr must be written to logic one and a sleep instruction must be executed. th e sm2, sm1, and sm 0 bits in the smcr register select which sleep mode (idle, adc noise reduction, power-down, power- save, or standby) will be activated by the sleep instru ction. see table 18 for a sum- mary. if an enabled interrupt occurs while the mcu is in a sleep mode, the mcu wakes up. the mcu is then halted for four cycles in addition to the start-up time, executes the interrupt routine, and resume s execution from the instruct ion following sleep. the con- tents of the register file and sram are unaltered when the device wakes up from sleep. if a reset occurs during sleep mode, the mcu wakes up and executes from the reset vector. figure 12 on page 24 presents the different clock systems in the atmega48/88/168, and their distribution. the figure is helpful in selecting an appropriate sleep mode. sleep mode control register ? smcr the sleep mode control register contains control bits for power management. bits 7..4 res: reserved bits these bits are unused bits in the atm ega48/88/168, and will alwa ys read as zero. bits 3..1 ? sm2..0: sleep mode select bits 2, 1, and 0 these bits select between the five available sleep modes as shown in table 18. note: 1. standby mode is only recommended for use with external crystals or resonators. bit 0 ? se: sleep enable the se bit must be written to logic one to make the mcu enter the sleep mode when the sleep instruction is executed. to avoid the mcu entering the sleep mode unless it is the programmer?s purpose, it is recommended to write the sleep enable (se) bit to one just before the execution of the sleep instruction and to cl ear it immediately after wak- ing up. bit 76543210 ????sm2sm1sm0sesmcr read/write r r r r r/w r/w r/w r/w initial value00000000 table 18. sleep mode select sm2 sm1 sm0 sleep mode 000idle 0 0 1 adc noise reduction 010power-down 011power-save 100reserved 101reserved 110standby (1) 111reserved
36 atmega48/88/168 2545d?avr?07/04 idle mode when the sm2..0 bits are written to 000, the sleep instruction makes the mcu enter idle mode, stopping the cpu but allowing the spi, usart, analog comparator, adc, 2-wire serial interface, timer/counters, watchdog, and the interrupt system to continue operating. this sleep mode basically halts clk cpu and clk flash , while allowing the other clocks to run. idle mode enables the mcu to wake up fr om external triggered interrupts as well as internal ones like the timer overflow and usart transmit complete interrupts. if wake-up from the analog comparator interrupt is not required, the analog comparator can be powered down by setting the acd bit in the analog comparator control and sta- tus register ? acsr. this will reduce power consumption in idle mode. if the adc is enabled, a conversion starts automatically when this mode is entered. adc noise reduction mode when the sm2..0 bits are written to 001, the sleep instruction makes the mcu enter adc noise reduction mode, stopping the cpu but allowing the adc, the external interrupts, the 2-wire serial interface address watch, timer/counter2, and the watchdog to continue operating (if enabled). this sleep mode basically halts clk i/o , clk cpu , and clk flash , while allowing the other clocks to run. this improves the noise environment for the adc, enabling higher resolution measure- ments. if the adc is enabled, a conversion starts automatically when this mode is entered. apart from the adc conversion complete interrupt, only an external reset, a watchdog system reset, a watchdog interrupt, a brown-out reset, a 2-wire serial interface address match, a timer/counter2 interrupt, an spm/eeprom ready interrupt, an external level interrupt on int0 or int1 or a pin change interrupt can wake up the mcu from adc noise reduction mode. power-down mode when the sm2..0 bits are written to 010, the sleep instruction makes the mcu enter power-down mode. in this mode, the external oscillator is stopped, while the external interrupts, the 2-wire serial interface address watch, and the watchdog continue operat- ing (if enabled). only an external reset, a watchdog system reset, a watchdog interrupt, a brown-out reset, a 2-wire serial interface address match, an external level interrupt on int0 or int1, or a pin change interrupt can wake up the mcu. this sleep mode basically halts all generated clocks, al lowing operation of asynchronous modules only. note that if a level triggered interrupt is used for wake-up from power-down mode, the changed level must be held for some time to wake up the mcu. refer to ?external inter- rupts? on page 80 for details. when waking up from power-down mode, there is a delay from the wake-up condition occurs until the wake-up becomes effective. this allows the clock to restart and become stable after having been stopped. the wake-up period is defined by the same cksel fuses that define the reset time-out period, as described in ?clock sources? on page 25. power-save mode when the sm2..0 bits are written to 011, the sleep instruction makes the mcu enter power-save mode. this mode is identica l to power-down, with one exception: if timer/counter2 is enabled, it will keep running during sl eep. the device can wake up from either timer overflow or output com pare event from timer/counter2 if the corre- sponding timer/counter2 interrupt enable bits are set in timsk2, and the global interrupt enable bit in sreg is set. if timer/counter2 is not running, power-down mode is recommended instead of power- save mode.
37 atmega48/88/168 2545d?avr?07/04 the timer/counter2 can be clocked both synchronously and asynchronously in power- save mode. if timer/counter2 is not usi ng the asynchronous clock, the timer/counter oscillator is stopped during sl eep. if timer/counte r2 is not using the synchronous clock, the clock source is stopped during sleep. note that even if the synchronous clock is run- ning in power-save, this clock is only available for timer/counter2. standby mode when the sm2..0 bits are 110 and an external crystal/resonator clock option is selected, the sleep instruction makes the mcu enter standby mode. this mode is identical to power-down with the exception that the oscillator is kept running. from standby mode, the device wakes up in six clock cycles. notes: 1. only recommended with external crystal or resonator selected as clock source. 2. if timer/counter2 is running in asynchronous mode. 3. for int1 and int0, only level interrupt. power reduction register the power reduction register, prr, provides a method to stop the clock to individual peripherals to reduce power consumption. the current state of the peripheral is frozen and the i/o registers can not be read or written. resources used by the peripheral when stopping the clock will remain occupied, hence the peripheral should in most cases be disabled before stopping the clock. waking up a module, which is done by clearing the bit in prr, puts the module in the same state as before shutdown. module shutdown can be used in idle mode and active mode to significantly reduce the overall power consumption. see ?power-d own supply current? on page 306 for exam- ples. in all other sleep modes, the clock is already stopped. power reduction register - prr bit 7 - prtwi: power reduction twi writing a logic one to this bit shuts down the twi by stopping the clock to the module. when waking up the twi again, the twi should be re initialized to ensure proper operation. table 19. active clock domains and wake-up sources in the different sleep modes. active clock domains oscillators wake-up sources sleep mode clk cpu clk flash clk io clk adc clk asy main clock source enabled timer oscillator enabled int1, int0 and pin change twi address match timer2 spm/eeprom ready adc wdt otheri/o idle xxx x x (2) x x x x xxx adc noise reduction x x x x (2) x (3) xxxxx power-down x (3) xx power-save x x x (3) xx x standby (1) xx (3) xx bit 765432 1 0 prtwi prtim2 prtim0 ? prtim1 prspi prusart0 pradc prr read/write r/w r/w r/w r r/w r/w r/w r/w initial value 0 0 0 0 0 0 0 0
38 atmega48/88/168 2545d?avr?07/04 bit 6 - prtim2: power reduction timer/counter2 writing a logic one to this bit shuts down the timer/counter2 module in synchronous mode (as2 is 0). when the timer/counter 2 is enabled, operation will continue like before the shutdown. bit 5 - prtim0: power reduction timer/counter0 writing a logic one to this bit shuts down the timer/counter0 module. when the timer/counter0 is enabl ed, operation will continue like before the shutdown. bit 4 - res: reserved bit this bit is reserved in atmega48/88 /168 and will always read as zero. bit 3 - prtim1: power reduction timer/counter1 writing a logic one to this bit shuts down the timer/counter1 module. when the timer/counter1 is enabl ed, operation will continue like before the shutdown. bit 2 - prspi: power reduction serial peripheral interface writing a logic one to this bit shuts down the serial peripheral interface by stopping the clock to the module. when waking up the sp i again, the spi should be re initialized to ensure proper operation. bit 1 - prusart0: power reduction usart0 writing a logic one to this bit shuts down the usart by stopping the clock to the mod- ule. when waking up the usart again, the us art should be re initialized to ensure proper operation. bit 0 - pradc: power reduction adc writing a logic one to this bit shuts down the adc. the adc must be disabled before shut down. the analog comparator cannot use the adc input mux when the adc is shut down. minimizing power consumption there are several possibilities to consider when trying to minimize the power consump- tion in an avr controlled system. in general, sleep modes should be used as much as possible, and the sleep mode should be selected so that as few as possible of the device?s functions are operating. all functi ons not needed should be disabled. in particu- lar, the following modules may need special consideration when trying to achieve the lowest possible power consumption. analog to digital converter if enabled, the adc will be enabled in all sleep modes. to save power, the adc should be disabled before entering any sleep mode. when the adc is turned off and on again, the next conversion will be an extended conversion. refer to ?analog-to-digital con- verter? on page 231 for details on adc operation. analog comparator when entering idle mode, the analog comparator should be disabled if not used. when entering adc noise reduction mode, the a nalog comparator should be disabled. in other sleep modes, the analog comparator is automatically disabled. however, if the analog comparator is set up to use the internal voltage reference as input, the analog comparator should be disabled in all sleep modes. otherwise, the internal voltage ref- erence will be enabled, independent of sleep mode. refer to ?analog comparator? on page 228 for details on how to configure the analog comparator. brown-out detector if the brown-out detector is not needed by the application, this module should be turned off. if the brown-out detector is enabled by the bodlevel fuses, it will be enabled in all sleep modes, and hence, always consume power. in the deeper sleep modes, this
39 atmega48/88/168 2545d?avr?07/04 will contribute significantly to the total curr ent consumption. refer to ?brown-out detec- tion? on page 43 for details on how to configure the brown-out detector. internal voltage reference the internal voltage reference will be enabled when needed by the brown-out detec- tion, the analog comparator or the adc. if these modules are disabled as described in the sections above, the internal voltage reference will be disabled and it will not be con- suming power. when turned on again, the user must allow the reference to start up before the output is used. if the reference is kept on in sleep mode, the output can be used immediately. refer to ?internal voltage reference? on page 45 for details on the start-up time. watchdog timer if the watchdog timer is not needed in the application, the module should be turned off. if the watchdog timer is enabled, it will be enabled in all sleep modes and hence always consume power. in the deeper sleep modes, this will contribute significantly to the total current consumption. refer to ?watchdog timer? on page 46 for details on how to configure the watchdog timer. port pins when entering a sleep mode, all port pins should be configured to use minimum power. the most important is then to ensure that no pins drive resistive loads. in sleep modes where both the i/o clock (clk i/o ) and the adc clock (clk adc ) are stopped, the input buff- ers of the device will be disabled. this ensures that no power is consumed by the input logic when not needed. in some cases, the input logic is needed for detecting wake-up conditions, and it will then be enabled. refer to the section ?digital input enable and sleep modes? on page 66 for details on which pins are enabled. if the input buffer is enabled and the input signal is left floating or have an analog signal level close to v cc /2, the input buffer will use excessive power. for analog input pins, the digital input buffe r should be disabled at all times. an analog signal level close to v cc /2 on an input pin can cause signi ficant current even in active mode. digital input buffers can be disabled by writing to the digital input disable regis- ters (didr1 and didr0). refer to ?digital input disable register 1 ? didr1? on page 230 and ?digital input disable register 0 ? didr0? on page 245 for details. on-chip debug system if the on-chip debug system is enabled by the dwen fuse and the chip enters sleep mode, the main clock source is enabled and hence always consumes power. in the deeper sleep modes, this will contribute significantly to the total current consumption.
40 atmega48/88/168 2545d?avr?07/04 system control and reset resetting the avr during reset, all i/o registers are set to their initial values, and the program starts exe- cution from the reset vector. for the atmega168, the instruction placed at the reset vector must be a jmp ? absolute jump ? instruction to the reset handling routine. for the atmega48 and atmega88, the instruction placed at the reset vector must be an rjmp ? relative jump ? instruction to the reset handling routine. if the program never enables an interrupt source, the interrupt vectors are not used, and regular program code can be placed at these locations. this is also the case if the reset vector is in the application section while the interrupt vectors are in the boot section or vice versa (atmega88/168 only). the circuit diagram in figure 16 shows the reset logic. table 20 defines the electrical parameters of the reset circuitry. the i/o ports of the avr are immediately reset to their initial state when a reset source goes active. this does not require any clock source to be running. after all reset sources have gone inactive, a delay counter is invoked, stretching the internal reset. this allows the power to reach a stable level before normal operation starts. the time-out period of the delay count er is defined by the user through the sut and cksel fuses. the different selections for the delay period are presented in ?clock sources? on page 25. reset sources the atmega48/88/168 has four sources of reset: power-on reset. the mcu is reset when the supply voltage is below the power-on reset threshold (v pot ). external reset. the mcu is reset when a low level is present on the reset pin for longer than the minimum pulse length. watchdog system reset. the mcu is reset when the watchdog timer period expires and the watchdog system reset mode is enabled. brown-out reset. the mcu is reset when the supply voltage v cc is below the brown-out reset threshold (v bot ) and the brown-out detector is enabled.
41 atmega48/88/168 2545d?avr?07/04 figure 16. reset logic notes: 1. values are guidelines only. actual values are tbd. 2. the power-on reset will not work unless the supply voltage has been below v pot (falling) table 20. reset characteristics (1) symbol parameter condition min typ max units v pot power-on reset threshold voltage (rising) tbd tbd tbd v power-on reset threshold voltage (falling) (2) tbd tbd tbd v v rst reset pin threshold voltage 0.1 0.9 v t rst minimum pulse width on reset pin 2.5 s mcu status register (mcusr) brown-out reset circuit bodlevel [2..0] delay counters cksel[3:0] ck timeout wdrf borf extrf porf data b u s clock generator spike filter pull-up resistor watchdog oscillator sut[1:0] power-on reset circuit rstdisbl
42 atmega48/88/168 2545d?avr?07/04 power-on reset a power-on reset (por) pulse is generated by an on-chip detection circuit. the detec- tion level is defined in table 20. the por is activated whenever v cc is below the detection level. the por circuit can be used to trigger the start-up reset, as well as to detect a failure in supply voltage. a power-on reset (por) circuit ensures that the device is reset from power-on. reach- ing the power-on reset threshold voltage invokes the delay counter, which determines how long the device is kept in reset after v cc rise. the reset signal is activated again, without any delay, when v cc decreases below t he detection level. figure 17. mcu start-up, reset tied to v cc figure 18. mcu start-up, reset extended externally external reset an external reset is generated by a low level on the reset pin. reset pulses longer than the minimum pulse width (see table 20) will generate a reset, even if the clock is not running. shorter pulses are not guaranteed to generate a reset. when the applied signal reaches the reset threshold voltage ? v rst ? on its positive edge, the delay counter starts the mcu after the time-out period ? t tout ? has expired. the external reset can be disabled by the rstdisbl fuse, see table 120 on page 273. v reset time-out internal reset t tout v pot v rst cc reset time-out internal reset t tout v pot v rst v cc
43 atmega48/88/168 2545d?avr?07/04 figure 19. external reset during operation brown-out detection atmega48/88/168 has an on-chip brown-out de tection (bod) circuit for monitoring the v cc level during operation by comparing it to a fixed trigger level. the trigger level for the bod can be selected by th e bodlevel fuses. the trigge r level has a hysteresis to ensure spike free brown-out detection. the hysteresis on the detection level should be interpreted as v bot+ = v bot + v hyst /2 and v bot- = v bot - v hyst /2. notes: 1. v bot may be below nominal minimum operating voltage for some devices. for devices where this is the case, t he device is tested down to v cc = v bot during the production test. this guar antees that a brown-out reset will occur before v cc drops to a voltage where correct operation of the microcontroller is no longer guaranteed. the test is performed using bo dlevel = 110 and bodlevel = 101 for atmega48v/88v/168v, and bodlevel = 101 and bodlevel = 101 for atmega48/88/168. 2. min/max values applicable for atmega48. when the bod is enabled, and v cc decreases to a value below the trigger level (v bot- in figure 20), the brown-out reset is immediately activated. when v cc increases above the trigger level (v bot+ in figure 20), the delay counter starts the mcu after the time- out period t tout has expired. cc table 21. bodlevel fuse coding (1) bodlevel 2..0 fuses min v bot typ v bot max v bot units 111 bod disabled 110 1.7 (2) 1.8 2.0 (2) v 101 2.5 (2) 2.7 2.9 (2) 100 4.1 (2) 4.3 4.5 (2) 011 reserved 010 001 000 table 22. brown-out characteristics symbol parameter min typ max units v hyst brown-out detector hysteresis 50 mv t bod min pulse width on brown-out reset ns
44 atmega48/88/168 2545d?avr?07/04 the bod circuit will only detect a drop in v cc if the voltage stays below the trigger level for longer than t bod given in table 20. figure 20. brown-out reset during operation watchdog system reset when the watchdog times out, it will generate a short rese t pulse of one ck cycle dura- tion. on the falling edge of this pulse, the delay timer starts counting the time-out period t tout . refer to page 46 for details on operation of the watchdog timer. figure 21. watchdog system reset during operation mcu status register ? mcusr the mcu status register provides information on which reset source caused an mcu reset. bit 7..4: res: reserved bits these bits are unused bits in the atm ega48/88/168, and will alwa ys read as zero. bit 3 ? wdrf: watchdog system reset flag this bit is set if a watchdog system reset o ccurs. the bit is reset by a power-on reset, or by writing a logic zero to the flag. bit 2 ? borf: brown-out reset flag this bit is set if a brown-out reset occurs. the bit is reset by a power-on reset, or by writing a logic zero to the flag. v cc reset time-out internal reset v bot- v bot+ t tout ck cc bit 76543210 ? ? ? ? wdrf borf extrf porf mcusr read/write r r r r r/w r/w r/w r/w initial value 0 0 0 0 see bit description
45 atmega48/88/168 2545d?avr?07/04 bit 1 ? extrf: external reset flag this bit is set if an external reset occurs. the bit is reset by a power-on reset, or by writing a logic zero to the flag. bit 0 ? porf: power-on reset flag this bit is set if a power-on reset occurs. th e bit is reset only by writing a logic zero to the flag. to make use of the reset flags to identify a reset condition, the user should read and then reset the mcusr as early as possible in the program. if the register is cleared before another reset occurs, the source of the reset can be found by examining the reset flags. internal voltage reference atmega48/88/168 features an internal bandgap reference. this reference is used for brown-out detection, and it can be used as an input to the analog comparator or the adc. voltage reference enable signals and start-up time the voltage reference has a start-up time that may influence the way it should be used. the start-up time is given in table 23. to save power, the reference is not always turned on. the reference is on during the following situations: 1. when the bod is enabled (by prog ramming the bodlevel [2..0] fuses). 2. when the bandgap reference is connected to the analog comparator (by setting the acbg bit in acsr). 3. when the adc is enabled. thus, when the bod is not enabled, after setting the acbg bit or enabling the adc, the user must always allow the reference to start up before the output from the analog com- parator or adc is used. to reduce power consumption in power-down mode, the user can avoid the three conditions above to ensure that the reference is turned off before entering power-down mode. note: 1. values are guidelines only. actual values are tbd. table 23. internal voltage reference characteristics (1) symbol parameter condi tion min typ max units v bg bandgap reference voltage tbd 1.0 1.1 1.2 v t bg bandgap reference start-up time tbd 40 70 s i bg bandgap reference current consumption tbd 10 tbd a
46 atmega48/88/168 2545d?avr?07/04 watchdog timer atmega48/88/168 has an enhanced watchdog timer (wdt). the main features are: clocked from separate on-chip oscillator 3 operating modes ? interrupt ? system reset ? interrupt and system reset selectable time-out pe riod from 16ms to 8s possible hardware fuse watchdog al ways on (wdton) for fail-safe mode figure 22. watchdog timer the watchdog timer (wdt) is a timer counting cycles of a separate on-chip 128 khz oscillator. the wdt gives an interrupt or a system reset when the counter reaches a given time-out value. in normal operation mo de, it is required that the system uses the wdr - watchdog timer reset - instruction to restart the counter before the time-out value is reached. if the system doesn't restart the counter, an interrupt or system reset will be issued. in interrupt mode, the wdt gives an interrupt when the timer expires. this interrupt can be used to wake the device from sleep-modes, and also as a general system timer. one example is to limit the maximum time allow ed for certain operations, giving an interrupt when the operation has run longer than expected. in system reset mode, the wdt gives a reset when the timer expires. this is typically used to prevent system hang-up in case of runaway code. the third mode, interrupt and system reset mode, combines the other two modes by first giving an interrupt and then switch to system reset mode. this mode will for instance allow a safe shutdown by saving critical para meters before a sys- tem reset. the watchdog always on (wdton) fuse, if programmed, will force the watchdog timer to system reset mode. with the fuse programmed the system reset mode bit (wde) and interrupt mode bit (wdie) are locked to 1 and 0 respectively. to further ensure pro- gram security, alterations to the watchdog set-up must follow timed sequences. the sequence for clearing wde and changing time-out configuration is as follows: 1. in the same operation, write a logic one to the watchdog change enable bit (wdce) and wde. a logic one must be written to wde regardless of the previ- ous value of the wde bit. 2. within the next four clock cycles, write the wde and watchdog prescaler bits (wdp) as desired, but with the wdce bit cleared. this must be done in one operation. 128khz oscillator osc/2k osc/4k osc/8k osc/16k osc/32k osc/64k osc/128k osc/256k osc/512k osc/1024k wdp0 wdp1 wdp2 wdp3 watchdog reset wde wdif wdie mcu reset interrupt
47 atmega48/88/168 2545d?avr?07/04 the following code example shows one assembly and one c function for turning off the watchdog timer. the example assumes that interrupts are controlled (e.g. by disabling interrupts globally) so that no interrupts wi ll occur during the execution of these functions. note: 1. the example code assumes that the pa rt specific header file is included. note: if the watchdog is accidentally enabled, for example by a runaway pointer or brown-out condition , the device will be reset and the watchdog timer will stay enabled. if the code is not set up to handle the watchdog, this might lead to an eternal loop of time-out resets. to avoid this situation, th e application software should always clear the assembly code example (1) wdt_off: ; turn off global interrupt cli ; reset watchdog timer wdr ; clear wdrf in mcusr in r16, mcusr andi r16, (0xff & (0< 48 atmega48/88/168 2545d?avr?07/04 watchdog system reset flag (wdrf) and the wde control bit in the initialisation rou- tine, even if the watchdog is not in use. the following code example shows one assembly and one c function for changing the time-out value of the watchdog timer. note: 1. the example code assumes that the pa rt specific header file is included. note: the watchdog timer should be reset before any change of the wdp bits, since a change in the wdp bits can result in a time -out when switching to a shorter time-out period. assembly code example (1) wdt_prescaler_change: ; turn off global interrupt cli ; reset watchdog timer wdr ; start timed sequence lds r16, wdtcsr ori r16, (1< 49 atmega48/88/168 2545d?avr?07/04 watchdog timer control register - wdtcsr bit 7 - wdif: watchdog interrupt flag this bit is set when a time-out occurs in the watchdog timer and the watchdog timer is configured for interrupt. wdif is cleared by hardware when executing the corresponding interrupt handling vector. alternatively, wdif is cleared by writing a logic one to the flag. when the i-bit in sreg and wdie are set, the watchdog time-out interrupt is executed. bit 6 - wdie: watchdog interrupt enable when this bit is written to one and the i-bit in the status register is set, the watchdog interrupt is enabled. if wde is cleared in combination with this setting, the watchdog timer is in interrupt mode, and the corresponding interrupt is executed if time-out in the watchdog timer occurs. if wde is set, the watchdog timer is in interrupt and system reset mode. the first time-out in the watchdog timer will set wdif. executing the corresponding interrupt vector will clear wdie and wdif automatically by hardware (the wa tchdog goes to sys- tem reset mode). this is useful for keeping the watchdog timer security while using the interrupt. to stay in interrupt and system reset mode, wdie must be set after each interrupt. this should however not be done within the interrupt service routine itself, as this might compromise the safety-function of the watchdog system reset mode. if the interrupt is not executed be fore the next time-out, a system reset will be applied. bit 4 - wdce: watchdog change enable this bit is used in timed sequences for changing wde and prescaler bits. to clear the wde bit, and/or change the prescaler bits, wdce must be set. once written to one, ha rdware will clear wdce after four clock cycles. bit 3 - wde: watchdog system reset enable wde is overridden by wdrf in mcusr. this means that wde is always set when wdrf is set. to clear wde, wdrf must be cl eared first. this feature ensures multiple resets during conditions causing failure, and a safe start-up after the failure. bit 5, 2..0 - wdp3..0: watchdog timer prescaler 3, 2, 1 and 0 the wdp3..0 bits determine the watchdog timer prescaling when the watchdog timer is running. the different prescaling values and their corresponding time-out periods are shown in table 25 on page 50. bit 76543210 wdif wdie wdp3 wdce wde wdp2 wdp1 wdp0 wdtcsr read/write r/w r/w r/w r/w r/w r/w r/w r/w initial value0000x000 table 24. watchdog timer configuration wdton wde wdie mode action on time-out 0 0 0 stopped none 0 0 1 interrupt mode interrupt 0 1 0 system reset mode reset 011 interrupt and system reset mode interrupt, then go to system reset mode 1 x x system reset mode reset
50 atmega48/88/168 2545d?avr?07/04 . table 25. watchdog timer prescale select wdp3 wdp2 wdp1 wdp0 number of wdt oscillator cycles typical time-out at v cc = 5.0v 0 0 0 0 2k (2048) cycles 16 ms 0 0 0 1 4k (4096) cycles 32 ms 0 0 1 0 8k (8192) cycles 64 ms 0 0 1 1 16k (16384) cycles 0.125 s 0 1 0 0 32k (32768) cycles 0.25 s 0 1 0 1 64k (65536) cycles 0.5 s 0 1 1 0 128k (131072) cycles 1.0 s 0 1 1 1 256k (262144) cycles 2.0 s 1 0 0 0 512k (524288) cycles 4.0 s 1 0 0 1 1024k (1048576) cycles 8.0 s 1010 reserved 1011 1100 1101 1110 1111
51 atmega48/88/168 2545d?avr?07/04 interrupts this section describes the specifics of the interrupt handling as performed in atmega48/88/168. for a general explanation of the avr interrupt handling, refer to ?reset and interrupt handling? on page 12. the interrupt vectors in atmega48, atmega88 and atmega168 are generally the same, with the following differences: each interrupt vector occupies two instruction words in atmega168, and one instruction word in atmega48 and atmega88. atmega48 does not have a separate boot loader section. in atmega88 and atmega168, the reset vector is affected by the bootrst fuse, and the interrupt vector start address is affected by the ivsel bit in mcucr. interrupt vectors in atmega48 table 26. reset and interrupt vectors in atmega48 vector no. program address source interrupt definition 1 0x000 reset external pin, power-on reset, brown-out reset and watchdog system reset 2 0x001 int0 external interrupt request 0 3 0x002 int1 external interrupt request 1 4 0x003 pcint0 pin change interrupt request 0 5 0x004 pcint1 pin change interrupt request 1 6 0x005 pcint2 pin change interrupt request 2 7 0x006 wdt watchdog time-out interrupt 8 0x007 timer2 compa timer/c ounter2 compare match a 9 0x008 timer2 compb timer/c ounter2 compare match b 10 0x009 timer2 ovf timer/counter2 overflow 11 0x00a timer1 capt timer/counter1 capture event 12 0x00b timer1 compa timer/counter1 compare match a 13 0x00c timer1 compb timer/c outner1 compare match b 14 0x00d timer1 ovf timer/counter1 overflow 15 0x00e timer0 compa timer/counter0 compare match a 16 0x00f timer0 compb timer/c ounter0 compare match b 17 0x010 timer0 ovf timer/counter0 overflow 18 0x011 spi, stc spi serial transfer complete 19 0x012 usart, rx usart rx complete 20 0x013 usart, udre usart, data register empty 21 0x014 usart, tx usart, tx complete 22 0x015 adc adc conversion complete 23 0x016 ee ready eeprom ready 24 0x017 analog comp analog comparator 25 0x018 twi 2-wire serial interface 26 0x019 spm ready store program memory ready
52 atmega48/88/168 2545d?avr?07/04 the most typical and general program setup for the reset and interrupt vector addresses in atmega48 is: address labels code comments 0x000 rjmp reset ; reset handler 0x001 rjmp ext_int0 ; irq0 handler 0x002 rjmp ext_int1 ; irq1 handler 0x003 rjmp pcint0 ; pcint0 handler 0x004 rjmp pcint1 ; pcint1 handler 0x005 rjmp pcint2 ; pcint2 handler 0x006 rjmp wdt ; watchdog timer handler 0x007 rjmp tim2_compa ; timer2 compare a handler 0x008 rjmp tim2_compb ; timer2 compare b handler 0x009 rjmp tim2_ovf ; timer2 overflow handler 0x00a rjmp tim1_capt ; timer1 capture handler 0x00b rjmp tim1_compa ; timer1 compare a handler 0x00c rjmp tim1_compb ; timer1 compare b handler 0x00d rjmp tim1_ovf ; timer1 overflow handler 0x00e rjmp tim0_compa ; timer0 compare a handler 0x00f rjmp tim0_compb ; timer0 compare b handler 0x010 rjmp tim0_ovf ; timer0 overflow handler 0x011 rjmp spi_stc ; spi transfer complete handler 0x012 rjmp usart_rxc ; usart, rx complete handler 0x013 rjmp usart_udre ; usart, udr empty handler 0x014 rjmp usart_txc ; usart, tx complete handler 0x015 rjmp adc ; adc conversion complete handler 0x016 rjmp ee_rdy ; eeprom ready handler 0x017 rjmp ana_comp ; analog comparator handler 0x018 rjmp twi ; 2-wire serial interface handler 0x019 rjmp spm_rdy ; store program memory ready handler ; 0x01areset: ldi r16, high(ramend); main program start 0x01b out sph,r16 ; set stack pointer to top of ram 0x01c ldi r16, low(ramend) 0x01d out spl,r16 0x01e sei ; enable interrupts 0x01f xxx ... ... ... ...
53 atmega48/88/168 2545d?avr?07/04 interrupt vectors in atmega88 notes: 1. when the bootrst fuse is programmed, the device will jump to the boot loader address at reset, see ?boot loader suppor t ? read-while-write self-programming, atmega88 and atmega168? on page 255. 2. when the ivsel bit in mcucr is set, interrupt vectors will be moved to the start of the boot flash section. the address of ea ch interrupt vector will then be the address in this table added to the start address of the boot flash section. table 28 shows reset and interrupt vectors placement for the various combinations of bootrst and ivsel settings. if the program never enables an inte rrupt source, the interrupt vectors are not used, and regular program code can be placed at these loca- tions. this is also the case if the reset ve ctor is in the applic ation section while the interrupt vectors are in the boot section or vice versa. table 27. reset and interrupt vectors in atmega88 vector no. program address (2) source interrup t definition 1 0x000 (1) reset external pin, power-on reset, brown-out reset and watchdog system reset 2 0x001 int0 external interrupt request 0 3 0x002 int1 external interrupt request 1 4 0x003 pcint0 pin change interrupt request 0 5 0x004 pcint1 pin change interrupt request 1 6 0x005 pcint2 pin change interrupt request 2 7 0x006 wdt watchdog time-out interrupt 8 0x007 timer2 compa timer /counter2 compare match a 9 0x008 timer2 compb timer /counter2 compare match b 10 0x009 timer2 ovf timer/counter2 overflow 11 0x00a timer1 capt timer/counter1 capture event 12 0x00b timer1 compa timer/counter1 compare match a 13 0x00c timer1 compb timer/coutner1 compare match b 14 0x00d timer1 ovf timer/counter1 overflow 15 0x00e timer0 compa timer/counter0 compare match a 16 0x00f timer0 compb timer /counter0 compare match b 17 0x010 timer0 ovf timer/counter0 overflow 18 0x011 spi, stc spi serial transfer complete 19 0x012 usart, rx usart rx complete 20 0x013 usart, udre usart, data register empty 21 0x014 usart, tx usart, tx complete 22 0x015 adc adc conversion complete 23 0x016 ee ready eeprom ready 24 0x017 analog comp analog comparator 25 0x018 twi 2-wire serial interface 26 0x019 spm ready store program memory ready
54 atmega48/88/168 2545d?avr?07/04 note: 1. the boot reset address is shown in table 109 on page 268. for the bootrst fuse ?1? means unprogrammed while ?0? means programmed. the most typical and general program setup for the reset and interrupt vector addresses in atmega88 is: address labels code comments 0x000 rjmp reset ; reset handler 0x001 rjmp ext_int0 ; irq0 handler 0x002 rjmp ext_int1 ; irq1 handler 0x003 rjmp pcint0 ; pcint0 handler 0x004 rjmp pcint1 ; pcint1 handler 0x005 rjmp pcint2 ; pcint2 handler 0x006 rjmp wdt ; watchdog timer handler 0x007 rjmp tim2_compa ; timer2 compare a handler 0x008 rjmp tim2_compb ; timer2 compare b handler 0x009 rjmp tim2_ovf ; timer2 overflow handler 0x00a rjmp tim1_capt ; timer1 capture handler 0x00b rjmp tim1_compa ; timer1 compare a handler 0x00c rjmp tim1_compb ; timer1 compare b handler 0x00d rjmp tim1_ovf ; timer1 overflow handler 0x00e rjmp tim0_compa ; timer0 compare a handler 0x00f rjmp tim0_compb ; timer0 compare b handler 0x010 rjmp tim0_ovf ; timer0 overflow handler 0x011 rjmp spi_stc ; spi transfer complete handler 0x012 rjmp usart_rxc ; usart, rx complete handler 0x013 rjmp usart_udre ; usart, udr empty handler 0x014 rjmp usart_txc ; usart, tx complete handler 0x015 rjmp adc ; adc conversion complete handler 0x016 rjmp ee_rdy ; eeprom ready handler 0x017 rjmp ana_comp ; analog comparator handler 0x018 rjmp twi ; 2-wire serial interface handler 0x019 rjmp spm_rdy ; store program memory ready handler ; 0x01areset: ldi r16, high(ramend); main program start 0x01b out sph,r16 ; set stack pointer to top of ram 0x01c ldi r16, low(ramend) table 28. reset and interrupt vectors placement in atmega88 (1) bootrst ivsel reset address interru pt vectors start address 1 0 0x000 0x001 1 1 0x000 boot reset address + 0x001 0 0 boot reset address 0x001 0 1 boot reset address boot reset address + 0x001
55 atmega48/88/168 2545d?avr?07/04 0x01d out spl,r16 0x01e sei ; enable interrupts 0x01f xxx ... ... ... ... when the bootrst fuse is unprogrammed, the boot section size set to 2k bytes and the ivsel bit in the mcucr register is set before any inte rrupts are enabled, the most typical and general program setup for the reset and interrupt vector addresses in atmega88 is: address labels code comments 0x000 reset: ldi r16,high(ramend); main program start 0x001 out sph,r16 ; set stack pointer to top of ram 0x002 ldi r16,low(ramend) 0x003 out spl,r16 0x004 sei ; enable interrupts 0x005 xxx ; .org 0xc01 0xc01 rjmp ext_int0 ; irq0 handler 0xc02 rjmp ext_int1 ; irq1 handler ... ... ... ; 0xc19 rjmp spm_rdy ; store program memory ready handler when the bootrst fuse is programmed and the boot section size set to 2k bytes, the most typical and general program setup for the reset and interrupt vector addresses in atmega88 is: address labels code comments .org 0x001 0x001 rjmp ext_int0 ; irq0 handler 0x002 rjmp ext_int1 ; irq1 handler ... ... ... ; 0x019 rjmp spm_rdy ; store program memory ready handler ; .org 0xc00 0xc00 reset: ldi r16,high(ramend); main program start 0xc01 out sph,r16 ; set stack pointer to top of ram 0xc02 ldi r16,low(ramend) 0xc03 out spl,r16 0xc04 sei ; enable interrupts 0xc05 xxx
56 atmega48/88/168 2545d?avr?07/04 when the bootrst fuse is programmed, the boot section size set to 2k bytes and the ivsel bit in the mcucr register is set befo re any interrupts are enabled, the most typ- ical and general program setup for the reset and interrupt vector addresses in atmega88 is: address labels code comments ; .org 0xc00 0xc00 rjmp reset ; reset handler 0xc01 rjmp ext_int0 ; irq0 handler 0xc02 rjmp ext_int1 ; irq1 handler ... ... ... ; 0xc19 rjmp spm_rdy ; store program memory ready handler ; 0xc1a reset: ldi r16,high(ramend); main program start 0xc1b out sph,r16 ; set stack pointer to top of ram 0xc1c ldi r16,low(ramend) 0xc1d out spl,r16 0xc1e sei ; enable interrupts 0xc1f xxx
57 atmega48/88/168 2545d?avr?07/04 interrupt vectors in atmega168 notes: 1. when the bootrst fuse is programmed, the device will jump to the boot loader address at reset, see ?boot loader suppor t ? read-while-write self-programming, atmega88 and atmega168? on page 255. 2. when the ivsel bit in mcucr is set, interrupt vectors will be moved to the start of the boot flash section. the address of ea ch interrupt vector will then be the address in this table added to the start address of the boot flash section. table 30 shows reset and interrupt vectors placement for the various combinations of bootrst and ivsel settings. if the program never enables an inte rrupt source, the interrupt vectors are not used, and regular program code can be placed at these loca- table 29. reset and interrupt vectors in atmega168 vectorno. program address (2) source interrup t definition 1 0x0000 (1) reset external pin, power-on reset, brown-out reset and watchdog system reset 2 0x0002 int0 external interrupt request 0 3 0x0004 int1 external interrupt request 1 4 0x0006 pcint0 pin change interrupt request 0 5 0x0008 pcint1 pin change interrupt request 1 6 0x000a pcint2 pin change interrupt request 2 7 0x000c wdt watchdog time-out interrupt 8 0x000e timer2 compa timer/counter2 compare match a 9 0x0010 timer2 compb timer/counter2 compare match b 10 0x0012 timer2 ovf timer/counter2 overflow 11 0x0014 timer1 capt timer/counter1 capture event 12 0x0016 timer1 compa timer /counter1 compare match a 13 0x0018 timer1 compb timer /coutner1 compare match b 14 0x001a timer1 ovf timer/counter1 overflow 15 0x001c timer0 compa timer/counter0 compare match a 16 0x001e timer0 compb timer/counter0 compare match b 17 0x0020 timer0 ovf timer/counter0 overflow 18 0x0022 spi, stc spi serial transfer complete 19 0x0024 usart, rx usart rx complete 20 0x0026 usart, udre usart, data register empty 21 0x0028 usart, tx usart, tx complete 22 0x002a adc adc conversion complete 23 0x002c ee ready eeprom ready 24 0x002e analog comp analog comparator 25 0x0030 twi 2-wire serial interface 26 0x0032 spm ready store program memory ready
58 atmega48/88/168 2545d?avr?07/04 tions. this is also the case if the reset ve ctor is in the applic ation section while the interrupt vectors are in the boot section or vice versa. note: 1. the boot reset address is shown in table 109 on page 268. for the bootrst fuse ?1? means unprogrammed while ?0? means programmed. the most typical and general program setup for the reset and interrupt vector addresses in atmega168 is: address labels code comments 0x0000 jmp reset ; reset handler 0x0002 jmp ext_int0 ; irq0 handler 0x0004 jmp ext_int1 ; irq1 handler 0x0006 jmp pcint0 ; pcint0 handler 0x0008 jmp pcint1 ; pcint1 handler 0x000a jmp pcint2 ; pcint2 handler 0x000c jmp wdt ; watchdog timer handler 0x000e jmp tim2_compa ; timer2 compare a handler 0x0010 jmp tim2_compb ; timer2 compare b handler 0x0012 jmp tim2_ovf ; timer2 overflow handler 0x0014 jmp tim1_capt ; timer1 capture handler 0x0016 jmp tim1_compa ; timer1 compare a handler 0x0018 jmp tim1_compb ; timer1 compare b handler 0x001a jmp tim1_ovf ; timer1 overflow handler 0x001c jmp tim0_compa ; timer0 compare a handler 0x001e jmp tim0_compb ; timer0 compare b handler 0x0020 jmp tim0_ovf ; timer0 overflow handler 0x0022 jmp spi_stc ; spi transfer complete handler 0x0024 jmp usart_rxc ; usart, rx complete handler 0x0026 jmp usart_udre ; usart, udr empty handler 0x0028 jmp usart_txc ; usart, tx complete handler 0x002a jmp adc ; adc conversion complete handler 0x002c jmp ee_rdy ; eeprom ready handler 0x002e jmp ana_comp ; analog comparator handler 0x0030 jmp twi ; 2-wire serial interface handler 0x0032 jmp spm_rdy ; store program memory ready handler ; 0x0033reset: ldi r16, high(ramend); main program start table 30. reset and interrupt vectors placement in atmega168 (1) bootrst ivsel reset address interr upt vectors start address 1 0 0x000 0x001 1 1 0x000 boot reset address + 0x0002 0 0 boot reset address 0x001 0 1 boot reset address boot reset address + 0x0002
59 atmega48/88/168 2545d?avr?07/04 0x0034 out sph,r16 ; set stack pointer to top of ram 0x0035 ldi r16, low(ramend) 0x0036 out spl,r16 0x0037 sei ; enable interrupts 0x0038 xxx ... ... ... ... when the bootrst fuse is unprogrammed, the boot section size set to 2k bytes and the ivsel bit in the mcucr register is set before any inte rrupts are enabled, the most typical and general program setup for the reset and interrupt vector addresses in atmega168 is: address labels code comments 0x0000 reset: ldi r16,high(ramend); main program start 0x0001 out sph,r16 ; set stack pointer to top of ram 0x0002 ldi r16,low(ramend) 0x0003 out spl,r16 0x0004 sei ; enable interrupts 0x0005 xxx ; .org 0xc02 0x1c02 jmp ext_int0 ; irq0 handler 0x1c04 jmp ext_int1 ; irq1 handler ... ... ... ; 0x1c32 jmp spm_rdy ; store program memory ready handler when the bootrst fuse is programmed and the boot section size set to 2k bytes, the most typical and general program setup for the reset and interrupt vector addresses in atmega168 is: address labels code comments .org 0x0002 0x0002 jmp ext_int0 ; irq0 handler 0x0004 jmp ext_int1 ; irq1 handler ... ... ... ; 0x0032 jmp spm_rdy ; store program memory ready handler ; .org 0x1c00 0x1c00 reset: ldi r16,high(ramend); main program start 0x1c01 out sph,r16 ; set stack pointer to top of ram 0x1c02 ldi r16,low(ramend) 0x1c03 out spl,r16 0x1c04 sei ; enable interrupts 0x1c05 xxx when the bootrst fuse is programmed, the boot section size set to 2k bytes and the ivsel bit in the mcucr register is set befo re any interrupts are enabled, the most typ- ical and general program setup for the reset and interrupt vector addresses in atmega168 is:
60 atmega48/88/168 2545d?avr?07/04 address labels code comments ; .org 0x1c00 0x1c00 jmp reset ; reset handler 0x1c02 jmp ext_int0 ; irq0 handler 0x1c04 jmp ext_int1 ; irq1 handler ... ... ... ; 0x1c32 jmp spm_rdy ; store program memory ready handler ; 0x1c33 reset: ldi r16,high(ramend); main program start 0x1c34 out sph,r16 ; set stack pointer to top of ram 0x1c35 ldi r16,low(ramend) 0x1c36 out spl,r16 0x1c37 sei ; enable interrupts 0x1c38 xxx moving interrupts between application and boot space, atmega88 and atmega168 the mcu control register controls the placement of the interrupt vector table. mcu control register ? mcucr bit 1 ? ivsel: interrupt vector select when the ivsel bit is cleared (zero), the interrupt vectors ar e placed at the start of the flash memory. when this bit is set (one), the interrupt vectors are moved to the begin- ning of the boot loader section of the flash. the actual address of the start of the boot flash section is determined by the bootsz fuses. refer to the section ?boot loader support ? read-while-write self-programming, atmega88 and atmega168? on page 255 for details. to avoid unintentional changes of interrupt vector tables, a special write procedure must be followed to change the ivsel bit: 1. write the interrupt vector change enable (ivce) bit to one. 2. within four cycles, write the desired value to ivsel while writing a zero to ivce. interrupts will automatically be disabled while this sequence is executed. interrupts are disabled in the cycle ivce is set, and they remain disabled until after the instruction fol- lowing the write to ivsel. if ivsel is not written, interrupts remain disabled for four cycles. the i-bit in the status register is unaffected by the automatic disabling. note: if interrupt vectors are placed in the boot loader section and boot lock bit blb02 is pro- grammed, interrupts are disabled while executing from the application section. if interrupt vectors are placed in the applicat ion section and boot lock bit blb12 is pro- gramed, interrupts are disabled while executin g from the boot loader section. refer to the section ?boot loader support ? read- while-write self-pro gramming, atmega88 and atmega168? on page 255 for details on boot lock bits. this bit is not available in atmega48. bit 76543210 ? ? ? pud ? ? ivsel ivce mcucr read/write r r r r/w r r r/w r/w initial value 0 0 0 0 0 0 0 0
61 atmega48/88/168 2545d?avr?07/04 bit 0 ? ivce: interrupt vector change enable the ivce bit must be written to logic one to enable change of the ivsel bit. ivce is cleared by hardware four cycles after it is written or when ivsel is written. setting the ivce bit will disable interrupts, as explain ed in the ivsel descri ption above. see code example below. this bit is not available in atmega48. assembly code example move_interrupts: ; enable change of interrupt vectors ldi r16, (1< 62 atmega48/88/168 2545d?avr?07/04 i/o-ports introduction all avr ports have true read-modify-write functionality when used as general digital i/o ports. this means that the direction of one port pin can be changed without uninten- tionally changing the direction of any other pin with the sbi and cbi instructions. the same applies when changing drive value (if c onfigured as output) or enabling/disabling of pull-up resistors (if configured as input). each output buffer has symmetrical drive characteristics with both high sink and source capability. the pin dr iver is strong enough to drive led displays directly. all port pins have individually selectable pull-up resistors with a supply-voltage invariant resistance. all i/o pins have protection diodes to both v cc and ground as indicated in figure 23. refer to ?electrical characteristics? on page 290 for a complete list of parameters. figure 23. i/o pin equivalent schematic all registers and bit references in this section are written in general form. a lower case ?x? represents the numbering letter for the port, and a lower case ?n? represents the bit number. however, when using the register or bit defines in a program, the precise form must be used. for example, portb3 for bit no. 3 in port b, here documented generally as portxn. the physical i/o registers and bit locations are listed in ?register descrip- tion for i/o ports? on page 79. three i/o memory address locations are allocated for each port, one each for the data register ? portx, data direction register ? ddrx, and the port input pins ? pinx. the port input pins i/o location is read only, while the data register and the data direction register are read/write. however, writing a logic one to a bit in t he pinx register, will result in a toggle in the corresponding bit in the data register. in addition, the pull-up disable ? pud bit in mcucr disables the pull- up function for all pins in all ports when set. using the i/o port as general dig ital i/o is described in ?ports as general digital i/o? on page 63. most port pins are multiplexed with alternate functions for the peripheral fea- tures on the device. how each alternate function interferes with the port pin is described in ?alternate port functions? on page 67. refer to the individual module sections for a full description of the alternate functions. c pin logic r pu see figure "general digital i/o" for details pxn
63 atmega48/88/168 2545d?avr?07/04 note that enabling the alternate function of some of the port pins does not affect the use of the other pins in the port as general digital i/o. ports as general digital i/o the ports are bi-directional i/o ports with optional internal pull-ups. figure 24 shows a functional description of one i/o-port pin, here generically called pxn. figure 24. general digital i/o (1) note: 1. wrx, wpx, wdx, rrx, rpx, and rdx are common to all pins within the same port. clk i/o , sleep, and pud are common to all ports. configuring the pin each port pin consists of three register bits: ddxn, portxn, and pinxn. as shown in ?register description for i/o ports? on page 79, the ddxn bits are accessed at the ddrx i/o address, the portxn bits at the portx i/o address, and the pinxn bits at the pinx i/o address. the ddxn bit in the ddrx register selects the direction of this pin. if ddxn is written logic one, pxn is configured as an output pin. if ddxn is written logic zero, pxn is config- ured as an input pin. if portxn is written logic one when the pin is configured as an input pin, the pull-up resistor is activated. to switch the pull-up resistor off, portxn has to be written logic zero or the pin has to be configured as an output pin. the port pins are tri-stated when reset condition becomes active, even if no clocks are running. if portxn is written logic one when the pin is configured as an output pin, the port pin is driven high (one). if portxn is written logic zero when the pin is configured as an out- put pin, the port pin is driven low (zero). clk rpx rrx rdx wdx pud synchronizer wdx: write ddrx wrx: write portx rrx: read portx register rpx: read portx pin pud: pullup disable clk i/o : i/o clock rdx: read ddrx d l q q reset reset q q d q q d clr portxn q q d clr ddxn pinxn data b u s sleep sleep: sleep control pxn i/o wpx 0 1 wrx wpx: write pinx register
64 atmega48/88/168 2545d?avr?07/04 toggling the pin writing a logic one to pinxn toggles the value of portxn, independent on the value of ddrxn. note that the sbi instruction can be used to toggle one single bit in a port. switching between input and output when switching between tri-state ({ddxn, portxn} = 0b00) and output high ({ddxn, portxn} = 0b11), an intermed iate state with either pull-up enabled {ddx n, portxn} = 0b01) or output low ({ddxn, portxn} = 0b10) must occur. normally, the pull-up enabled state is fully acceptable, as a high -impedant environment will not notice the dif- ference between a strong high driver and a pull-up. if this is not the case, the pud bit in the mcucr register can be set to di sable all pull-ups in all ports. switching between input with pull-up and output low generates the same problem. the user must use either the tri-state ({ddxn, portxn} = 0b00) or the output high state ({ddxn, portxn} = 0b11) as an intermediate step. table 31 summarizes the control signals for the pin value. reading the pin value independent of the setting of data direction bit ddxn, the port pin can be read through the pinxn register bit. as shown in figure 24, the pinxn register bit and the preceding latch constitute a synchronizer. this is needed to avoid metastability if the physical pin changes value near the edge of the internal cl ock, but it also introduces a delay. figure 25 shows a timing diagram of the synchroni zation when reading an externally applied pin value. the maximum and minimum propagation delays are denoted t pd,max and t pd,min respectively. figure 25. synchronization when reading an externally applied pin value table 31. port pin configurations ddxn portxn pud (in mcucr) i/o pull-up comment 0 0 x input no tri-state (hi-z) 0 1 0 input yes pxn will source current if ext. pulled low. 0 1 1 input no tri-state (hi-z) 1 0 x output no output low (sink) 1 1 x output no output high (source) xxx in r17, pinx 0x00 0xff instructions sync latch pinxn r17 xxx system clk t pd, max t pd, min
65 atmega48/88/168 2545d?avr?07/04 consider the clock period starting shortly afte r the first falling edge of the system clock. the latch is closed when the clock is low, and goes transparent when the clock is high, as indicated by the shaded region of t he ?sync latch? signal. the signal value is latched when the system clock goes low. it is clocked into the pinxn register at the suc- ceeding positive clock edge. as indicated by the two arrows tpd,max and tpd,min, a single signal transition on the pin will be delayed between ? and 1? system clock period depending upon the time of assertion. when reading back a software assigned pin value, a nop instruction must be inserted as indicated in figure 26. the out instruction sets the ?sync latch? signal at the positive edge of the clock. in this case, the delay tpd through the synchronizer is 1 system clock period. figure 26. synchronization when reading a software assigned pin value the following code example shows how to set port b pins 0 and 1 high, 2 and 3 low, and define the port pins from 4 to 7 as input with pull-ups assigned to port pins 6 and 7. the resulting pin values are read back again, but as previously discussed, a nop instruction is included to be able to read back the value recently assigned to some of the pins. out portx, r16 nop in r17, pinx 0xff 0x00 0xff system clk r16 instructions sync latch pinxn r17 t pd
66 atmega48/88/168 2545d?avr?07/04 note: 1. for the assembly program, two tempor ary registers are used to minimize the time from pull-ups are set on pins 0, 1, 6, and 7, until the direction bits are correctly set, defining bit 2 and 3 as low and redefini ng bits 0 and 1 as strong high drivers. digital input enable and sleep modes as shown in figure 24, the digital input si gnal can be clamped to ground at the input of the schmitt trigger. the signal denoted sleep in the figure, is set by the mcu sleep controller in power-down mode, power-save mode, and standby mode to avoid high power consumption if some input signals are left floating, or have an analog signal level close to v cc /2. sleep is overridden for port pins enabled as external interrup t pins. if the external inter- rupt request is not enabled, sleep is active also for these pins. sleep is also overridden by various other alternate functions as described in ?alternate port func- tions? on page 67. if a logic high level (?one?) is present on an asynchronous external interrupt pin config- ured as ?interrupt on rising edge, falling edge, or any logic change on pin? while the external interrupt is not enabled, the corresponding exte rnal interrupt flag will be set when resuming from the above mentioned sle ep mode, as the clamping in these sleep mode produces the requested logic change. unconnected pins if some pins are unused, it is recommended to ensure that these pins have a defined level. even though most of the digital inputs are disabled in the deep sleep modes as assembly code example (1) ... ; define pull-ups and set outputs high ; define directions for port pins ldi r16,(1< 67 atmega48/88/168 2545d?avr?07/04 described above, floating inputs should be av oided to reduce current consumption in all other modes where the digital inputs are en abled (reset, active mode and idle mode). the simplest method to ensure a defined level of an unused pin, is to enable the internal pull-up. in this case, the pull-up will be disabled during reset. if low power consumption during reset is important, it is recommended to use an external pull-up or pull-down. connecting unused pins directly to v cc or gnd is not recommended, since this may cause excessive currents if the pin is accidentally configured as an output. alternate port functions most port pins have alternate functions in addition to being general digital i/os. figure 27 shows how the port pin control signals fr om the simplified figure 24 can be overrid- den by alternate functions. the overriding signals may not be present in all port pins, but the figure serves as a generic description app licable to all port pins in the avr micro- controller family. figure 27. alternate port functions (1) note: 1. wrx, wpx, wdx, rrx, rpx, and rdx are common to all pins within the same port. clk i/o , sleep, and pud are common to all ports. all other signals are unique for each pin. clk rpx rrx wrx rdx wdx pud synchronizer wdx: write ddrx wrx: write portx rrx: read portx register rpx: read portx pin pud: pullup disable clk i/o : i/o clock rdx: read ddrx d l q q set clr 0 1 0 1 0 1 dixn aioxn dieoexn pvovxn pvoexn ddovxn ddoexn puoexn puovxn puoexn: pxn pull-up override enable puovxn: pxn pull-up override value ddoexn: pxn data direction override enable ddovxn: pxn data direction override value pvoexn: pxn port value override enable pvovxn: pxn port value override value dixn: digital input pin n on portx aioxn: analog input/output pin n on portx reset reset q q d clr q q d clr q q d clr pinxn portxn ddxn data b u s 0 1 dieovxn sleep dieoexn: pxn digital input-enable override enable dieovxn: pxn digital input-enable override value sleep: sleep control pxn i/o 0 1 ptoexn ptoexn: pxn, port toggle override enable wpx: write pinx wpx
68 atmega48/88/168 2545d?avr?07/04 table 32 summarizes the function of the overriding signals. the pin and port indexes from figure 27 are not shown in the succeed ing tables. the overriding signals are gen- erated internally in the modules having the alternate function. the following subsections shortly describe the alternate functions for each port, and relate the overriding signals to the alternat e function. refer to the alternate function description for further details. table 32. generic description of overriding signals for alternate functions signal name full name description puoe pull-up override enable if this signal is set, the pull-up enable is controlled by the puov signal. if this signal is cleared, the pull-up is enabled when {ddxn, portxn, pud} = 0b010. puov pull-up override value if puoe is set, the pull-up is enabled/disabled when puov is set/cleared, regardless of the setting of the ddxn, portxn, and pud register bits. ddoe data direction override enable if this signal is set, the output driver enable is controlled by the ddov signal. if this signal is cleared, the output driver is enabled by the ddxn register bit. ddov data direction override value if ddoe is set, the output driver is enabled/disabled when ddov is set/cleared, regardless of the setting of the ddxn register bit. pvoe port value override enable if this signal is set and the output driver is enabled, the port value is controlled by the pvov signal. if pvoe is cleared, and the output driver is enabled, the port value is controlled by the portxn register bit. pvov port value override value if pvoe is set, the port value is set to pvov, regardless of the setting of the portxn register bit. ptoe port toggle override enable if ptoe is set, the portxn register bit is inverted. dieoe digital input enable override enable if this bit is set, the digital input enable is controlled by the dieov signal. if this signal is cleared, the digital input enable is determined by mcu state (normal mode, sleep mode). dieov digital input enable override value if dieoe is set, the digital input is enabled/disabled when dieov is set/cleared, regardless of the mcu state (normal mode, sleep mode). di digital input this is the digital input to alternate functions. in the figure, the signal is connected to the output of the schmitt trigger but before the synchronizer. unless the digital input is used as a clock source, the module with the alternate function will use its own synchronizer. aio analog input/output this is the analog input/output to/from alternate functions. the signal is connected directly to the pad, and can be used bi-directionally.
69 atmega48/88/168 2545d?avr?07/04 mcu control register ? mcucr bit 4 ? pud: pull-up disable when this bit is written to one, the pull-ups in the i/o ports are disabled even if the ddxn and portxn registers are configured to enable the pull-ups ({ddxn, portxn} = 0b01). see ?configuring the pin? on page 63 for more details about this feature. alternate functions of port b the port b pins with alternate functions are shown in table 33. the alternate pin configuration is as follows: xtal2/tosc2/pcint7 ? port b, bit 7 xtal2: chip clock oscillator pin 2. used as clock pin for cr ystal oscillator or low-fre- quency crystal oscillator. when used as a cl ock pin, the pin can not be used as an i/o pin. tosc2: timer oscillator pin 2. used only if internal calibrated rc oscillator is selected as chip clock source, and the asynchronous timer is enabled by the correct setting in assr. when the as2 bit in assr is set (one ) and the exclk bit is cleared (zero) to enable asynchronous clocking of timer/counter2 using the crystal oscillator, pin pb7 is disconnected from the port, a nd becomes the inverting output of the oscillator amplifier. in this mode, a crystal oscilla tor is connected to this pin, and the pin cannot be used as an i/o pin. bit 7 6 5 4 3 2 1 0 ? ? ?pud ? ? ivsel ivce mcucr read/write r r r r/w r r r/w r/w initial value 0 0 0 0 0 0 0 0 table 33. port b pins alternate functions port pin alternate functions pb7 xtal2 ( chip clock oscillator pin 2 ) tosc2 ( timer oscillator pin 2 ) pcint7 (pin change interrupt 7) pb6 xtal1 ( chip clock oscillator pin 1 or external clock input ) tosc1 ( timer oscillator pin 1 ) pcint6 (pin change interrupt 6) pb5 sck (spi bus master clock input) pcint5 (pin change interrupt 5) pb4 miso (spi bus master input/slave output) pcint4 (pin change interrupt 4) pb3 mosi (spi bus master output/slave input) oc2a (timer/counter2 output compare match a output) pcint3 (pin change interrupt 3) pb2 ss (spi bus master slave select) oc1b (timer/counter1 output compare match b output) pcint2 (pin change interrupt 2) pb1 oc1a (timer/counter1 output compare match a output) pcint1 (pin change interrupt 1) pb0 icp1 (timer/counter1 input capture input) clko (divided system clock output) pcint0 (pin change interrupt 0)
70 atmega48/88/168 2545d?avr?07/04 pcint7: pin change interrupt source 7. the pb7 pin can serve as an external interrupt source. if pb7 is used as a clock pin, ddb 7, portb7 and pinb7 will all read 0. xtal1/tosc1/pcint6 ? port b, bit 6 xtal1: chip clock oscillator pin 1. used for all chip clock sources except internal cali- brated rc oscillator. when used as a clock pin, the pin can not be used as an i/o pin. tosc1: timer oscillator pin 1. used only if internal calibrated rc oscillator is selected as chip clock source, and the asynchronous timer is enabled by the correct setting in assr. when the as2 bit in assr is set (one) to enable asynchronous clocking of timer/counter2, pin pb6 is disconnected from the port, and becomes the input of the inverting oscillator amp lifier. in this mode, a crystal oscillator is connected to this pin, and the pin can not be used as an i/o pin. pcint6: pin change interrupt source 6. the pb6 pin can serve as an external interrupt source. if pb6 is used as a clock pin, ddb 6, portb6 and pinb6 will all read 0. sck/pcint5 ? port b, bit 5 sck: master clock output, slave clock input pin for spi channel. when the spi is enabled as a slave, this pin is configured as an input regardless of the setting of ddb5. when the spi is enabled as a master, the da ta direction of this pin is controlled by ddb5. when the pin is forced by the spi to be an input, the pull-up can still be con- trolled by the portb5 bit. pcint5: pin change interrupt source 5. the pb5 pin can serve as an external interrupt source. miso/pcint4 ? port b, bit 4 miso: master data input, slave data output pin for spi channel. when the spi is enabled as a master, this pin is configured as an input regardless of the setting of ddb4. when the spi is enabled as a slave, th e data direction of this pin is controlled by ddb4. when the pin is forced by the spi to be an input, the pull-up can still be con- trolled by the portb4 bit. pcint4: pin change interrupt source 4. the pb4 pin can serve as an external interrupt source. mosi/oc2/pcint3 ? port b, bit 3 mosi: spi master data output, slave data input for spi channel. when the spi is enabled as a slave, this pin is configured as an input regardless of the setting of ddb3. when the spi is enabled as a master, the da ta direction of this pin is controlled by ddb3. when the pin is forced by the spi to be an input, the pull-up can still be con- trolled by the portb3 bit. oc2, output compare match output: the pb3 pin can serve as an external output for the timer/counter2 compare match. the pb3 pin has to be configured as an output (ddb3 set (one)) to serve this function. the oc2 pin is also the output pin for the pwm mode timer function. pcint3: pin change interrupt source 3. the pb3 pin can serve as an external interrupt source. ss /oc1b/pcint2 ? port b, bit 2 ss : slave select input. when the spi is enabled as a slave, this pin is configured as an input regardless of the setting of ddb2. as a slave, the spi is activated when this pin is
71 atmega48/88/168 2545d?avr?07/04 driven low. when the spi is enabled as a master , the data direction of this pin is con- trolled by ddb2. when the pin is forced by the spi to be an input, the pull-up can still be controlled by the portb2 bit. oc1b, output compare match output: the pb2 pin can serve as an external output for the timer/counter1 compare match b. the pb2 pin has to be configured as an output (ddb2 set (one)) to serve this function. the oc1b pin is also the output pin for the pwm mode timer function. pcint2: pin change interrupt source 2. the pb2 pin can serve as an external interrupt source. oc1a/pcint1 ? port b, bit 1 oc1a, output compare match output: the pb1 pin can serve as an external output for the timer/counter1 compare match a. the pb1 pin has to be configured as an output (ddb1 set (one)) to serve this function. the oc1a pin is also the output pin for the pwm mode timer function. pcint1: pin change interrupt source 1. the pb1 pin can serve as an external interrupt source. icp1/clko/pcint0 ? port b, bit 0 icp1, input capture pin: the pb0 pin can act as an input capture pin for timer/counter1. clko, divided system clock: the divided system clock can be output on the pb0 pin. the divided system clock will be output if t he ckout fuse is programmed, regardless of the portb0 and ddb0 settings. it will also be output during reset. pcint0: pin change interrupt source 0. the pb0 pin can serve as an external interrupt source. table 34 and table 35 relate the alternate functions of port b to the overriding signals shown in figure 27 on page 67. spi mstr input and spi slave output constitute the miso signal, while mosi is divide d into spi mstr output and spi slave input.
72 atmega48/88/168 2545d?avr?07/04 notes: 1. intrc means that one of the inter nal rc oscillators are selected (by the cksel fuses), extck means that external cloc k is selected (by the cksel fuses). table 34. overriding signals for alternate functions in pb7..pb4 signal name pb7/xtal2/ tosc2/pcint7 (1) pb6/xtal1/ tosc1/pcint6 (1) pb5/sck/ pcint5 pb4/miso/ pcint4 puoe intrc extck + as2 intrc + as2 spe mstr spe mstr puov 0 0 portb5 pud portb4 pud ddoe intrc extck + as2 intrc + as2 spe mstr spe mstr ddov 0 0 0 0 pvoe 0 0 spe mstr spe mstr pvov 0 0 sck output spi slave output dieoe intrc extck + as2 + pcint7 pcie0 intrc + as2 + pcint6 pcie0 pcint5 pcie0 pcint4 pcie0 dieov (intrc + extck) as2 intrc as2 11 di pcint7 input pcint6 input pcint5 input sck input pcint4 input spi mstr input aio oscillator output oscillator/clock input ?? table 35. overriding signals for alternate functions in pb3..pb0 signal name pb3/mosi/ oc2/pcint3 pb2/ss / oc1b/pcint2 pb1/oc1a/ pcint1 pb0/icp1/ pcint0 puoe spe mstr spe mstr 00 puov portb3 pud portb2 pud 00 ddoe spe mstr spe mstr 00 ddov 0 0 0 0 pvoe spe mstr + oc2a enable oc1b enable oc1a enable 0 pvov spi mstr output + oc2a oc1b oc1a 0 dieoe pcint3 pcie0 pci nt2 pcie0 pcint1 pcie0 pcint0 pcie0 dieov1111 di pcint3 input spi slave input pcint2 input spi ss pcint1 input pcint0 input icp1 input aio????
73 atmega48/88/168 2545d?avr?07/04 alternate functions of port c the port c pins with alternate functions are shown in table 36. the alternate pin configuration is as follows: reset /pcint14 ? port c, bit 6 reset , reset pin: when the rstdisbl fuse is programmed, this pin functions as a normal i/o pin, and the part will have to rely on power-on reset and brown-out reset as its reset sources. when the rstdisbl fuse is unprogrammed, the reset circuitry is connected to the pin, and the pin can not be used as an i/o pin. if pc6 is used as a reset pin, ddc6, portc6 and pinc6 will all read 0. pcint14: pin change interrupt source 14. t he pc6 pin can serve as an external inter- rupt source. scl/adc5/pcint13 ? port c, bit 5 scl, 2-wire serial interface clock: when the twen bit in twcr is set (one) to enable the 2-wire serial interface, pin pc5 is disconnected from the port and becomes the serial clock i/o pin for the 2-wire serial interface. in this mode, there is a spike filter on the pin to suppress spikes shorter than 50 ns on the input signal, and the pin is driven by an open drain driver with slew-rate limitation. pc5 can also be used as adc input channel 5. note that adc input channel 5 uses dig- ital power. pcint13: pin change interrupt source 13. t he pc5 pin can serve as an external inter- rupt source. sda/adc4/pcint12 ? port c, bit 4 sda, 2-wire serial interface data: when t he twen bit in twcr is set (one) to enable the 2-wire serial interface, pin pc4 is disconnected from the port and becomes the serial data i/o pin for the 2-wire serial interf ace. in this mode, there is a spike filter on the pin to suppress spikes shorter than 50 ns on the input signal, and the pin is driven by an open drain driver with slew-rate limitation. table 36. port c pins alternate functions port pin alternate function pc6 reset (reset pin) pcint14 (pin change interrupt 14) pc5 adc5 (adc input channel 5) scl (2-wire serial bus clock line) pcint13 (pin change interrupt 13) pc4 adc4 (adc input channel 4) sda (2-wire serial bus data input/output line) pcint12 (pin change interrupt 12) pc3 adc3 (adc input channel 3) pcint11 (pin change interrupt 11) pc2 adc2 (adc input channel 2) pcint10 (pin change interrupt 10) pc1 adc1 (adc input channel 1) pcint9 (pin change interrupt 9) pc0 adc0 (adc input channel 0) pcint8 (pin change interrupt 8)
74 atmega48/88/168 2545d?avr?07/04 pc4 can also be used as adc input channel 4. note that adc input channel 4 uses dig- ital power. pcint12: pin change interrupt source 12. t he pc4 pin can serve as an external inter- rupt source. adc3/pcint11 ? port c, bit 3 pc3 can also be used as adc input channel 3. note that adc input channel 3 uses analog power. pcint11: pin change interrupt source 11. t he pc3 pin can serve as an external inter- rupt source. adc2/pcint10 ? port c, bit 2 pc2 can also be used as adc input channel 2. note that adc input channel 2 uses analog power. pcint10: pin change interrupt source 10. t he pc2 pin can serve as an external inter- rupt source. adc1/pcint9 ? port c, bit 1 pc1 can also be used as adc input channel 1. note that adc input channel 1 uses analog power. pcint9: pin change interrupt source 9. the pc1 pin can serve as an external interrupt source. adc0/pcint8 ? port c, bit 0 pc0 can also be used as adc input channel 0. note that adc input channel 0 uses analog power. pcint8: pin change interrupt source 8. the pc0 pin can serve as an external interrupt source. table 37 and table 38 relate the alternate functions of port c to the overriding signals shown in figure 27 on page 67. note: 1. when enabled, the 2-wire serial interface enables slew-rate controls on the output pins pc4 and pc5. this is not shown in the figure. in addition, spike filters are con- table 37. overriding signals for alternate functions in pc6..pc4 (1) signal name pc6/reset /pcint14 pc5/scl/adc5/pcint 13 pc4/sda/adc4/pcint12 puoe rstdisbl twen twen puov 1 portc5 pud portc4 pud ddoe rstdisbl twen twen ddov 0 scl_out sda_out pvoe 0 twen twen pvov 0 0 0 dieoe rstdisbl + pcint14 pcie1 pcint13 pcie1 + adc5d pcint12 pcie1 + adc4d dieov rstdisbl pcint13 pcie1 pcint12 pcie1 di pcint14 input pcint13 input pcint12 input aio reset input adc5 input / scl input adc4 input / sda input
75 atmega48/88/168 2545d?avr?07/04 nected between the aio outputs shown in the port figure and the digital logic of the twi module. alternate functions of port d the port d pins with alternate functions are shown in table 39. the alternate pin configuration is as follows: ain1/oc2b/pcint23 ? port d, bit 7 table 38. overriding signals for alternate functions in pc3..pc0 signal name pc3/adc3/ pcint11 pc2/adc2/ pcint10 pc1/adc1/ pcint9 pc0/adc0/ pcint8 puoe0000 puov0000 ddoe 0 0 0 0 ddov 0 0 0 0 pvoe0000 pvov0000 dieoe pcint11 pcie1 + adc3d pcint10 pcie1 + adc2d pcint9 pcie1 + adc1d pcint8 pcie1 + adc0d dieov pcint11 pcie1 pcint10 pc ie1 pcint9 pcie1 pcint8 pcie1 di pcint11 input pcint10 input pcint9 input pcint8 input aio adc3 input adc2 input adc1 input adc0 input table 39. port d pins alternate functions port pin alternate function pd7 ain1 (analog comparator negative input) pcint23 (pin change interrupt 23) pd6 ain0 (analog comparator positive input) oc0a (timer/counter0 output compare match a output) pcint22 (pin change interrupt 22) pd5 t1 (timer/counter 1 external counter input) oc0b (timer/counter0 output compare match b output) pcint21 (pin change interrupt 21) pd4 xck (usart external clock input/output) t0 (timer/counter 0 external counter input) pcint20 (pin change interrupt 20) pd3 int1 (external interrupt 1 input) oc2b (timer/counter2 output compare match b output) pcint19 (pin change interrupt 19) pd2 int0 (external interrupt 0 input) pcint18 (pin change interrupt 18) pd1 txd (usart output pin) pcint17 (pin change interrupt 17) pd0 rxd (usart input pin) pcint16 (pin change interrupt 16)
76 atmega48/88/168 2545d?avr?07/04 ain1, analog comparator negative input. configure the port pin as input with the inter- nal pull-up switched off to avoid the digital port function from interfering with the function of the analog comparator. pcint23: pin change interrupt source 23. t he pd7 pin can serve as an external inter- rupt source. ain0/oc0a/pcint22 ? port d, bit 6 ain0, analog comparator positive input. configure the port pin as input with the internal pull-up switched off to avoid the digital port function from interfering with the function of the analog comparator. oc0a, output compare match output: the pd6 pin can serve as an external output for the timer/counter0 compare match a. the pd6 pin has to be configured as an output (ddd6 set (one)) to serve this function. the oc0a pin is also the output pin for the pwm mode timer function. pcint22: pin change interrupt source 22. t he pd6 pin can serve as an external inter- rupt source. t1/oc0b/pcint21 ? port d, bit 5 t1, timer/counter1 counter source. oc0b, output compare match output: the pd5 pin can serve as an external output for the timer/counter0 compare match b. the pd5 pin has to be configured as an output (ddd5 set (one)) to serve this function. the oc0b pin is also the output pin for the pwm mode timer function. pcint21: pin change interrupt source 21. t he pd5 pin can serve as an external inter- rupt source. xck/t0/pcint20 ? port d, bit 4 xck, usart external clock. t0, timer/counter0 counter source. pcint20: pin change interrupt source 20. t he pd4 pin can serve as an external inter- rupt source. int1/oc2b/pcint19 ? port d, bit 3 int1, external interrupt source 1: the pd3 pin can serve as an external interrupt source. oc2b, output compare match output: the pd3 pin can serve as an external output for the timer/counter0 compare match b. the pd3 pin has to be configured as an output (ddd3 set (one)) to serve this function. the oc2b pin is also the output pin for the pwm mode timer function. pcint19: pin change interrupt source 19. t he pd3 pin can serve as an external inter- rupt source. int0/pcint18 ? port d, bit 2 int0, external interrupt source 0: the pd2 pin can serve as an external interrupt source. pcint18: pin change interrupt source 18. t he pd2 pin can serve as an external inter- rupt source.
77 atmega48/88/168 2545d?avr?07/04 txd/pcint17 ? port d, bit 1 txd, transmit data (data output pin for the usart). when the usart transmitter is enabled, this pin is configured as an output regardless of the value of ddd1. pcint17: pin change interrupt source 17. t he pd1 pin can serve as an external inter- rupt source. rxd/pcint16 ? port d, bit 0 rxd, receive data (data input pin for the usart). when the usart receiver is enabled this pin is configured as an inpu t regardless of the value of ddd0. when the usart forces this pin to be an input, the pull-up can still be controlled by the portd0 bit. pcint16: pin change interrupt source 16. t he pd0 pin can serve as an external inter- rupt source. table 40 and table 41 relate the alternate functions of port d to the overriding signals shown in figure 27 on page 67. table 40. overriding signals for alternate functions pd7..pd4 signal name pd7/ain1 /pcint23 pd6/ain0/ oc0a/pcint22 pd5/t1/oc0b/ pcint21 pd4/xck/ t0/pcint20 puoe0000 puo0000 ddoe 0 0 0 0 ddov 0 0 0 0 pvoe 0 oc0a enable oc0b enable umsel pvov 0 oc0a oc0b xck output dieoe pcint23 pcie2 pcint22 pc ie2 pcint21 pcie2 pcint20 pcie2 dieov1111 di pcint23 input pcint22 input pcint21 input t1 input pcint20 input xck input t0 input aio ain1 input ain0 input ? ?
78 atmega48/88/168 2545d?avr?07/04 table 41. overriding signals for alternate functions in pd3..pd0 signal name pd3/oc2b/int1/ pcint19 pd2/int0/ pcint18 pd1/txd/ pcint17 pd0/rxd/ pcint16 puoe 0 0 txen rxen puo 0 0 0 portd0 pud ddoe 0 0 txen rxen ddov 0 0 1 0 pvoe oc2b enable 0 txen 0 pvov oc2b 0 txd 0 dieoe int1 enable + pcint19 pcie2 int0 enable + pcint18 pcie1 pcint17 pcie2 pcint16 pcie2 dieov1111 di pcint19 input int1 input pcint18 input int0 input pcint17 input pcint16 input rxd aio????
79 atmega48/88/168 2545d?avr?07/04 register description for i/o ports the port b data register ? portb the port b data direction register ? ddrb the port b input pins address ? pinb the port c data register ? portc the port c data direction register ? ddrc the port c input pins address ? pinc the port d data register ? portd the port d data direction register ? ddrd the port d input pins address ? pind bit 76543210 portb7 portb6 portb5 portb4 portb3 portb2 portb1 portb0 portb read/write r/w r/w r/w r/w r/w r/w r/w r/w initial value00000000 bit 76543210 ddb7 ddb6 ddb5 ddb4 ddb3 ddb2 ddb1 ddb0 ddrb read/write r/w r/w r/w r/w r/w r/w r/w r/w initial value00000000 bit 76543210 pinb7 pinb6 pinb5 pinb4 pi nb3 pinb2 pinb1 pinb0 pinb read/writerrrrrrrr initial value n/a n/a n/a n/a n/a n/a n/a n/a bit 76543210 ? portc6 portc5 portc4 portc3 portc2 portc1 portc0 portc read/write r r/w r/w r/w r/w r/w r/w r/w initial value00000000 bit 76543210 ? ddc6 ddc5 ddc4 ddc3 ddc2 ddc1 ddc0 ddrc read/write r r/w r/w r/w r/w r/w r/w r/w initial value00000000 bit 76543210 ? pinc6 pinc5 pinc4 pinc3 pinc2 pinc1 pinc0 pinc read/writerrrrrrrr initial value 0 n/a n/a n/a n/a n/a n/a n/a bit 76543210 portd7 portd6 portd5 portd4 portd3 portd2 portd1 portd0 portd read/write r/w r/w r/w r/w r/w r/w r/w r/w initial value00000000 bit 76543210 ddd7 ddd6 ddd5 ddd4 ddd3 ddd2 ddd1 ddd0 ddrd read/write r/w r/w r/w r/w r/w r/w r/w r/w initial value00000000 bit 76543210 pind7 pind6 pind5 pind4 pi nd3 pind2 pind1 pind0 pind read/writerrrrrrrr initial value n/a n/a n/a n/a n/a n/a n/a n/a
80 atmega48/88/168 2545d?avr?07/04 external interrupts the external interrupts are triggered by the int0 and int1 pins or any of the pcint23..0 pins. observe that, if enabled, the interrupts will trigger even if the int0 and int1 or pcint23..0 pins are configured as outputs. this feature provides a way of gen- erating a software interrupt. the pin change interrupt pci2 will trigger if any enabled pcint23..16 pin toggles. the pin change interrupt pci1 will trigger if any enabled pcint14..8 pin toggles. the pin change interrupt pci0 will trig ger if any enabled pcint7..0 pin toggles. the pcmsk2, pcm sk1 and pcmsk0 registers control which pins contribute to the pin change interrupts. pin change interrupts on pcint23..0 are detected asynchronously. this implies that these interrupts can be used for waking the part also from sleep modes other than idle mode. the int0 and int1 inte rrupts can be triggered by a fallin g or rising edge or a low level. this is set up as indicated in the specification for the external interrupt control register a ? eicra. when the int0 or int1 interrupts are enabled and are configured as level triggered, the interrup ts will trigger as long as the pin is held low. note that recognition of falling or rising edge interrupts on int0 or int1 requires the presence of an i/o clock, described in ?clock systems and their distri bution? on page 24. low level interrupt on int0 and int1 is detected asynchronously. this implies that this interrupt can be used for waking the part also from sleep modes other than idle mode. the i/o clock is halted in all sleep modes except idle mode. note that if a level triggered interrupt is used for wake-up from power-down, the required level must be held long enough for the mcu to co mplete the wake-up to trigger the level interrupt. if the level disappears before the end of the start-up time, the mcu will still wake up, but no interrupt will be generated. the start-up time is defined by the sut and cksel fuses as descr ibed in ?system clock and clock options? on page 24. external interrupt control register a ? eicra the external interrupt control register a contains control bits for interrupt sense control. bit 7..4 ? res: reserved bits these bits are unused bits in the atm ega48/88/168, and will alwa ys read as zero. bit 3, 2 ? isc11, isc10: interrupt sense control 1 bit 1 and bit 0 the external interrupt 1 is activated by the external pin int1 if the sreg i-flag and the corresponding interrupt mask are set. the level and edges on the external int1 pin that activate the interrupt are defined in tabl e 42. the value on the int1 pin is sampled before detecting edges. if edge or toggle interrupt is selected, pulses that last longer than one clock period will generate an interrupt. shorter pulses are not guaranteed to generate an interrupt. if low level interrupt is selected, the low level must be held until the completion of the currently executing instruction to generate an interrupt. bit 76543210 ????isc11isc10isc01isc00eicra read/write r r r r r/w r/w r/w r/w initial value00000000
81 atmega48/88/168 2545d?avr?07/04 bit 1, 0 ? isc01, isc00: interrupt sense control 0 bit 1 and bit 0 the external interrupt 0 is activated by the external pin int0 if the sreg i-flag and the corresponding interrupt mask are set. the level and edges on the external int0 pin that activate the interrupt are defined in tabl e 43. the value on the int0 pin is sampled before detecting edges. if edge or toggle interrupt is selected, pulses that last longer than one clock period will generate an interrupt. shorter pulses are not guaranteed to generate an interrupt. if low level interrupt is selected, the low level must be held until the completion of the currently executing instruction to generate an interrupt. external interrupt mask register ? eimsk bit 7..2 ? res: reserved bits these bits are unused bits in the atm ega48/88/168, and will alwa ys read as zero. bit 1 ? int1: external interrupt request 1 enable when the int1 bit is set (one) and the i-bit in the status register (sreg) is set (one), the external pin interrupt is enabled. the interrupt sense control1 bits 1/0 (isc11 and isc10) in the external interrupt control register a (eicra) define whether the external interrupt is activated on rising and/or falling edge of the int1 pin or level sensed. activity on the pin will cause an interrupt request even if int1 is configured as an output. the corresponding interrupt of external interrupt request 1 is executed from the int1 inter- rupt vector. bit 0 ? int0: external interrupt request 0 enable when the int0 bit is set (one) and the i-bit in the status register (sreg) is set (one), the external pin interrupt is enabled. the interrupt sense control0 bits 1/0 (isc01 and isc00) in the external interrupt control register a (eicra) define whether the external interrupt is activated on rising and/or falling edge of the int0 pin or level sensed. activity on the pin will cause an interrupt request even if int0 is configured as an output. the corresponding interrupt of external interrupt request 0 is executed from the int0 inter- rupt vector. table 42. interrupt 1 sense control isc11 isc10 description 0 0 the low level of int1 generates an interrupt request. 0 1 any logical change on int1 generates an interrupt request. 1 0 the falling edge of int1 generates an interrupt request. 1 1 the rising edge of int1 generates an interrupt request. table 43. interrupt 0 sense control isc01 isc00 description 0 0 the low level of int0 generates an interrupt request. 0 1 any logical change on int0 generates an interrupt request. 1 0 the falling edge of int0 generates an interrupt request. 1 1 the rising edge of int0 generates an interrupt request. bit 76543210 ??????int1int0eimsk read/writerrrrrrr/wr/w initial value 0 0 0 0 0 0 0 0
82 atmega48/88/168 2545d?avr?07/04 external interrupt flag register ? eifr bit 7..2 ? res: reserved bits these bits are unused bits in the atm ega48/88/168, and will alwa ys read as zero. bit 1 ? intf1: external interrupt flag 1 when an edge or logic change on the int1 pin triggers an interrupt request, intf1 becomes set (one). if the i-bit in sreg and the int1 bit in eimsk are set (one), the mcu will jump to the correspondi ng interrupt vector. the flag is cleared when the inter- rupt routine is executed. alternatively, the fl ag can be cleared by writing a logical one to it. this flag is always cl eared when int1 is configured as a level interrupt. bit 0 ? intf0: external interrupt flag 0 when an edge or logic change on the int0 pin triggers an interrupt request, intf0 becomes set (one). if the i-bit in sreg and the int0 bit in eimsk are set (one), the mcu will jump to the correspondi ng interrupt vector. the flag is cleared when the inter- rupt routine is executed. alternatively, the fl ag can be cleared by writing a logical one to it. this flag is always cl eared when int0 is configured as a level interrupt. pin change interrupt control register - pcicr bit 7..3 - res: reserved bits these bits are unused bits in the atm ega48/88/168, and will alwa ys read as zero. bit 2 - pcie2: pin change interrupt enable 2 when the pcie2 bit is set (one) and the i-bit in the status register (sreg) is set (one), pin change interrupt 2 is enabled. any change on any enabled pcint23..16 pin will cause an interrupt. the corresponding interrup t of pin change interrupt request is exe- cuted from the pci2 interrupt vector. pcin t23..16 pins are enabled individually by the pcmsk2 register. bit 1 - pcie1: pin change interrupt enable 1 when the pcie1 bit is set (one) and the i-bit in the status register (sreg) is set (one), pin change interrupt 1 is enabled. any change on any enabled pcint14..8 pin will cause an interrupt. the corresponding interrup t of pin change interrupt request is exe- cuted from the pci1 interrupt vector. pcin t14..8 pins are enabled individually by the pcmsk1 register. bit 0 - pcie0: pin change interrupt enable 0 when the pcie0 bit is set (one) and the i-bit in the status register (sreg) is set (one), pin change interrupt 0 is enabled. any change on any enabled pcint7..0 pin will cause an interrupt. the corresponding interrupt of pin change interrupt request is executed from the pci0 interrupt vector. pcint7..0 pi ns are enabled individually by the pcmsk0 register. bit 76543210 ? ? ? ? ? ? intf1 intf0 eifr read/writerrrrrrr/wr/w initial value 0 0 0 0 0 0 0 0 bit 76543210 ? ? ? ? ? pcie2 pcie1 pcie0 pcicr read/write r r r r r r/w r/w r/w initial value 0 0 0 0 0 0 0 0
83 atmega48/88/168 2545d?avr?07/04 pin change interrupt flag register - pcifr bit 7..3 - res: reserved bits these bits are unused bits in the atm ega48/88/168, and will alwa ys read as zero. bit 2 - pcif2: pin change interrupt flag 2 when a logic change on any pcint23..16 pin triggers an interrupt request, pcif2 becomes set (one). if the i-bit in sreg and the pcie2 bit in pcicr are set (one), the mcu will jump to the correspondi ng interrupt vector. the flag is cleared when the inter- rupt routine is executed. alternatively, the fl ag can be cleared by writing a logical one to it. bit 1 - pcif1: pin change interrupt flag 1 when a logic change on any pcint14..8 pin triggers an interrupt request, pcif1 becomes set (one). if the i-bit in sreg and the pcie1 bit in pcicr are set (one), the mcu will jump to the correspondi ng interrupt vector. the flag is cleared when the inter- rupt routine is executed. alternatively, the fl ag can be cleared by writing a logical one to it. bit 0 - pcif0: pin change interrupt flag 0 when a logic change on any pcint7..0 pin triggers an interrupt request, pcif0 becomes set (one). if the i-bit in sreg and the pcie0 bit in pcicr are set (one), the mcu will jump to the correspondi ng interrupt vector. the flag is cleared when the inter- rupt routine is executed. alternatively, the fl ag can be cleared by writing a logical one to it. pin change mask register 2 ? pcmsk2 bit 7..0 ? pcint23..16: pin change enable mask 23..16 each pcint23..16-bit selects whether pin change interrupt is enabled on the corre- sponding i/o pin. if pcint23..16 is set and the pcie2 bit in pcicr is set, pin change interrupt is enabled on the corresponding i/o pin. if pcint23..16 is cleared, pin change interrupt on the corresponding i/o pin is disabled. pin change mask register 1 ? pcmsk1 bit 7 ? res: reserved bit this bit is an unused bit in the atmega4 8/88/168, and will always read as zero. bit 6..0 ? pcint14..8: pin change enable mask 14..8 each pcint14..8-bit selects whether pin change interrupt is enabled on the correspond- ing i/o pin. if pcint14..8 is set and the pcie1 bit in pcicr is set, pin change interrupt bit 76543210 ? ? ? ? ? pcif2 pcif1 pcif0 pcifr read/write r r r r r r/w r/w r/w initial value 0 0 0 0 0 0 0 0 bit 76543210 pcint23 pcint22 pcint21 pcint20 pcint19 pcint18 pcint17 pcint16 pcmsk2 read/write r/w r/w r/w r/w r/w r/w r/w r/w initial value00000000 bit 76543210 ? pcint14 pcint13 pcint12 pcint11 pcint10 pcint9 pcint8 pcmsk1 read/write r r/w r/w r/w r/w r/w r/w r/w initial value00000000
84 atmega48/88/168 2545d?avr?07/04 is enabled on the corresponding i/o pin. if pcint14..8 is cleared, pin change interrupt on the corresponding i/o pin is disabled. pin change mask register 0 ? pcmsk0 bit 7..0 ? pcint7..0: pin change enable mask 7..0 each pcint7..0 bit selects whether pin change interrupt is enabled on the correspond- ing i/o pin. if pcint7..0 is set and the pcie0 bit in pcicr is set, pin change interrupt is enabled on the corresponding i/o pin. if pcint7..0 is cleared, pin change interrupt on the corresponding i/o pin is disabled. bit 76543210 pcint7 pcint6 pcint5 pcint4 pcint3 pcint2 pcint1 pcint0 pcmsk0 read/write r/w r/w r/w r/w r/w r/w r/w r/w initial value 0 0 0 0 0 0 0 0
85 atmega48/88/168 2545d?avr?07/04 8-bit timer/counter0 with pwm timer/counter0 is a general purpose 8-bi t timer/counter module, with two independent output compare units, and with pwm support. it allows accurate program execution timing (event management) and wave generation. the main features are: two independent output compare units double buffered out put compare registers clear timer on compar e match (auto reload) glitch free, phase correct pulse width modulator (pwm) variable pwm period frequency generator three independent interrupt sources (tov0, ocf0a, and ocf0b) overview a simplified block diagram of the 8-bit time r/counter is shown in figure 28. for the actual placement of i/o pins, refer to ?pinout atmega48/88/168? on page 2. cpu acces- sible i/o registers, including i/o bits and i/o pins, are shown in bold. the device- specific i/o register and bit locations are listed in the ?8-bit timer/counter register description? on page 95. the prtim0 bit in ?power reduction register - prr? on page 37 must be written to zero to enable timer/counter0 module. figure 28. 8-bit timer/counter block diagram definitions many register and bit references in this section are written in general form. a lower case ?n? replaces the timer/counter number, in this case 0. a lower case ?x? replaces the output compare unit, in this case compare unit a or co mpare unit b. however, when using the register or bit defines in a pr ogram, the precise form must be used, i.e., tcnt0 for accessing timer/counter0 counter value and so on. the definitions in table 44 are also used extensively throughout the document. timer/counter data b u s ocrna ocrnb = = tcntn waveform generation waveform generation ocna ocnb = fixed top value control logic = 0 top bottom count clear direction tovn (int.req.) ocna (int.req.) ocnb (int.req.) tccrna tccrnb clk tn prescaler t/c oscillator clk i/o tosc1 tosc2
86 atmega48/88/168 2545d?avr?07/04 registers the timer/counter (tcnt0) and output compare registers (ocr0a and ocr0b) are 8-bit registers. interrupt request (abbreviated to int.req. in the figure) signals are all vis- ible in the timer interrupt flag register (t ifr0). all interrupts are individually masked with the timer interrupt mask register (timsk0). tifr0 and timsk0 are not shown in the figure. the timer/counter can be clocked internally, via the prescaler, or by an external clock source on the t0 pin. the clock select logic block controls which clock source and edge the timer/counter uses to increment (or decrement) its value. the timer/counter is inactive when no cloc k source is selected. the output from the clock select logic is referred to as the timer clock (clk t0 ). the double buffered output compare registers (ocr0a and ocr0b) are compared with the timer/counter value at all times. the result of the compare can be used by the waveform generator to generate a pwm or variable frequency output on the output compare pins (oc0a and oc0b). see ?using the output compare unit? on page 114. for details. the compare match event will also set the compare flag (ocf0a or ocf0b) which can be used to generate an output compare interrupt request. timer/counter clock sources the timer/counter can be clocked by an intern al or an external clock source. the clock source is selected by the clock select lo gic which is controlled by the clock select (cs02:0) bits located in the timer/counte r control register (tccr0b). for details on clock sources and pre scaler, see ?timer/counter0 and timer/counter1 prescalers? on page 102. counter unit the main part of the 8-bit timer/counter is the programmable bi-directional counter unit. figure 29 shows a block diagram of the counter and its surroundings. figure 29. counter unit block diagram signal description (internal signals): count increment or decrement tcnt0 by 1. direction select between increment and decrement. table 44. definitions bottom the counter reaches the bottom when it becomes 0x00. max the counter reaches its maximum when it becomes 0xff (decimal 255). top the counter reaches the top when it becomes equal to the highest value in the count sequence. the top value can be assigned to be the fixed value 0xff (max) or the value stored in the ocr0a register. the assignment is dependent on the mode of operation. data bus tcntn control logic count tovn (int.req.) clock select top tn edge detector ( from prescaler ) clk tn bottom direction clear
87 atmega48/88/168 2545d?avr?07/04 clear clear tcnt0 (set all bits to zero). clk t n timer/counter clock, referred to as clk t0 in the following. top signalize that tcnt0 has reached maximum value. bottom signalize that tcnt0 has reached minimum value (zero). depending of the mode of operation used, the counter is cleared, incremented, or dec- remented at each timer clock (clk t0 ). clk t0 can be generated from an external or internal clock source, selected by the clock select bits (cs02:0). when no clock source is selected (cs02:0 = 0) the timer is stopped. however, the tcnt0 value can be accessed by the cpu, regardless of whether clk t0 is present or not. a cpu write overrides (has priority over) all counter clear or count operations. the counting sequence is determined by the setting of the wgm01 and wgm00 bits located in the timer/counter control register (tccr0a) and the wgm02 bit located in the timer/counter control register b (tccr0b). there are close connections between how the counter behaves (counts) and how waveforms are generated on the output compare outputs oc0a and oc0b. for more details about advanced counting sequences and waveform generation, see ?modes of operation? on page 90. the timer/counter overflow flag (tov0) is set according to the mode of operation selected by the wgm02:0 bits. tov0 can be used for generating a cpu interrupt. output compare unit the 8-bit comparator continuously compares tcnt0 with the output compare regis- ters (ocr0a and ocr0b). whenever tcnt0 equals ocr0a or ocr0b, the comparator signals a match. a match will set the output compare flag (ocf0a or ocf0b) at the next timer clock cycle. if the corresponding interrupt is enabled, the out- put compare flag generates an output compare interrupt. the output compare flag is automatically cleared when the interrupt is executed. alternatively, the flag can be cleared by software by writing a logical one to its i/o bit location. the waveform gener- ator uses the match signal to generate an output according to operating mode set by the wgm02:0 bits and compare output mode (com0x1:0) bits. the max and bottom sig- nals are used by the waveform generator for handling the special cases of the extreme values in some modes of operation (?modes of operation? on page 90). figure 30 shows a block diagram of the output compare unit.
88 atmega48/88/168 2545d?avr?07/04 figure 30. output compare unit, block diagram the ocr0x registers are double buffered when using any of the pulse width modula- tion (pwm) modes. for the normal and clear timer on compare (ctc) modes of operation, the double buffering is disabled. the double buffering synchronizes the update of the ocr0x compare registers to either top or bottom of the counting sequence. the synchronization prevents the occurrence of odd-length, non-symmetrical pwm pulses, thereby making the output glitch-free. the ocr0x register access may seem comple x, but this is not case. when the double buffering is enabled, the cpu has access to the ocr0x buffer register, and if double buffering is disabled the cp u will access the ocr0x directly. force output compare in non-pwm waveform generation modes, the match output of the comparator can be forced by writing a one to the force output compare (foc0x) bit. forcing compare match will not set the ocf0x flag or reload /clear the timer, but the oc0x pin will be updated as if a real compare match had occurred (the com0x1:0 bits settings define whether the oc0x pin is set, cleared or toggled). compare match blocking by tcnt0 write all cpu write operations to the tcnt0 register will block any compare match that occur in the next timer clock cycle, even w hen the timer is stopped. this feature allows ocr0x to be initialized to the same value as tcnt0 without triggering an interrupt when the timer/counter clock is enabled. using the output compare unit since writing tcnt0 in any mode of operat ion will block all compare matches for one timer clock cycle, there are risks involved when changing tcnt0 when using the output compare unit, independently of whether the timer/counter is running or not. if the value written to tcnt0 equals the ocr0x va lue, the compare match will be missed, resulting in incorrect waveform generation . similarly, do not write the tcnt0 value equal to bottom when the counter is downcounting. the setup of the oc0x should be performed before setting the data direction register for the port pin to output. the easiest way of setting the oc0x value is to use the force output compare (foc0x) strobe bits in normal mode. the oc0x registers keep their values even when changing between waveform generation modes. ocfn x (int.req.) = (8-bit comparator ) ocrnx ocnx data bus tcntn wgmn1:0 waveform generator top focn comnx1:0 bottom
89 atmega48/88/168 2545d?avr?07/04 be aware that the com0x1:0 bits are not double buffered together with the compare value. changing the com0x1:0 bi ts will take effect immediately. compare match output unit the compare output mode (com0x1:0) bits have two functions. the waveform gener- ator uses the com0x1:0 bits for defining the output compare (oc0x) state at the next compare match. also, the com0x1:0 bits control the oc0x pin output source. figure 31 shows a simplified schematic of the logic affected by the com0x1:0 bit setting. the i/o registers, i/o bits, and i/o pins in the figur e are shown in bold. only the parts of the general i/o port control registers (ddr and port) that are affected by the com0x1:0 bits are shown. when referring to the oc0x state, the reference is for the internal oc0x register, not the oc0x pin. if a system reset occur, the oc0x register is reset to ?0?. figure 31. compare match output unit, schematic the general i/o port function is overridden by the output compare (oc0x) from the waveform generator if either of the com0x1:0 bits are set. however, the oc0x pin direction (input or output) is still controlled by the data dir ection register (ddr) for the port pin. the data direction register bit for the oc0x pin (ddr_oc0x) must be set as output before the oc0x value is visible on the pin. the port override function is indepen- dent of the waveform generation mode. the design of the output compare pin logic allows initialization of the oc0x state before the output is enabled. note that some com0x1:0 bit settings are reserved for certain modes of operation. see ?8-bit timer/counter register description? on page 95. compare output mode and waveform generation the waveform generator uses the com0x1:0 bits differently in normal, ctc, and pwm modes. for all modes, setting the com0x1:0 = 0 tells the waveform generator that no action on the oc0x register is to be performed on the next compare match. for com- pare output actions in the non-pwm modes refer to table 45 on page 95. for fast pwm mode, refer to table 46 on page 96, and for phase correct pwm refer to table 47 on page 96. a change of the com0x1:0 bits state will have effect at the first co mpare match after the bits are written. for non-pwm modes, the action can be forced to have immediate effect by using the foc0x strobe bits. port ddr dq dq ocnx pin ocnx dq waveform generator comnx1 comnx0 0 1 data b u s focn clk i/o
90 atmega48/88/168 2545d?avr?07/04 modes of operation the mode of operation, i.e., the behavior of the timer/counter and the output compare pins, is defined by the combination of the waveform generation mode (wgm02:0) and compare output mode (com0x1:0) bits. the compare output mode bits do not affect the counting sequence, while the waveform generation mode bits do. the com0x1:0 bits control whether the pwm output generated should be inverted or not (inverted or non-inverted pwm). for non-pwm modes the com0x1:0 bits control whether the out- put should be set, cleared, or toggled at a compare match (see ?compare match output unit? on page 89.). for detailed timing information refer to ?timer/counter timing diagrams? on page 94. normal mode the simplest mode of operation is the normal mode (wgm02:0 = 0). in this mode the counting direction is always up (incrementing), and no counter clear is performed. the counter simply overruns when it passes its maximum 8-bit value (top = 0xff) and then restarts from the bottom (0x00). in normal operation the timer/counter overflow flag (tov0) will be set in the same timer cloc k cycle as the tcnt0 becomes zero. the tov0 flag in this case behaves like a ninth bit, except that it is only set, not cleared. however, combined with the timer overflow interrupt that automatically clears the tov0 flag, the timer resolution can be increased by software. there are no special cases to consider in the normal mode, a new counter value can be written anytime. the output compare unit can be used to generate interrupts at some given time. using the output compare to generate waveforms in normal mode is not recommended, since this will occupy too much of the cpu time. clear timer on compare match (ctc) mode in clear timer on compare or ctc mode (wgm02:0 = 2), the ocr0a register is used to manipulate the counter resolution. in ctc mode the counter is cleared to zero when the counter value (tcnt0) matches the ocr0 a. the ocr0a defines the top value for the counter, hence also its resolution. this mode allows greater control of the compare match output frequency. it also simplifies th e operation of counting external events. the timing diagram for the ctc mode is shown in figure 32. the counter value (tcnt0) increases until a compare match occurs between tcnt0 and ocr0a, and then counter (tcnt0) is cleared. figure 32. ctc mode, timing diagram an interrupt can be generated each time the counter value reaches the top value by using the ocf0a flag. if the interrupt is enabled, the interrupt handler routine can be used for updating the top value. however, changing top to a value close to bottom when the counter is running with none or a low prescaler value must be done with care since the ctc mode does not have the double buffering feature. if the new value written to ocr0a is lower than the current value of tcnt0, the counter will miss the compare tcntn ocn (toggle) ocnx interrupt flag set 1 4 period 2 3 (comnx1:0 = 1)
91 atmega48/88/168 2545d?avr?07/04 match. the counter will then have to count to its maxi mum value (0xff) and wrap around starting at 0x00 before the compare match can occur. for generating a waveform output in ctc mode, the oc0a output can be set to toggle its logical level on each compare match by setting the compare output mode bits to tog- gle mode (com0a1:0 = 1). the oc0a value w ill not be visible on the port pin unless the data direction for the pin is set to outp ut. the waveform gene rated will have a maximum frequency of f oc0 = f clk_i/o /2 when ocr0a is set to zero (0x00). the waveform frequency is defined by the following equation: the n variable represents the prescale factor (1, 8, 64, 256, or 1024). as for the normal mode of operation, the tov0 flag is set in the same timer clock cycle that the counter counts from max to 0x00. fast pwm mode the fast pulse width modulation or fast pwm mode (wgm02:0 = 3 or 7) provides a high frequency pwm waveform generation option. the fast pwm differs from the other pwm option by its single-slope operation. the counter counts from bottom to top then restarts from bottom. top is defined as 0xff when wgm2:0 = 3, and ocr0a when wgm2:0 = 7. in non-inverting compare output mode, the output compare (oc0x) is cleared on the compare match between tcnt0 and ocr0x, and set at bottom. in inverting compare output mode, the output is set on compare match and cleared at bottom. due to the single-slope operation, the operating frequency of the fast pwm mode can be twice as high as the phase co rrect pwm mode that use dual-slope opera- tion. this high frequency makes the fast pwm mode well suited for power regulation, rectification, and dac applications. high fr equency allows physicall y small sized exter- nal components (coils, capacitors), and therefore reduces total system cost. in fast pwm mode, the counter is incremented until the counter value matches the top value. the counter is then cleared at the following timer clock cycle. the timing diagram for the fast pwm mode is shown in figure 33. the tcnt0 value is in the timing diagram shown as a histogram for illustrating the single-slope operation. the diagram includes non-inverted and inverted pwm outputs. the small horizontal line marks on the tcnt0 slopes represent compare matches between ocr0x and tcnt0. figure 33. fast pwm mode, timing diagram f ocnx f clk_i/o 2 n 1 ocrnx + () ?? ------------------------------------------------- - = tcntn ocrnx update and tovn interrupt flag set 1 period 2 3 ocn ocn (comnx1:0 = 2) (comnx1:0 = 3) ocrnx interrupt flag set 4 5 6 7
92 atmega48/88/168 2545d?avr?07/04 the timer/counter overflow flag (tov0) is set each time the counter reaches top. if the interrupt is enabled, the interrupt handler routine can be used for updating the com- pare value. in fast pwm mode, the compare unit allows generation of pwm waveforms on the oc0x pins. setting the com0x1:0 bits to two will produce a non-inverted pwm and an inverted pwm output can be generated by setting the com0x1:0 to three: setting the com0a1:0 bits to one allows the oc0a pin to toggle on compare matches if the wgm02 bit is set. this option is not available for the oc0b pin (see table 49 on page 97). the actual oc 0x value will only be visible on the por t pin if the data direction for the port pin is set as output. the pwm waveform is generated by setting (or clearing) the oc0x register at the compare match between ocr0x and tcnt0, and clearing (or set- ting) the oc0x register at the timer clock cycle the counter is cleared (changes from top to bottom). the pwm frequency for the output can be calculated by the following equation: the n variable represents the prescale factor (1, 8, 64, 256, or 1024). the extreme values for the ocr0a register represents special cases when generating a pwm waveform output in the fast pwm mode. if the ocr0a is set equal to bottom, the output will be a narro w spike for each max+1 timer cl ock cycle. setting the ocr0a equal to max will result in a constantly high or low output (depending on t he polarity of the output set by the com0a1:0 bits.) a frequency (with 50% duty cycle) waveform output in fast pwm mode can be achieved by setting oc0x to toggle its logical level on each compare match (com0x1:0 = 1). the waveform generated will have a maximum frequency of f oc0 = f clk_i/o /2 when ocr0a is set to zero. this feature is similar to t he oc0a toggle in ctc mode, except the double buffer feature of the output compare unit is enabled in the fast pwm mode. phase correct pwm mode the phase correct pwm mode (wgm02:0 = 1 or 5) provides a high resolution phase correct pwm waveform generation option. the phase correct pwm mode is based on a dual-slope operation. the counter counts repeatedly from bottom to top and then from top to bottom. top is defined as 0xff when wgm2:0 = 1, and ocr0a when wgm2:0 = 5. in non-inverting compare output mode, the output compare (oc0x) is cleared on the compare match between tcnt0 and ocr0x while upcounting, and set on the compare match while downcounting. in inverting output compare mode, the operation is inverted. the dual-slope operation has lower maximum operation frequency than single slope operation. however, due to the symmetric feature of the dual-slope pwm modes, these modes are preferred for motor control applications. in phase correct pwm mode the counter is incremented until the counter value matches top. when the counter reaches top, it changes the count direction. the tcnt0 value will be equal to top for one timer clock cycle. the timing diagram for the phase correct pwm mode is shown on figure 34. the tcnt0 value is in the timing diagram shown as a histogram for illustrating the dual-slope operation. the diagra m includes non-inverted and inverted pwm outputs. the small horizontal line marks on the tcnt0 slopes repre- sent compare matches between ocr0x and tcnt0. f ocnxpwm f clk_i/o n 256 ? ------------------ =
93 atmega48/88/168 2545d?avr?07/04 figure 34. phase correct pwm mode, timing diagram the timer/counter overflow flag (tov0) is set each time the counter reaches bot- tom. the interrupt flag can be used to generate an interrupt each time the counter reaches the bottom value. in phase correct pwm mode, the compare unit allows generation of pwm waveforms on the oc0x pins. setting the com0x1:0 bits to two will produce a non-inverted pwm. an inverted pwm output can be generated by setting the com0x1:0 to three: setting the com0a0 bits to one allows the oc0a pin to toggle on compare matches if the wgm02 bit is set. this option is not available for the oc0b pin (see table 50 on page 97). the actual oc0x value will only be visible on the port pin if the data direction for the port pin is set as output. the pwm waveform is generated by clearing (or setting) the oc0x register at the compare match between ocr0x and tcnt0 when the counter incre- ments, and setting (or clearing) the oc0x register at compare match between ocr0x and tcnt0 when the counter decrements. the pwm frequency for the output when using phase correct pwm can be calculated by the following equation: the n variable represents the prescale factor (1, 8, 64, 256, or 1024). the extreme values for the ocr0a register represent special cases when generating a pwm waveform output in the phase correct pwm mode. if the ocr0a is set equal to bottom, the output will be cont inuously low and if set e qual to max the output will be continuously high for non-inverted pwm mode. for inverted pw m the output will have the opposite logic values. at the very start of period 2 in figure 34 ocnx has a transition from high to low even though there is no compare match. the point of this transition is to guarantee symmetry around bottom. there are two cases that give a transition without compare match. ocrnx changes its value from max, like in figure 34. when the ocr0a value is max the ocn pin value is the same as the result of a down-counting compare tovn interrupt flag set ocnx interrupt flag set 1 2 3 tcntn period ocnx ocnx (comnx1:0 = 2) (comnx1:0 = 3) ocrnx update f ocnxpcpwm f clk_i/o n 510 ? ------------------ =
94 atmega48/88/168 2545d?avr?07/04 match. to ensure symmetry around bottom the ocnx value at max must correspond to the result of an up-counting compare match. the timer starts counting from a value higher than the one in ocrnx, and for that reason misses the compare match and hence the ocnx change that would have happened on the way up. timer/counter timing diagrams the timer/counter is a synchronous design and the timer clock (clk t0 ) is therefore shown as a clock enable signal in the followi ng figures. the figures include information on when interrupt flags are set. figure 35 c ontains timing data for basic timer/counter operation. the figure shows the count sequence close to the max value in all modes other than phase correct pwm mode. figure 35. timer/counter timing diagram, no prescaling figure 36 shows the same timing data, but with the prescaler enabled. figure 36. timer/counter timing diagram, with prescaler (f clk_i/o /8) figure 37 shows the setting of ocf0b in all modes and ocf0a in all modes except ctc mode and pwm mode, where ocr0a is top. figure 37. timer/counter timing diagram, setting of ocf0x, with prescaler (f clk_i/o /8) clk tn (clk i/o /1) tovn clk i/o tcntn max - 1 max bottom bottom + 1 tovn tcntn max - 1 max bottom bottom + 1 clk i/o clk tn (clk i/o /8) ocfnx ocrnx tcntn ocrnx value ocrnx - 1 ocrnx ocrnx + 1 ocrnx + 2 clk i/o clk tn (clk i/o /8)
95 atmega48/88/168 2545d?avr?07/04 figure 38 shows the setting of ocf0a and the clearing of tcnt0 in ctc mode and fast pwm mode where ocr0a is top. figure 38. timer/counter timing diagram, clear timer on compare match mode, with prescaler (f clk_i/o /8) 8-bit timer/counter register description timer/counter control register a ? tccr0a bits 7:6 ? com0a1:0: compare match output a mode these bits control the output compare pin (oc0a) behavior. if one or both of the com0a1:0 bits are set, the oc0a output overrides the normal port functionality of the i/o pin it is connected to. ho wever, note that t he data direction register (ddr) bit cor- responding to the oc0a pin must be set in order to enable the output driver. when oc0a is connected to the pin, the func tion of the com0a1:0 bits depends on the wgm02:0 bit setting. table 45 shows the com0a1:0 bit functionality when the wgm02:0 bits are set to a normal or ctc mode (non-pwm). ocfnx ocrnx tcntn (ctc) top top - 1 top bottom bottom + 1 clk i/o clk tn (clk i/o /8) bit 7 6 5 4 3 2 1 0 com0a1 com0a0 com0b1 com0b0 ? ? wgm01 wgm00 tccr0a read/write r/w r/w r/w r/w r r r/w r/w initial value 0 0 0 0 0 0 0 0 table 45. compare output mode, non-pwm mode com0a1 com0a0 description 0 0 normal port operation, oc0a disconnected. 0 1 toggle oc0a on compare match 1 0 clear oc0a on compare match 1 1 set oc0a on compare match
96 atmega48/88/168 2545d?avr?07/04 table 46 shows the com0a1:0 bit functionality when the wgm01:0 bits are set to fast pwm mode. note: 1. a special case occurs when ocr0a equ als top and com0a1 is set. in this case, the compare match is ignored, but the set or clear is done at top. see ?fast pwm mode? on page 91 for more details. table 47 shows the com0a1:0 bit functi onality when the wgm02:0 bits are set to phase correct pwm mode. note: 1. a special case occurs when ocr0a equ als top and com0a1 is set. in this case, the compare match is ignored, but the set or clear is done at top. see ?phase cor- rect pwm mode? on page 119 for more details. bits 5:4 ? com0b1:0: compare match output b mode these bits control the output compare pin (oc0b) behavior. if one or both of the com0b1:0 bits are set, the oc0b output overrides the normal port functionality of the i/o pin it is connected to. ho wever, note that t he data direction register (ddr) bit cor- responding to the oc0b pin must be set in order to enable the output driver. when oc0b is connected to the pin, the func tion of the com0b1:0 bits depends on the wgm02:0 bit setting. table 48 shows the com0b1:0 bit functionality when the wgm02:0 bits are set to a normal or ctc mode (non-pwm). table 46. compare output mode, fast pwm mode (1) com0a1 com0a0 description 0 0 normal port operation, oc0a disconnected. 0 1 wgm02 = 0: normal port operation, oc0a disconnected. wgm02 = 1: toggle oc0a on compare match. 1 0 clear oc0a on compare match, set oc0a at top 1 1 set oc0a on compare match, clear oc0a at top table 47. compare output mode, phase correct pwm mode (1) com0a1 com0a0 description 0 0 normal port operation, oc0a disconnected. 0 1 wgm02 = 0: normal port operation, oc0a disconnected. wgm02 = 1: toggle oc0a on compare match. 1 0 clear oc0a on compare match when up-counting. set oc0a on compare match when down-counting. 1 1 set oc0a on compare match when up-counting. clear oc0a on compare match when down-counting. table 48. compare output mode, non-pwm mode com0b1 com0b0 description 0 0 normal port operation, oc0b disconnected. 0 1 toggle oc0b on compare match 1 0 clear oc0b on compare match 1 1 set oc0b on compare match
97 atmega48/88/168 2545d?avr?07/04 table 49 shows the com0b1:0 bit functionality when the wgm02:0 bits are set to fast pwm mode. note: 1. a special case occurs when ocr0b equ als top and com0b1 is set. in this case, the compare match is ignored, but the set or clear is done at top. see ?fast pwm mode? on page 91 for more details. table 50 shows the com0b1:0 bit functi onality when the wgm02:0 bits are set to phase correct pwm mode. note: 1. a special case occurs when ocr0b equ als top and com0b1 is set. in this case, the compare match is ignored, but the set or clear is done at top. see ?phase cor- rect pwm mode? on page 92 for more details. bits 3, 2 ? res: reserved bits these bits are reserved bits in the atme ga48/88/168 and will al ways read as zero. bits 1:0 ? wgm01:0: waveform generation mode combined with the wgm02 bit found in the tccr0b register, these bits control the counting sequence of the counter, the source for maximum (top) counter value, and what type of waveform generation to be used, see table 51. modes of operation sup- ported by the timer/counter unit are: normal mode (counter), clear timer on compare match (ctc) mode, and two types of pulse width modulation (pwm) modes (see ?modes of operation? on page 90). table 49. compare output mode, fast pwm mode (1) com0b1 com0b0 description 0 0 normal port operation, oc0b disconnected. 01reserved 1 0 clear oc0b on compare match, set oc0b at top 1 1 set oc0b on compare match, clear oc0b at top table 50. compare output mode, phase correct pwm mode (1) com0b1 com0b0 description 0 0 normal port operation, oc0b disconnected. 01reserved 1 0 clear oc0b on compare match when up-counting. set oc0b on compare match when down-counting. 1 1 set oc0b on compare match when up-counting. clear oc0b on compare match when down-counting. table 51. waveform generation mode bit description mode wgm02 wgm01 wgm00 timer/count er mode of operation top update of ocrx at tov flag set on (1)(2) 0 0 0 0 normal 0xff immediate max 1 0 0 1 pwm, phase correct 0xff top bottom 2 0 1 0 ctc ocra immediate max 3 0 1 1 fast pwm 0xff top max
98 atmega48/88/168 2545d?avr?07/04 notes: 1. max = 0xff 2. bottom = 0x00 timer/counter control register b ? tccr0b bit 7 ? foc0a: force output compare a the foc0a bit is only active when the wgm bits specify a non-pwm mode. however, for ensuring compatib ility with future devices, this bit must be set to zero when tccr0b is written when operating in pw m mode. when writing a logical one to the foc0a bit, an immediate compare match is forced on the waveform generation unit. the oc0a output is changed according to its com0a1:0 bits setting. note that the foc0a bit is implemented as a strobe. therefore it is the value present in the com0a1:0 bits that determines the effect of the forced compare. a foc0a strobe will not generate any interrup t, nor will it clear the timer in ctc mode using ocr0a as top. the foc0a bit is always read as zero. bit 6 ? foc0b: force output compare b the foc0b bit is only active when the wgm bits specify a non-pwm mode. however, for ensuring compatib ility with future devices, this bit must be set to zero when tccr0b is written when operating in pw m mode. when writing a logical one to the foc0b bit, an immediate compare match is forced on the waveform generation unit. the oc0b output is changed according to its com0b1:0 bits setting. note that the foc0b bit is implemented as a strobe. therefore it is the value present in the com0b1:0 bits that determines the effect of the forced compare. a foc0b strobe will not generate any interrup t, nor will it clear the timer in ctc mode using ocr0b as top. the foc0b bit is always read as zero. bits 5:4 ? res: reserved bits these bits are reserved bits in the atme ga48/88/168 and will al ways read as zero. bit 3 ? wgm02: waveform generation mode see the description in the ?timer/counter control register a ? tccr0a? on page 95. 41 0 0reserved ? ? ? 5 1 0 1 pwm, phase correct ocra top bottom 61 1 0reserved ? ? ? 7 1 1 1 fast pwm ocra top top table 51. waveform generation mode bit description (continued) mode wgm02 wgm01 wgm00 timer/count er mode of operation top update of ocrx at tov flag set on (1)(2) bit 7 6 5 4 3 2 1 0 foc0a foc0b ? ? wgm02 cs02 cs01 cs00 tccr0b read/write w w r r r r r/w r/w initial value 0 0 0 0 0 0 0 0
99 atmega48/88/168 2545d?avr?07/04 bits 2:0 ? cs02:0: clock select the three clock select bits select the cloc k source to be used by the timer/counter. if external pin modes are used for the timer/counter0, transitions on the t0 pin will clock the counter even if the pin is configured as an output. this feature allows software control of the counting. timer/counter register ? tcnt0 the timer/counter register gives direct access, both for read and write operations, to the timer/counter unit 8-bit counter. writing to the tcnt0 register blocks (removes) the compare match on the following timer clock. modifying the counter (tcnt0) while the counter is running, introduces a risk of missing a compare match between tcnt0 and the ocr0x registers. output compare register a ? ocr0a the output compare register a contains an 8-bit value that is continuously compared with the counter value (tcnt0). a match can be used to generate an output compare interrupt, or to generate a waveform output on the oc0a pin. output compare register b ? ocr0b the output compare register b contains an 8-bit value that is continuously compared with the counter value (tcnt0). a match can be used to generate an output compare interrupt, or to generate a waveform output on the oc0b pin. table 52. clock select bit description cs02 cs01 cs00 description 0 0 0 no clock source (timer/counter stopped) 001clk i/o /(no prescaling) 010clk i/o /8 (from prescaler) 011clk i/o /64 (from prescaler) 100clk i/o /256 (from prescaler) 101clk i/o /1024 (from prescaler) 1 1 0 external clock source on t0 pin. clock on falling edge. 1 1 1 external clock source on t0 pin. clock on rising edge. bit 76543210 tcnt0[7:0] tcnt0 read/write r/w r/w r/w r/w r/w r/w r/w r/w initial value00000000 bit 76543210 ocr0a[7:0] ocr0a read/write r/w r/w r/w r/w r/w r/w r/w r/w initial value00000000 bit 76543210 ocr0b[7:0] ocr0b read/write r/w r/w r/w r/w r/w r/w r/w r/w initial value00000000
100 atmega48/88/168 2545d?avr?07/04 timer/counter interrupt mask register ? timsk0 bits 7..3 ? res: reserved bits these bits are reserved bits in the atme ga48/88/168 and will al ways read as zero. bit 2 ? ocie0b: timer/counter output compare match b interrupt enable when the ocie0b bit is written to one, and the i-bit in the status register is set, the timer/counter compare match b interrupt is enabled. the corresponding interrupt is executed if a compare match in timer/counter occurs, i.e., when the ocf0b bit is set in the timer/counter interrupt flag register ? tifr0. bit 1 ? ocie0a: timer/counter0 output compare match a interrupt enable when the ocie0a bit is written to one, and the i-bit in the status register is set, the timer/counter0 compare match a interrupt is enabled. the corresponding interrupt is executed if a compare match in timer/counter0 occurs, i.e., when the ocf0a bit is set in the timer/counter 0 interrupt flag register ? tifr0. bit 0 ? toie0: timer/counter0 overflow interrupt enable when the toie0 bit is written to one, and the i-bit in the status register is set, the timer/counter0 overflow interrupt is enable d. the corresponding interrupt is executed if an overflow in timer/counter0 occurs, i.e., when the tov0 bit is set in the timer/counter 0 interrupt flag register ? tifr0. timer/counter 0 interrupt flag register ? tifr0 bits 7..3 ? res: reserved bits these bits are reserved bits in the atme ga48/88/168 and will al ways read as zero. bit 2 ? ocf0b: timer/counter 0 output compare b match flag the ocf0b bit is set when a compare match occurs between the timer/counter and the data in ocr0b ? output compare register0 b. ocf0b is cleared by hardware when executing the corresponding interrupt handling vector. alternatively, ocf0b is cleared by writing a logic one to the flag. when the i-bit in sreg, ocie0b (timer/counter compare b match interrupt enable), and ocf0b are set, the timer/counter compare match interrupt is executed. bit 1 ? ocf0a: timer/counter 0 output compare a match flag the ocf0a bit is set when a compare match occurs between the timer/counter0 and the data in ocr0a ? output compare register0. ocf0a is cleared by hardware when executing the corresponding interrupt handling vector. alternatively, ocf0a is cleared by writing a logic one to the flag. when t he i-bit in sreg, ocie0a (timer/counter0 compare match interrupt enable), and ocf0a are set, the timer/counter0 compare match interrupt is executed. bit 0 ? tov0: timer/counter0 overflow flag the bit tov0 is set when an overflow occu rs in timer/counter0. tov0 is cleared by hardware when executing the corresponding interrupt handling vector. alternatively, tov0 is cleared by writing a logic one to the flag. when the sreg i-bit, toie0 bit 76543 210 ?????ocie0bocie0atoie0timsk0 read/write r r r r r r/w r/w r/w initial value00000 000 bit 76543210 ?????ocf0bocf0atov0tifr0 read/writerrrrrr/wr/wr/w initial value00000000
101 atmega48/88/168 2545d?avr?07/04 (timer/counter0 overflow interrupt enable), and tov0 are set, the timer/counter0 overflow interrupt is executed. the setting of this flag is dependent of the wgm02:0 bit setting. refer to table 51, ?waveform generation mode bit description? on page 97.
102 atmega48/88/168 2545d?avr?07/04 timer/counter0 and timer/counter1 prescalers timer/counter1 and timer/counter0 share the same prescaler module, but the timer/counters can have different prescaler settings. the description below applies to both timer/counter1 and timer/counter0. internal clock source the timer/counter can be clocked directly by the system clock (by setting the csn2:0 = 1). this provides the fastes t operation, with a maximum timer/counter clock frequency equal to system clock frequency (f clk_i/o ). alternatively, one of four taps from the pres- caler can be used as a clock source. the prescaled clock has a frequency of either f clk_i/o /8, f clk_i/o /64, f clk_i/o /256, or f clk_i/o /1024. prescaler reset the prescaler is free running, i.e., operates independently of the clock select logic of the timer/counter, and it is shared by timer/counter1 and timer/counter0. since the prescaler is not affected by the timer/counter?s clock select, the state of the prescaler will have implications for situations where a prescaled clock is used. one example of prescaling artifacts occurs when the timer is enabled and clocked by the prescaler (6 > csn2:0 > 1). the number of system clock cycles from when the timer is enabled to the first count occurs can be from 1 to n+1 system clock cycles, where n equals the pres- caler divisor (8, 64, 256, or 1024). it is possible to use the prescaler reset fo r synchronizing the timer/counter to program execution. however, care must be taken if the other timer/counter that shares the same prescaler also uses prescaling. a pr escaler reset will affect the prescaler period for all timer/counters it is connected to. external clock source an external clock source applied to the t1/t0 pin can be used as timer/counter clock (clk t1 /clk t0 ). the t1/t0 pin is sampled once every system clock cycle by the pin syn- chronization logic. the synchronized (sampled) signal is then passed through the edge detector. figure 39 shows a functional equivalent block diagram of the t1/t0 synchroni- zation and edge detector logic. the registers are clocked at the positive edge of the internal system clock ( clk i/o ). the latch is transparent in the high period of the internal system clock. the edge detector generates one clk t1 /clk t 0 pulse for each positive (csn2:0 = 7) or neg- ative (csn2:0 = 6) edge it detects. figure 39. t1/t0 pin sampling the synchronization and edge detector logic introduces a delay of 2.5 to 3.5 system clock cycles from an edge has been applied to the t1/t0 pin to the counter is updated. enabling and disabling of the clock input must be done when t1/t0 has been stable for at least one system clock cycle, otherwise it is a risk that a false timer/counter clock pulse is generated. each half period of the exte rnal clock applied must be longer than one system clock cycle to ensure correct sampling. the external clock must be guaranteed to have less than half the system clock frequency (f extclk < f clk_i/o /2) given a 50/50% duty cycle. since tn_sync (to clock select logic) edge detector synchronization dq dq le dq tn clk i/o
103 atmega48/88/168 2545d?avr?07/04 the edge detector uses sampling, the maximum frequency of an external clock it can detect is half the sampling frequency (nyquist sampling theorem). however, due to vari- ation of the system clock freq uency and duty cycle caused by oscillator source (crystal, resonator, and capacitors) tolerances, it is recommended that maximum frequency of an external clock source is less than f clk_i/o /2.5. an external clock source can not be prescaled. figure 40. prescaler for timer/counter0 and timer/counter1 (1) note: 1. the synchronization logic on the input pins ( t1/t0) is shown in figure 39. general timer/counter control register ? gtccr bit 7 ? tsm: timer/counter synchronization mode writing the tsm bit to one activates the timer/counter synchronization mode. in this mode, the value that is written to the psrasy and psr sync bits is kept, hence keep- ing the corresponding prescaler reset signals asserted. this ensures that the corresponding timer/counters are halted and can be configured to the same value with- out the risk of one of them advancing during configuration. when the tsm bit is written to zero, the psrasy and psrsync bits are cleared by hardware, and the timer/counters start counting simultaneously. bit 0 ? psrsync: prescaler reset when this bit is one, timer/counter1 and ti mer/counter0 prescaler will be reset. this bit is normally cleared immediately by hardware, except if the tsm bit is set. note that timer/counter1 and timer/counter0 share the same prescaler and a reset of this pres- caler will affect both timers. psrsync clear clk t1 clk t0 t1 t0 clk i/o synchronization synchronization bit 765432 1 0 tsm ? ? ? ? ? psrasy psrsync gtccr read/writer/wrrrrrr/wr/w initial value000000 0 0
104 atmega48/88/168 2545d?avr?07/04 16-bit timer/counter1 with pwm the 16-bit timer/counter unit allows accura te program execution timing (event man- agement), wave generation, and signal timing measurement. the main features are: true 16-bit design (i.e., allows 16-bit pwm) two independent output compare units double buffered out put compare registers one input capture unit input capture noise canceler clear timer on compar e match (auto reload) glitch-free, phase correct pu lse width modulator (pwm) variable pwm period frequency generator external event counter four independent interrupt sources (tov1, ocf1a, ocf1b, and icf1) overview most register and bit references in this se ction are written in general form. a lower case ?n? replaces the timer/counter number, and a lower case ?x? replaces the output com- pare unit channel. however, when using the register or bit defines in a program, the precise form must be used, i.e., tcnt1 for accessing timer/counter1 counter value and so on. a simplified block diagram of the 16-bit time r/counter is shown in figure 41. for the actual placement of i/o pins, refer to ?pinout atmega48/88/168? on page 2. cpu acces- sible i/o registers, including i/o bits and i/o pins, are shown in bold. the device- specific i/o register and bit locations are listed in the ?16-bit timer/counter register description? on page 125. the prtim1 bit in ?power reduction register - prr? on page 37 must be written to zero to enable timer/counter1 module.
105 atmega48/88/168 2545d?avr?07/04 figure 41. 16-bit timer/counter block diagram (1) note: 1. refer to figure 1 on page 2, table 33 on page 69 and table 39 on page 75 for timer/counter1 pin placement and description. registers the timer/counter (tcnt1), output compare registers (ocr1a/b), and input capture register (icr1) are all 16-bit registers. special procedures must be followed when accessing the 16-bit registers. these procedur es are described in the section ?access- ing 16-bit registers? on page 106. the timer/counter control registers (tccr1a/b) are 8-bit registers and have no cpu access restrictions. interrupt requests (abbreviated to int.req. in the figure) signals are all visible in the timer interrupt flag register (tifr1). all interrupts are in dividually masked with the timer interrupt mask register (timsk1). tifr1 and timsk1 are not shown in the figure. the timer/counter can be clocked internally, via the prescaler, or by an external clock source on the t1 pin. the clock select logic block controls which clock source and edge the timer/counter uses to increment (or decrement) its value. the timer/counter is inactive when no cloc k source is selected. the output from the clock select logic is referred to as the timer clock (clk t 1 ). the double buffered output compare registers (ocr1a/b) are compared with the timer/counter value at all time. the result of the compare can be used by the waveform generator to generate a pwm or variable frequency output on the output compare pin (oc1a/b). see ?output compare units? on page 113.. the co mpare match event will clock select timer/counter data b u s ocrna ocrnb icrn = = tcntn waveform generation waveform generation ocna ocnb noise canceler icpn = fixed top values edge detector control logic = 0 top bottom count clear direction tovn (int.req.) ocna (int.req.) ocnb (int.req.) icfn (int.req.) tccrna tccrnb ( from analog comparator ouput ) tn edge detector ( from prescaler ) clk tn
106 atmega48/88/168 2545d?avr?07/04 also set the compare match flag (ocf1a/b) which can be used to generate an output compare interrupt request. the input capture register can capture the timer/counter value at a given external (edge triggered) event on either the input capture pin (icp1) or on the analog compar- ator pins (see ?analog comparator? on page 228.) the input capture unit includes a digital filtering unit (noise canceler) for reducing the chance of capturing noise spikes. the top value, or maximum timer/counter value, can in some modes of operation be defined by either the ocr1a register, the icr1 register, or by a set of fixed values. when using ocr1a as top value in a pwm mode, the ocr1a register can not be used for generating a pwm output. however, the top value will in this case be double buffered allowing the top value to be changed in run time. if a fixed top value is required, the icr1 register can be used as an alternative, freeing the ocr1a to be used as pwm output. definitions the following definitions are used extensively throughout the section: accessing 16-bit registers the tcnt1, ocr1a/b, and icr1 are 16-bit registers that can be accessed by the avr cpu via the 8-bit data bus. the 16-bit register must be byte accessed using two read or write operations. each 16-bit timer has a single 8-bit register for temporary storing of the high byte of the 16-bit access. the same temporary register is shared between all 16-bit registers within each 16-bit timer. accessing the low byte triggers the 16-bit read or write operation. when the low byte of a 16-bit register is written by the cpu, the high byte stored in the temporary register, and the low byte written are both copied into the 16-bit register in the same clock cycle. when the low byte of a 16-bit register is read by the cpu, the high byte of the 16-bit register is copied into the temporary register in the same clock cycle as the low byte is read. not all 16-bit accesses uses the temporary register for the high byte. reading the ocr1a/b 16-bit registers does not involve using the temporary register. to do a 16-bit write, the high byte must be written before the low byte. for a 16-bit read, the low byte must be read before the high byte. the following code examples show how to access the 16-bit timer registers assuming that no interrupts updates the temporary register. the same principle can be used directly for accessing the ocr1a/b and icr1 registers. note that when using ?c?, the compiler handles the 16-bit access. table 53. definitions bottom the counter reaches the bottom when it becomes 0x0000. max the counter reaches its max imum when it becomes 0xffff (decimal 65535). top the counter reaches the top when it becomes equal to the highest value in the count sequence. the top value can be assigned to be one of the fixed values: 0x00ff, 0x01ff, or 0x03ff, or to the value stored in the ocr1a or icr1 regis- ter. the assignment is dependent of the mode of operation.
107 atmega48/88/168 2545d?avr?07/04 note: 1. the example code assumes that th e part specific header file is included. for i/o registers located in extended i/o map, ?in?, ?out?, ?s bis?, ?sbic?, ?cbi?, and ?sbi? instructions must be replaced with instructions that allow access to extended i/o. typically ?lds? and ?sts? combined with ?sbrs?, ?sbrc?, ?sbr?, and ?cbr?. the assembly code example returns the tcnt1 value in the r17:r16 register pair. it is important to notice that accessing 16-bit registers are atomic operations. if an inter- rupt occurs between the two instructions accessing the 16-bit register, and the interrupt code updates the temporary register by accessing the same or any other of the 16-bit timer registers, then the result of the access outside the interrupt will be corrupted. therefore, when both the main code and the interrupt code update the temporary regis- ter, the main code must disable the interrupts during the 16-bit access. assembly code examples (1) ... ; set tcnt 1 to 0x01ff ldi r17,0x01 ldi r16,0xff out tcnt 1 h,r17 out tcnt 1 l,r16 ; read tcnt 1 into r17:r16 in r16,tcnt 1 l in r17,tcnt 1 h ... c code examples (1) unsigned int i; ... /* set tcnt 1 to 0x01ff */ tcnt 1 = 0x1ff; /* read tcnt 1 into i */ i = tcnt 1 ; ...
108 atmega48/88/168 2545d?avr?07/04 the following code examples show how to do an atomic read of the tcnt1 register contents. reading any of the ocr1a/b or icr1 registers can be done by using the same principle. note: 1. the example code assumes that th e part specific header file is included. for i/o registers located in extended i/o map, ?in?, ?out?, ?s bis?, ?sbic?, ?cbi?, and ?sbi? instructions must be replaced with instructions that allow access to extended i/o. typically ?lds? and ?sts? combined with ?sbrs?, ?sbrc?, ?sbr?, and ?cbr?. the assembly code example returns the tcnt1 value in the r17:r16 register pair. assembly code example (1) tim16_readtcnt 1 : ; save global interrupt flag in r18,sreg ; disable interrupts cli ; read tcnt 1 into r17:r16 in r16,tcnt 1 l in r17,tcnt 1 h ; restore global interrupt flag out sreg,r18 ret c code example (1) unsigned int tim16_readtcnt 1 ( void ) { unsigned char sreg; unsigned int i; /* save global interrupt flag */ sreg = sreg; /* disable interrupts */ _cli(); /* read tcnt 1 into i */ i = tcnt 1 ; /* restore global interrupt flag */ sreg = sreg; return i; }
109 atmega48/88/168 2545d?avr?07/04 the following code examples show how to do an atomic write of the tcnt1 register contents. writing any of the ocr1a/b or icr1 registers can be done by using the same principle. note: 1. the example code assumes that th e part specific header file is included. for i/o registers located in extended i/o map, ?in?, ?out?, ?s bis?, ?sbic?, ?cbi?, and ?sbi? instructions must be replaced with instructions that allow access to extended i/o. typically ?lds? and ?sts? combined with ?sbrs?, ?sbrc?, ?sbr?, and ?cbr?. the assembly code example requires that the r17:r16 register pair contains the value to be written to tcnt1. reusing the temporary high byte register if writing to more than one 16-bit register where the high byte is the same for all registers written, then the high byte only needs to be written once. however, note that the same rule of atomic operation described pr eviously also applies in this case. assembly code example (1) tim16_writetcnt 1 : ; save global interrupt flag in r18,sreg ; disable interrupts cli ; set tcnt 1 to r17:r16 out tcnt 1 h,r17 out tcnt 1 l,r16 ; restore global interrupt flag out sreg,r18 ret c code example (1) void tim16_writetcnt 1 ( unsigned int i ) { unsigned char sreg; unsigned int i; /* save global interrupt flag */ sreg = sreg; /* disable interrupts */ _cli(); /* set tcnt 1 to i */ tcnt 1 = i; /* restore global interrupt flag */ sreg = sreg; }
110 atmega48/88/168 2545d?avr?07/04 timer/counter clock sources the timer/counter can be clocked by an intern al or an external clock source. the clock source is selected by the clock sele ct logic which is controlled by the clock select (cs12:0) bits located in the timer/counter cont rol register b (tccr1b). for details on clock sources and pre scaler, see ?timer/counter0 and timer/counter1 prescalers? on page 102. counter unit the main part of the 16-bit timer/counter is the programmable 16-bit bi-directional counter unit. figure 42 shows a block diagram of the counter and its surroundings. figure 42. counter unit block diagram signal description (internal signals): count increment or decrement tcnt1 by 1. direction select between increment and decrement. clear clear tcnt1 (set all bits to zero). clk t 1 timer/counter clock. top signalize that tcnt1 has reached maximum value. bottom signalize that tcnt1 has reached minimum value (zero). the 16-bit counter is mapped into two 8-bit i/o memory locations: counter high (tcnt1h) containing the upper eight bits of the counter, and counter low (tcnt1l) containing the lower eight bits. the tcnt1h register can only be indirectly accessed by the cpu. when the cpu does an access to the tcnt1h i/o location, the cpu accesses the high byte temporary register (temp). the temporary register is updated with the tcnt1h value when the tcnt1l is read, and tcnt1h is updated with the temporary register value when tcnt1l is writ ten. this allows the cpu to read or write the entire 16-bit counter value within one cl ock cycle via the 8-bit data bus. it is impor- tant to notice that there are special cases of writing to the tcnt1 register when the counter is counting that will give unpredictable results. the special cases are described in the sections where they are of importance. depending on the mode of operation used, the counter is cleared, incremented, or dec- remented at each timer clock (clk t 1 ). the clk t 1 can be generated from an external or internal clock source, selected by the clock select bits (cs12:0). when no clock source is selected (cs12:0 = 0) the timer is stopped. however, the tcnt1 value can be accessed by the cpu, independent of whether clk t 1 is present or not. a cpu write over- rides (has priority over) all counter clear or count operations. the counting sequence is determined by the setting of the waveform generation mode bits (wgm13:0) located in the timer/counter control registers a and b (tccr1a and tccr1b). there are close connections between how the counter behaves (counts) and temp (8-bit) data bus (8-bit) tcntn (16-bit counter) tcntnh (8-bit) tcntnl (8-bit) control logic count clear direction tovn (int.req.) clock select top bottom tn edge detector ( from prescaler ) clk tn
111 atmega48/88/168 2545d?avr?07/04 how waveforms are generated on the output compare outputs oc1x. for more details about advanced counting sequences and waveform generation, see ?modes of opera- tion? on page 115. the timer/counter overflow flag (tov1) is set according to the mode of operation selected by the wgm13:0 bits. tov1 can be used for generating a cpu interrupt. input capture unit the timer/counter incorporates an input capt ure unit that can capture external events and give them a time-stamp indicating time of occurrence. the external signal indicating an event, or multiple events, can be applied via the icp1 pin or alternatively, via the analog-comparator unit. the time-stamps c an then be used to calculate frequency, duty- cycle, and other features of the signal a pplied. alternatively the time-stamps can be used for creating a log of the events. the input capture unit is illustrated by the block diagram shown in figure 43. the ele- ments of the block diagram that are not directly a part of the input capture unit are gray shaded. the small ?n? in register and bit names indicates the timer/counter number. figure 43. input capture unit block diagram when a change of the logic level (an event) occurs on the input capture pin (icp1), alternatively on the analog comparator output (aco), and this change confirms to the setting of the edge de tector, a capture will be triggered. when a captur e is triggered, the 16-bit value of the counter (tcnt1) is written to the input capture register (icr1). the input capture flag (icf1) is set at the same system clock as the tcnt1 value is copied into icr1 register. if enabled (icie1 = 1), the input capture flag generates an input capture interrupt. the icf1 flag is automatic ally cleared when the interrupt is executed. alternatively the icf1 flag can be cleared by software by writing a logical one to its i/o bit location. reading the 16-bit value in the input capture register (icr1) is done by first reading the low byte (icr1l) and then the high byte (icr1h). when the low byte is read the high byte is copied into the high byte tempor ary register (temp). when the cpu reads the icr1h i/o location it will a ccess the temp register. icfn (int.req.) analog comparator write icrn (16-bit register) icrnh (8-bit) noise canceler icpn edge detector temp (8-bit) data bus (8-bit) icrnl (8-bit) tcntn (16-bit counter) tcntnh (8-bit) tcntnl (8-bit) acic* icnc ices aco*
112 atmega48/88/168 2545d?avr?07/04 the icr1 register can only be written when using a waveform generation mode that utilizes the icr1 register for defining the counter?s top value. in these cases the waveform generation mode (wgm13:0) bits must be set before the top value can be written to the icr1 register. when writing the icr1 register the high byte must be writ- ten to the icr1h i/o location before the low byte is written to icr1l. for more information on how to access the 16-bit registers refer to ?accessing 16-bit registers? on page 106. input capture trigger source the main trigger source for the input capture unit is the input capture pin (icp1). timer/counter1 can alternatively use the a nalog comparator output as trigger source for the input capture unit. the analog comparat or is selected as trigger source by set- ting the analog comparator input capture (acic) bit in the analog comparator control and status register (acsr). be aware that changing trigger source can trigger a cap- ture. the input capture flag must therefore be cleared after the change. both the input capture pin (icp1) and the analog comparator output (aco) inputs are sampled using the same technique as for the t1 pin (figure 39 on page 102). the edge detector is also identical. ho wever, when the noise canceler is enabled, additional logic is inserted before the edge detector, which increases the delay by four system clock cycles. note that the input of the noise canceler and edge detector is always enabled unless the timer/counter is set in a waveform generation mode that uses icr1 to define top. an input capture can be triggered by software by contro lling the port of the icp1 pin. noise canceler the noise canceler improves noise immunity by using a simple digital filtering scheme. the noise canceler input is monitored over four samples, and all four must be equal for changing the output that in turn is used by the edge detector. the noise canceler is enabled by setting the input capture noise canceler (icnc1) bit in timer/counter cont rol register b (tccr1b). when enabled the noise canceler intro- duces additional four system clock cycles of delay from a change applied to the input, to the update of the icr1 register. the noise canceler uses the system clock and is there- fore not affected by the prescaler. using the input capture unit the main challenge when using the input capture unit is to assign enough processor capacity for handling the incoming events. the time between two events is critical. if the processor has not read the captured value in the icr1 register before the next event occurs, the icr1 will be overwritten with a new va lue. in this case the result of the cap- ture will be incorrect. when using the input capture interrupt, the icr1 register should be read as early in the interrupt handler routine as possible. even though the input capture interrupt has rela- tively high priority, the maximum interrupt response time is dependent on the maximum number of clock cycles it takes to handle any of the other interrupt requests. using the input capture unit in any mode of operation when the top value (resolution) is actively changed during operation, is not recommended. measurement of an external signal?s duty cycle requires that the trigger edge is changed after each capture. changing the edge sensing must be done as early as possible after the icr1 register has been read. after a change of the edge, the input capture flag (icf1) must be cleared by software (writing a logical one to the i/o bit location). for measuring frequency only, the clearing of the icf1 flag is not required (if an interrupt handler is used).
113 atmega48/88/168 2545d?avr?07/04 output compare units the 16-bit comparator continuously compares tcnt1 with the output compare regis- ter (ocr1x). if tcnt equals ocr1x the comparator signals a match. a match will set the output compare flag (ocf1x) at the next timer clock cycle . if enabled (ocie1x = 1), the output compare flag generates an output compare interrupt. the ocf1x flag is automatically cleared when the interrup t is executed. altern atively the ocf1x flag can be cleared by software by writing a logical one to its i/o bit location. the waveform generator uses the match signal to generate an output according to operating mode set by the waveform generation mode (wgm13:0) bits and compare output mode (com1x1:0) bits. the top and bottom signals are used by the waveform generator for handling the special cases of the extreme values in some modes of operation (see ?modes of operation? on page 115.) a special feature of output compare unit a allows it to define the timer/counter top value (i.e., counter resolution). in addition to the counter resolution, the top value defines the period time for waveforms generated by the waveform generator. figure 44 shows a block diagram of the output compare unit. the small ?n? in the regis- ter and bit names indicates the device number (n = 1 for timer/counter 1), and the ?x? indicates output compare unit (a/b). the elements of the block diagram that are not directly a part of the output compare unit are gray shaded. figure 44. output compare unit, block diagram the ocr1x register is double buffered when using any of the twelve pulse width mod- ulation (pwm) modes. for the normal and clear timer on compare (ctc) modes of operation, the double buffering is disabled. the double buffering synchronizes the update of the ocr1x compare register to either top or bottom of the counting sequence. the synchronization prevents the occurrence of odd-length, non-symmetrical pwm pulses, thereby making the output glitch-free. the ocr1x register access may seem comple x, but this is not case. when the double buffering is enabled, the cpu has access to the ocr1x buffer register, and if double buffering is disabled the cpu will access the ocr1x directly. the co ntent of the ocr1x ocfnx (int.req.) = (16-bit comparator ) ocrnx buffer (16-bit register) ocrnxh buf. (8-bit) ocnx temp (8-bit) data bus (8-bit) ocrnxl buf. (8-bit) tcntn (16-bit counter) tcntnh (8-bit) tcntnl (8-bit) comnx1:0 wgmn3:0 ocrnx (16-bit register) ocrnxh (8-bit) ocrnxl (8-bit) waveform generator top bottom
114 atmega48/88/168 2545d?avr?07/04 (buffer or compare) register is only changed by a write operation (the timer/counter does not update this register automatically as the tcnt1 and icr1 register). therefore ocr1x is not read via the high byte temporary register (temp). however, it is a good practice to read the low byte first as when accessing other 16-bit registers. writing the ocr1x registers must be done via the temp register since the compare of all 16 bits is done continuously. the high by te (ocr1xh) has to be written first. when the high byte i/o location is written by the cpu, the temp register will be updated by the value written. then when the low by te (ocr1xl) is written to the lower eight bits, the high byte will be copied into the upper 8-bits of either the ocr1x buffer or ocr1x compare reg- ister in the same system clock cycle. for more information of how to access the 16-bit registers refer to ?accessing 16-bit registers? on page 106. force output compare in non-pwm waveform generation modes, the match output of the comparator can be forced by writing a one to the force output compare (foc1x) bit. forcing compare match will not set the ocf1x flag or reload /clear the timer, but the oc1x pin will be updated as if a real compare match had occurred (the com11:0 bits settings define whether the oc1x pin is set, cleared or toggled). compare match blocking by tcnt1 write all cpu writes to the tcnt1 register will bl ock any compare match that occurs in the next timer clock cycle, even when the timer is stopped. this feature allows ocr1x to be initialized to the same value as tcnt1 without triggering an interrupt when the timer/counter clock is enabled. using the output compare unit since writing tcnt1 in any mode of operat ion will block all compare matches for one timer clock cycle, there are risks involved when changing tcnt1 when using any of the output compare channels, independent of whether the timer/counter is running or not. if the value written to tcnt1 equals th e ocr1x value, the compare match will be missed, resulting in incorrect waveform generation. do not write the tcnt1 equal to top in pwm modes with variable top valu es. the compare match for the top will be ignored and the counter will cont inue to 0xffff. sim ilarly, do not writ e the tcnt1 value equal to bottom when the counter is downcounting. the setup of the oc1x should be performed before setting the data direction register for the port pin to output. the easiest way of setting the oc1x value is to use the force output compare (foc1x) strobe bits in normal mode. the oc1x register keeps its value even when changing between waveform generation modes. be aware that the com1x1:0 bits are not double buffered together with the compare value. changing the com1x1:0 bi ts will take effect immediately. compare match output unit the compare output mode (com1x1:0) bits have two functions. the waveform gener- ator uses the com1x1:0 bits for defining the output compare (oc1x) state at the next compare match. secondly the com1x1:0 bits control the oc1x pin output source. fig- ure 45 shows a simplified schematic of the logic affected by the com1x1:0 bit setting. the i/o registers, i/o bits, and i/o pins in the figure are shown in bold. only the parts of the general i/o port control registers (ddr and port) that are affected by the com1x1:0 bits are shown. when referring to the oc1x state, the reference is for the internal oc1x register, not the oc1x pin. if a system reset occur, the oc1x register is reset to ?0?.
115 atmega48/88/168 2545d?avr?07/04 figure 45. compare match output unit, schematic the general i/o port function is overridden by the output compare (oc1x) from the waveform generator if either of the com1x1:0 bits are set. however, the oc1x pin direction (input or output ) is still controlled by the data direction register (ddr) for the port pin. the data direction register bit for the oc1x pin (ddr_oc1x) must be set as output before the oc1x value is visible on th e pin. the port override function is generally independent of the waveform generation mode, but there are some exceptions. refer to table 54, table 55 and table 56 for details. the design of the output compare pin logic allows initialization of the oc1x state before the output is enabled. note that some com1x1:0 bit settings are reserved for certain modes of operation. see ?16-bit timer/counter register description? on page 125. the com1x1:0 bits have no effect on the input capture unit. compare output mode and waveform generation the waveform generator uses the com1x1:0 bits differently in normal, ctc, and pwm modes. for all modes, setting the com1x1:0 = 0 tells the waveform generator that no action on the oc1x register is to be performed on the next compare match. for com- pare output actions in the non-pwm modes refer to table 54 on page 125. for fast pwm mode refer to table 55 on page 125, and for phase correct and phase and fre- quency correct pwm refer to table 56 on page 126. a change of the com1x1:0 bits state will have effect at the first co mpare match after the bits are written. for non-pwm modes, the action can be forced to have immediate effect by using the foc1x strobe bits. modes of operation the mode of operation, i.e., the behavior of the timer/counter and the output compare pins, is defined by the combination of the waveform generation mode (wgm13:0) and compare output mode (com1x1:0) bits. the compare output mode bits do not affect the counting sequence, while the waveform generation mode bits do. the com1x1:0 bits control whether the pwm output generated should be inverted or not (inverted or non-inverted pwm). for non-pwm modes the com1x1:0 bits control whether the out- port ddr dq dq ocnx pin ocnx dq waveform generator comnx1 comnx0 0 1 data b u s focnx clk i/o
116 atmega48/88/168 2545d?avr?07/04 put should be set, cleared or toggle at a compare match (see ?compare match output unit? on page 114.) for detailed timing information refer to ?timer/counter timing diagrams? on page 123. normal mode the simplest mode of operation is the normal mode (wgm13:0 = 0). in this mode the counting direction is always up (incrementing), and no counter clear is performed. the counter simply overruns when it passes its maximum 16-bit value (max = 0xffff) and then restarts from the bottom (0x0000). in normal operation the timer/counter over- flow flag (tov1) will be set in the same time r clock cycle as the tcnt1 becomes zero. the tov1 flag in this case behaves like a 17th bit, except that it is only set, not cleared. however, combined with the timer overflow interrupt that automatically clears the tov1 flag, the timer resolution can be increased by software. there are no special cases to consider in the normal mode, a new counter value can be written anytime. the input capture unit is easy to use in normal mode. however, observe that the maxi- mum interval between the external events must not exceed the resolution of the counter. if the interval between events are too long, the timer overflow interrupt or the prescaler must be used to extend the resolution for the capture unit. the output compare units can be used to generate interrupts at some given time. using the output compare to generate waveforms in normal mode is not recommended, since this will occupy too much of the cpu time. clear timer on compare match (ctc) mode in clear timer on compare or ctc mode (wgm13:0 = 4 or 12), the ocr1a or icr1 register are used to manipulate the counte r resolution. in ctc mode the counter is cleared to zero when the counter value (tcnt1) matches either the ocr1a (wgm13:0 = 4) or the icr1 (wgm13:0 = 12). the ocr1a or icr1 define the top value for the counter, hence also its resolution. this mode allows greater control of the compare match output frequency. it also simplifies th e operation of counting external events. the timing diagram for the ctc mode is shown in figure 46. the counter value (tcnt1) increases until a compare match occurs with either ocr1a or icr1, and then counter (tcnt1) is cleared. figure 46. ctc mode, timing diagram an interrupt can be generated at each time the counter value reaches the top value by either using the ocf1a or icf1 flag according to the register used to define the top value. if the interrupt is enabled, the interrupt handler routine can be used for updating the top value. however, changing the top to a value close to bottom when the counter is running with none or a low prescaler value must be done with care since the ctc mode does not have the double buffering feature. if the new value written to tcntn ocna (toggle) ocna interrupt flag set or icfn interrupt flag set (interrupt on top) 1 4 period 2 3 (comna1:0 = 1)
117 atmega48/88/168 2545d?avr?07/04 ocr1a or icr1 is lower than the current value of tcnt1, the counter will miss the com- pare match. the counter will then have to count to its maximum value (0xffff) and wrap around starting at 0x0000 before the compare match can occur. in many cases this feature is not desirable. an alternative will then be to use the fast pwm mode using ocr1a for defining top (wgm13:0 = 15) since the ocr1a then will be double buffered. for generating a waveform output in ctc mode, the oc1a output can be set to toggle its logical level on each compare match by setting the compare output mode bits to tog- gle mode (com1a1:0 = 1). the oc1a value w ill not be visible on the port pin unless the data direction for the pin is set to output (ddr_oc1a = 1). the waveform generated will have a maximum frequency of f oc 1 a = f clk_i/o /2 when ocr1a is set to zero (0x0000). the waveform frequency is defined by the following equation: the n variable represents the prescaler factor (1, 8, 64, 256, or 1024). as for the normal mode of operation, the tov1 flag is set in the same timer clock cycle that the counter counts from max to 0x0000. fast pwm mode the fast pulse width modulation or fast pwm mode (wgm13:0 = 5, 6, 7, 14, or 15) pro- vides a high frequency pwm waveform generation option. the fast pwm differs from the other pwm options by its single-slope operation. the counter counts from bottom to top then restarts from bottom. in non-inverting compare output mode, the output compare (oc1x) is set on the compare match between tcnt1 and ocr1x, and cleared at top. in inverting compare output mode output is cleared on compare match and set at top. due to the single-slope operation, the operating frequency of the fast pwm mode can be twice as high as the phase correct and phase and frequency correct pwm modes that use dual-slope operation. this high frequency makes the fast pwm mode well suited for power regulation, re ctification, and dac applications. high fre- quency allows physically sm all sized external components (coils, capacitors), hence reduces total system cost. the pwm resolution for fast pwm can be fixed to 8-, 9-, or 10-bit, or defined by either icr1 or ocr1a. the minimum resolution allowed is 2-bit (icr1 or ocr1a set to 0x0003), and the maximum resolution is 16-bit (icr1 or ocr1a set to max). the pwm resolution in bits can be calculated by using the following equation: in fast pwm mode the counter is incremented until the counter value matches either one of the fixed values 0x00ff, 0x01ff, or 0x03ff (wgm13:0 = 5, 6, or 7), the value in icr1 (wgm13:0 = 14), or th e value in ocr1a (wgm13:0 = 15). the counter is then cleared at the following timer clock cycle. the timing diagram for the fast pwm mode is shown in figure 47. the figure shows fast pwm mode when ocr1a or icr1 is used to define top. the tcnt1 value is in the ti ming diagram shown as a histogram for illus- trating the single-slope operation. the diagram includes non-inverted and inverted pwm outputs. the small horizontal line marks on the tcnt1 slopes represent compare matches between ocr1x and tc nt1. the oc1x interrupt fl ag will be set when a com- pare match occurs. f ocna f clk_i/o 2 n 1 ocrna + () ?? -------------------------------------------------- - = r fpwm top 1 + () log 2 () log ---------------------------------- - =
118 atmega48/88/168 2545d?avr?07/04 figure 47. fast pwm mode, timing diagram the timer/counter overflow flag (tov1) is set each time the counter reaches top. in addition the oc1a or icf1 flag is set at t he same timer clock cy cle as tov1 is set when either ocr1a or icr1 is used for defining the top value. if one of the interrupts are enabled, the interrupt handler routine can be used for updating the top and com- pare values. when changing the top value the program must ensure that the new top value is higher or equal to the value of all of the compare registers. if the top value is lower than any of the compare registers, a compare match will never occur between the tcnt1 and the ocr1x. note that when using fixed top values the unused bits are masked to zero when any of the ocr1x registers are written. the procedure for updating icr1 differs from updating ocr1a when used for defining the top value. the icr1 register is not dou ble buffered. this means that if icr1 is changed to a low value when the counter is running with none or a low prescaler value, there is a risk that the new icr1 value written is lower than the current value of tcnt1. the result will then be that the counter will miss the comp are match at the top value. the counter will then have to count to t he max value (0xffff) and wrap around start- ing at 0x0000 before the compare match can occur. the ocr1a register however, is double buffered. this feature allows the ocr1a i/o location to be written anytime. when the ocr1a i/o location is written the value written will be put into the ocr1a buffer register. the ocr1a compare register will then be updated with the value in the buffer register at the next timer clo ck cycle the tcnt1 matches top. the update is done at the same timer clock cycle as the tcnt1 is cleared and the tov1 flag is set. using the icr1 register for defining top works well when using fixed top values. by using icr1, the ocr1a register is free to be used for generating a pwm output on oc1a. however, if the base pwm frequency is actively changed (by changing the top value), using the ocr1a as top is clearly a better choice due to its double buffer feature. in fast pwm mode, the compare units allow generation of pwm waveforms on the oc1x pins. setting the com1x1:0 bits to two will produce a non-inverted pwm and an inverted pwm output can be generated by setting the com1x1:0 to three (see table on page 125). the actual oc1x value will only be vi sible on the port pin if the data direction for the port pin is set as output (ddr_oc1x). the pwm waveform is generated by set- ting (or clearing) the oc1x register at the compare match between ocr1x and tcnt1, tcntn ocrnx/top update and tovn interrupt flag set and ocna interrupt flag set or icfn interrupt flag set (interrupt on top) 1 7 period 2 3 4 5 6 8 ocnx ocnx (comnx1:0 = 2) (comnx1:0 = 3)
119 atmega48/88/168 2545d?avr?07/04 and clearing (or setting) the oc1x register at the timer clock cycle the counter is cleared (changes from top to bottom). the pwm frequency for the output can be calculated by the following equation: the n variable represents the prescaler divider (1, 8, 64, 256, or 1024). the extreme values for the ocr1x register represents special cases when generating a pwm waveform output in the fast pwm mode. if the ocr1x is set equal to bottom (0x0000) the output will be a na rrow spike for each top+1 timer clock cycle. setting the ocr1x equal to top will result in a constant high or low output (depending on the polar- ity of the output set by the com1x1:0 bits.) a frequency (with 50% duty cycle) waveform output in fast pwm mode can be achieved by setting oc1a to toggle its logical leve l on each compare match (com1a1:0 = 1). this applies only if ocr1a is used to define the top value (wgm13:0 = 15). the wave- form generated will have a maximum frequency of f oc 1 a = f clk_i/o /2 when ocr1a is set to zero (0x0000). this feature is similar to the oc1a toggle in ctc mode, except the dou- ble buffer feature of the output compare unit is enabled in the fast pwm mode. phase correct pwm mode the phase correct pulse width modulation or phase correct pwm mode (wgm13:0 = 1, 2, 3, 10, or 11) provides a high resolution phase correct pwm waveform generation option. the phase correct pwm mode is, like the phase and frequency correct pwm mode, based on a dual-slope operation. the counter counts repeatedly from bottom (0x0000) to top and then from top to bottom. in non-inverting compare output mode, the output compare (oc1x) is cleared on the compare match between tcnt1 and ocr1x while upcounting, and set on the compare match while downcounting. in inverting output compare mode, the operation is inverted. the dual-slope operation has lower maximum operation frequency than single slope operation. however, due to the symmetric feature of the dual-slope pwm modes, these modes are preferred for motor control applications. the pwm resolution for the phase correct pwm mode can be fixed to 8-, 9-, or 10-bit, or defined by either icr1 or ocr1a. the minimum resolution allowed is 2-bit (icr1 or ocr1a set to 0x0003), and the maximum resolution is 16-bit (icr1 or ocr1a set to max). the pwm resolution in bits can be calculated by using the following equation: in phase correct pwm mode the counter is incremented until the counter value matches either one of the fixed values 0x00ff, 0x01ff, or 0x03ff (wgm13:0 = 1, 2, or 3), the value in icr1 (wgm13:0 = 10 ), or the value in ocr1a (wgm13:0 = 11). the counter has then reached the top and changes the count direction. the tcnt1 value will be equal to top for one timer clock cycle. the timing diagram for the phase correct pwm mode is shown on figure 48. the figure shows phase correct pwm mode when ocr1a or icr1 is used to define top. the tcnt1 value is in the timing diagram shown as a histogram for illustrating the dual-slope operation. the di agram includes non-inverted and inverted pwm outputs. the small horizontal line marks on the tcnt1 slopes repre- sent compare matches between ocr1x and tcnt1. the oc1x interrupt flag will be set when a compare match occurs. f ocnxpwm f clk_i/o n 1 top + () ? ---------------------------------- - = r pcpwm top 1 + () log 2 () log ---------------------------------- - =
120 atmega48/88/168 2545d?avr?07/04 figure 48. phase correct pwm mode, timing diagram the timer/counter overflow flag (tov1) is set each time the counter reaches bot- tom. when either ocr1a or icr1 is used for defining the top value, the oc1a or icf1 flag is set accordingly at the same timer clock cycle as the ocr1x registers are updated with the double buffer value (at top). the interrupt flags can be used to gen- erate an interrupt each time the counter reaches the top or bottom value. when changing the top value the program must ensure that the new top value is higher or equal to the value of all of the compare registers. if the top value is lower than any of the compare registers, a compare match will never occur between the tcnt1 and the ocr1x. note that when using fixed top values, the unused bits are masked to zero when any of the ocr1x registers are written. as the third period shown in figure 48 illustrates, changing the top ac tively while the timer/ counter is running in the phase correct mode can result in an unsymmetrical output. the reason for this can be found in the time of update of the ocr1x register. since the ocr1x update occurs at top, the pwm period starts and ends at top. this implies that the length of the fall- ing slope is determined by the previous top value, while the length of the rising slope is determined by the new top value. when these two values differ the two slopes of the period will differ in length. the difference in length gives the unsymme trical result on the output. it is recommended to use the phase and frequency correct mode instead of the phase correct mode when changing the top value wh ile the timer/counter is running. when using a static top value there are practically no differences between the two modes of operation. in phase correct pwm mode, the compare units allow generation of pwm waveforms on the oc1x pins. setting the com1x1:0 bits to two will produce a non-inverted pwm and an inverted pwm output can be generated by setting the com1x1:0 to three (see table on page 126). the actual oc1x value will only be visible on the port pin if the data direc- tion for the port pin is set as output (ddr_oc1x). the pwm waveform is generated by setting (or clearing) the oc1x register at the compare match between ocr1x and tcnt1 when the counter increments, and clearing (or setting) the oc1x register at compare match between ocr1x and tcnt1 when the counter decrements. the pwm ocrnx/top update and ocna interrupt flag set or icfn interrupt flag set (interrupt on top) 1 2 3 4 tovn interrupt flag set (interrupt on bottom) tcntn period ocnx ocnx (comnx1:0 = 2) (comnx1:0 = 3)
121 atmega48/88/168 2545d?avr?07/04 frequency for the output when using phase correct pwm can be calculated by the fol- lowing equation: the n variable represents the prescaler divider (1, 8, 64, 256, or 1024). the extreme values for the ocr1x register represent special cases when generating a pwm waveform output in the phase correct pwm mode. if the ocr1x is set equal to bottom the output will be continuously low and if set equal to top the output will be continuously high for non-inverted pwm mode. for inverted pw m the output will have the opposite logic values. if ocr1a is us ed to define the top value (wgm13:0 = 11) and com1a1:0 = 1, the oc1a output will toggle with a 50% duty cycle. phase and frequency correct pwm mode the phase and frequency correct pulse width modulation, or phase and frequency cor- rect pwm mode (wgm13:0 = 8 or 9) provides a high resolution phase and frequency correct pwm waveform generation option. the phase and frequency correct pwm mode is, like the phase correct pwm mode, based on a dual-slope operation. the counter counts repeatedly from bottom (0x0000) to top and then from top to bot- tom. in non-inverting compare output mode, the output compare (oc1x) is cleared on the compare match between tcnt1 and ocr1x while upcounting, and set on the compare match while downcounting. in inverting compare output mode, the operation is inverted. the dual-slope operation gives a lower maximum operation frequency com- pared to the single-slope operation. however, due to the symmetric feature of the dual- slope pwm modes, these modes are preferred for motor control applications. the main difference between the phase correct, and the phase and frequency correct pwm mode is the time the ocr1x register is updated by the ocr1x buffer register, (see figure 48 and figure 49). the pwm resolution for the phase and frequency correct pwm mode can be defined by either icr1 or ocr1a. the minimum resolution allowed is 2-bit (icr1 or ocr1a set to 0x0003), and the maximum resolution is 16-bit (icr1 or ocr1a set to max). the pwm resolution in bits can be calculated using the following equation: in phase and frequency correct pwm mode the counter is incremented until the counter value matches either the value in icr1 (wgm13:0 = 8), or the value in ocr1a (wgm13:0 = 9). the counter has then reached the top and changes the count direc- tion. the tcnt1 value will be equal to top for one ti mer clock cycle. the timing diagram for the phase correct and frequency correct pwm mode is shown on figure 49. the figure shows phase and frequency correct pwm mode when ocr1a or icr1 is used to define top. the tcnt1 value is in the timing diagram shown as a histogram for illustrating the dual-slope operation. the diagram includes non-inverted and inverted pwm outputs. the small horizontal line marks on the tcnt1 slopes represent compare matches between ocr1x and tc nt1. the oc1x interrupt fl ag will be set when a com- pare match occurs. f ocnxpcpwm f clk_i/o 2 ntop ?? --------------------------- - = r pfcpwm top 1 + () log 2 () log ---------------------------------- - =
122 atmega48/88/168 2545d?avr?07/04 figure 49. phase and frequency correct pwm mode, timing diagram the timer/counter overflow flag (tov1) is set at the same timer clock cycle as the ocr1x registers are updated with the double buffer value (at bottom). when either ocr1a or icr1 is used for defining the to p value, the oc1a or icf1 flag set when tcnt1 has reached top. the interrupt flags can then be used to generate an interrupt each time the counter reaches the top or bottom value. when changing the top value the program must ensure that the new top value is higher or equal to the value of all of the compare registers. if the top value is lower than any of the compare registers, a compare match will never occur between the tcnt1 and the ocr1x. as figure 49 shows the output generated is, in contrast to the phase correct mode, sym- metrical in all periods. since the ocr1x registers are updated at bottom, the length of the rising and the falling slopes will always be equal. this gives symmetrical output pulses and is therefore frequency correct. using the icr1 register for defining top works well when using fixed top values. by using icr1, the ocr1a register is free to be used for generating a pwm output on oc1a. however, if the base pwm frequency is actively changed by changing the top value, using the ocr1a as top is clearl y a better choice due to its double buffer feature. in phase and frequency correct pwm mode, the compare units allow generation of pwm waveforms on the oc1x pins. setting the com1x1:0 bits to two will produce a non-inverted pwm and an inverted pwm output can be generated by setting the com1x1:0 to three (see tabl e on page 126). the actual oc1x value will only be visible on the port pin if the data direction for the port pin is set as output (ddr_oc1x). the pwm waveform is generated by setting (or clearing) the oc1x register at the compare match between ocr1x and tcnt1 when the counter increments, and clearing (or set- ting) the oc1x register at compare match between ocr1x and tcnt1 when the counter decrements. the pwm frequency for the output when using phase and fre- quency correct pwm can be calculated by the following equation: ocrnx/top updateand tovn interrupt flag set (interrupt on bottom) ocna interrupt flag set or icfn interrupt flag set (interrupt on top) 1 2 3 4 tcntn period ocnx ocnx (comnx1:0 = 2) (comnx1:0 = 3) f ocnxpfcpwm f clk_i/o 2 ntop ?? --------------------------- - =
123 atmega48/88/168 2545d?avr?07/04 the n variable represents the prescaler divider (1, 8, 64, 256, or 1024). the extreme values for the ocr1x register represents special cases when generating a pwm waveform output in the phase correct pwm mode. if the ocr1x is set equal to bottom the output will be continuously low and if set equal to top the output will be set to high for non-inverted pwm mode. for inverted pwm the output will have the opposite logic values. if ocr1a is used to define the top value (wgm13:0 = 9) and com1a1:0 = 1, the oc1a output will toggle with a 50% duty cycle. timer/counter timing diagrams the timer/counter is a synchronous design and the timer clock (clk t1 ) is therefore shown as a clock enable signal in the followi ng figures. the figures include information on when interrupt flags are set, and when the ocr1x register is updated with the ocr1x buffer value (only for mo des utilizing double buffering ). figure 50 shows a timing diagram for the setting of ocf1x. figure 50. timer/counter timing diagram, setting of ocf1x, no prescaling figure 51 shows the same timing data, but with the prescaler enabled. figure 51. timer/counter timing diagram, setting of ocf1x, with prescaler (f clk_i/o /8) figure 52 shows the count sequence close to top in various modes. when using phase and frequency correct pwm mode the ocr1x register is updated at bottom. the timing diagrams will be th e same, but top should be replaced by bottom, top-1 by clk tn (clk i/o /1) ocfnx clk i/o ocrnx tcntn ocrnx value ocrnx - 1 ocrnx ocrnx + 1 ocrnx + 2 ocfnx ocrnx tcntn ocrnx value ocrnx - 1 ocrnx ocrnx + 1 ocrnx + 2 clk i/o clk tn (clk i/o /8)
124 atmega48/88/168 2545d?avr?07/04 bottom+1 and so on. the same renaming applies for modes that set the tov1 flag at bottom. figure 52. timer/counter timing diagram, no prescaling figure 53 shows the same timing data, but with the prescaler enabled. figure 53. timer/counter timing diagram, with prescaler (f clk_i/o /8) tovn (fpwm) and icfn (if used as top) ocrnx (update at top) tcntn (ctc and fpwm) tcntn (pc and pfc pwm) top - 1 top top - 1 top - 2 old ocrnx value new ocrnx value top - 1 top bottom bottom + 1 clk tn (clk i/o /1) clk i/o tovn (fpwm) and icf n (if used as top) ocrnx (update at top) tcntn (ctc and fpwm) tcntn (pc and pfc pwm) top - 1 top top - 1 top - 2 old ocrnx value new ocrnx value top - 1 top bottom bottom + 1 clk i/o clk tn (clk i/o /8)
125 atmega48/88/168 2545d?avr?07/04 16-bit timer/counter register description timer/counter1 control register a ? tccr1a bit 7:6 ? com1a1:0: compare output mode for channel a bit 5:4 ? com1b1:0: compare output mode for channel b the com1a1:0 and com1b1:0 control the output compare pins (oc1a and oc1b respectively) behavior. if one or both of t he com1a1:0 bits are written to one, the oc1a output overrides the normal port functionality of the i/o pin it is connected to. if one or both of the com1b1:0 bit are written to one, the oc1b output overrides the normal port functionality of the i/o pin it is connected to. however, note that the data direction reg- ister (ddr) bit corresponding to the oc1a or oc1b pin must be set in order to enable the output driver. when the oc1a or oc1b is connected to the pin, the function of the com1x1:0 bits is dependent of the wgm13:0 bits setting. table 54 shows the com1x1:0 bit functionality when the wgm13:0 bits are set to a normal or a ctc mode (non-pwm). table 55 shows the com1x1:0 bit functionalit y when the wgm13:0 bits are set to the fast pwm mode. note: 1. a special case occurs when ocr1a/ocr1b equals top and com1a1/com1b1 is set. in this case the compare match is ig nored, but the set or clear is done at top. see ?fast pwm mode? on page 117. for more details. bit 7 6 5 4 3210 com1a1 com1a0 com1b1 com1b0 ? ? wgm11 wgm10 tccr1a read/write r/w r/w r/w r/w r r r/w r/w initial value 0 0 0 0 0 0 0 0 table 54. compare output mode, non-pwm com1a1/com1b1 com1a0/com1b0 description 0 0 normal port operation, oc1a/oc1b disconnected. 0 1 toggle oc1a/oc1b on compare match. 1 0 clear oc1a/oc1b on compare match (set output to low level). 1 1 set oc1a/oc1b on com pare match (set output to high level). table 55. compare output mode, fast pwm (1) com1a1/com1b1 com1a0/com1b0 description 0 0 normal port operation, oc1a/oc1b disconnected. 0 1 wgm13:0 = 14 or 15: toggle oc1a on compare match, oc1b disconnected (normal port operation). for all other wgm1 settings, normal port operation, oc1a/oc1b disconnected. 1 0 clear oc1a/oc1b on compare match, set oc1a/oc1b at top 1 1 set oc1a/oc1b on compare match, clear oc1a/oc1b at top
126 atmega48/88/168 2545d?avr?07/04 table 56 shows the com1x1:0 bit functionalit y when the wgm13:0 bits are set to the phase correct or the phase and frequency correct, pwm mode. note: 1. a special case occurs when ocr1a/ocr1b equals top and com1a1/com1b1 is set. see ?phase correct pwm mode? on page 119. for more details. bit 1:0 ? wgm11:0: waveform generation mode combined with the wgm13:2 bits found in the tccr1b register, these bits control the counting sequence of the counter, the source for maximum (top) counter value, and what type of waveform generation to be used, see table 57. modes of operation sup- ported by the timer/counter unit are: normal mode (counter), clear timer on compare match (ctc) mode, and three types of pu lse width modulation (pwm) modes. (see ?modes of operation? on page 115.). table 56. compare output mode, phase correct and phase and frequency correct pwm (1) com1a1/com1b1 com1a0/com1b0 description 0 0 normal port operation, oc1a/oc1b disconnected. 0 1 wgm13:0 = 8, 9, 10 or 11: toggle oc1a on compare match, oc1b disconnected (normal port operation). for all other wgm1 settings, normal port operation, oc1a/oc1b disconnected. 1 0 clear oc1a/oc1b on compare match when up- counting. set oc1a/oc1b on compare match when downcounting. 1 1 set oc1a/oc1b on compare match when up- counting. clear oc1a/oc1b on compare match when downcounting.
127 atmega48/88/168 2545d?avr?07/04 note: 1. the ctc1 and pwm11:0 bit defin ition names are obsolete. use the wgm 12:0 definitions. however, the functionality and location of these bits are compatible with previous versions of the timer. timer/counter1 control register b ? tccr1b bit 7 ? icnc1: input capture noise canceler setting this bit (to one) activates the input capture noise canceler. when the noise can- celer is activated, the input from the input capture pin (icp1) is filtered. the filter function requires four successive equal valued samples of the icp1 pin for changing its output. the input capture is therefore delaye d by four oscillator cycles when the noise canceler is enabled. bit 6 ? ices1: input capture edge select this bit selects which edge on the input capture pin (icp1) that is used to trigger a cap- ture event. when the ices1 bit is written to zero, a falling (negative) edge is used as trigger, and when the ices1 bit is written to one, a rising (positive) edge will trigger the capture. when a capture is triggered according to t he ices1 setting, the counter value is copied into the input capture register (icr1). t he event will also set the input capture flag (icf1), and this can be used to cause an input capture interrupt, if this interrupt is enabled. table 57. waveform generation mode bit description (1) mode wgm13 wgm12 (ctc1) wgm11 (pwm11) wgm10 (pwm10) timer/counter mode of operation top update of ocr1 x at tov1 flag set on 0 0 0 0 0 normal 0xffff immediate max 1 0 0 0 1 pwm, phase correct, 8-bit 0x00ff top bottom 2 0 0 1 0 pwm, phase correct, 9-bit 0x01ff top bottom 3 0 0 1 1 pwm, phase correct, 10-bit 0x03ff top bottom 4 0 1 0 0 ctc ocr1a immediate max 5 0 1 0 1 fast pwm, 8-bit 0x00ff top top 6 0 1 1 0 fast pwm, 9-bit 0x01ff top top 7 0 1 1 1 fast pwm, 10-bit 0x03ff top top 8 1 0 0 0 pwm, phase and frequency correct icr1 bottom bottom 9 1 0 0 1 pwm, phase and frequency correct ocr1a bottom bottom 10 1 0 1 0 pwm, phase correct icr1 top bottom 11 1 0 1 1 pwm, phase correct ocr1a top bottom 12 1 1 0 0 ctc icr1 immediate max 13 1 1 0 1 (reserved) ? ? ? 14 1 1 1 0 fast pwm icr1 top top 15 1 1 1 1 fast pwm ocr1a top top bit 7654 3210 icnc1 ices1 ? wgm13 wgm12 cs12 cs11 cs10 tccr1b read/write r/w r/w r r/w r/w r/w r/w r/w initial value 0 0 0 0 0 0 0 0
128 atmega48/88/168 2545d?avr?07/04 when the icr1 is used as top value (see description of the wgm13:0 bits located in the tccr1a and the tccr1b register), t he icp1 is disconnected and consequently the input capture function is disabled. bit 5 ? reserved bit this bit is reserved for future use. for ensuring compatibility with future devices, this bit must be written to zero when tccr1b is written. bit 4:3 ? wgm13:2: waveform generation mode see tccr1a register description. bit 2:0 ? cs12:0: clock select the three clock select bits select the clock source to be used by the timer/counter, see figure 50 and figure 51. if external pin modes are used for the timer/counter1, transitions on the t1 pin will clock the counter even if the pin is configured as an output. this feature allows software control of the counting. timer/counter1 control register c ? tccr1c bit 7 ? foc1a: force output compare for channel a bit 6 ? foc1b: force output compare for channel b the foc1a/foc1b bits are only active w hen the wgm13:0 bits specifies a non-pwm mode. however, for ensu ring compatibility with future devices, these bits must be set to zero when tccr1a is written when operating in a pwm mo de. when writing a logical one to the foc1a/foc1b bit, an immediate compare match is forced on the waveform generation unit. the oc1a/oc1b output is changed according to its com1x1:0 bits setting. note that the foc1a/foc1b bits are implemented as strobes. therefore it is the value present in the com1x1:0 bits that determine the effect of the forced compare. a foc1a/foc1b strobe will not ge nerate any interrup t nor will it clear the timer in clear timer on compare match (ctc) mode using ocr1a as top. the foc1a/foc1b bits are always read as zero. table 58. clock select bit description cs12 cs11 cs10 description 0 0 0 no clock source (timer/counter stopped). 001clk i/o /1 (no prescaling) 010clk i/o /8 (from prescaler) 011clk i/o /64 (from prescaler) 100clk i/o /256 (from prescaler) 101clk i/o /1024 (from prescaler) 1 1 0 external clock source on t1 pin. clock on falling edge. 1 1 1 external clock source on t1 pin. clock on rising edge. bit 7654 3210 foc1a foc1b ? ? ? ? ? ? tccr1c read/write r/w r/w r r r r r r initial value 0 0 0 0 0 0 0 0
129 atmega48/88/168 2545d?avr?07/04 timer/counter1 ? tcnt1h and tcnt1l the two timer/counter i/o locations (tcnt1h and t cnt1l, combined tcnt1) give direct access, both for read and for write operations, to the timer/counter unit 16-bit counter. to ensure that both the high and low bytes are read and written simultaneously when the cpu accesses these registers, the access is performed using an 8-bit tempo- rary high byte register (temp). this temporary register is shared by all the other 16-bit registers. see ?accessing 16-bit registers? on page 106. modifying the counter (tcnt1) while the counter is running introduces a risk of missing a compare match between tcnt1 and one of the ocr1x registers. writing to the tcnt1 register blocks (removes) the compare match on the following timer clock for all compare units. output compare register 1 a ? ocr1ah and ocr1al output compare register 1 b ? ocr1bh and ocr1bl the output compare registers contain a 16- bit value that is continuously compared with the counter value (tcnt1). a match can be used to generate an output compare interrupt, or to generate a waveform output on the oc1x pin. the output compare registers are 16-bit in size. to ensure that both the high and low bytes are written simultaneously when the cpu writes to these registers, the access is performed using an 8-bit temporary high byte register (temp). this temporary register is shared by all the other 16-bit registers. see ?accessing 16-bit registers? on page 106. input capture register 1 ? icr1h and icr1l the input capture is updated with the counter (tcnt1) value each time an event occurs on the icp1 pin (or optionally on the analog comparator output for timer/counter1). the input capture can be used for defining the counter top value. the input capture register is 16-bit in size. to ensure that both the high and low bytes are read simultaneously when the cpu accesses these registers, the access is per- formed using an 8-bit temporary high byte re gister (temp). this temporary register is shared by all the other 16-bit registers. see ?accessing 16-bit registers? on page 106. bit 76543210 tcnt1[15:8] tcnt1h tcnt1[7:0] tcnt1l read/write r/w r/w r/w r/w r/w r/w r/w r/w initial value00000000 bit 76543210 ocr1a[15:8] ocr1ah ocr1a[7:0] ocr1al read/write r/w r/w r/w r/w r/w r/w r/w r/w initial value00000000 bit 76543210 ocr1b[15:8] ocr1bh ocr1b[7:0] ocr1bl read/write r/w r/w r/w r/w r/w r/w r/w r/w initial value00000000 bit 76543210 icr1[15:8] icr1h icr1[7:0] icr1l read/write r/w r/w r/w r/w r/w r/w r/w r/w initial value00000000
130 atmega48/88/168 2545d?avr?07/04 timer/counter1 interrupt mask register ? timsk1 bit 7, 6 ? res: reserved bits these bits are unused bits in the atm ega48/88/168, and will alwa ys read as zero. bit 5 ? icie1: timer/counter1, input capture interrupt enable when this bit is written to one, and the i-flag in the status register is set (interrupts glo- bally enabled), the timer/counter1 input capture interrupt is enabled. the corresponding interrupt vector (see ?interrupts? on page 51) is executed when the icf1 flag, located in tifr1, is set. bit 4, 3 ? res: reserved bits these bits are unused bits in the atm ega48/88/168, and will alwa ys read as zero. bit 2 ? ocie1b: timer/counter1, output compare b match interrupt enable when this bit is written to one, and the i-flag in the status register is set (interrupts glo- bally enabled), the timer/counter1 output compare b match interrupt is enabled. the corresponding interrupt vector (see ?interrupts? on page 51) is executed when the ocf1b flag, located in tifr1, is set. bit 1 ? ocie1a: timer/counter1, output compare a match interrupt enable when this bit is written to one, and the i-flag in the status register is set (interrupts glo- bally enabled), the timer/counter1 output compare a match interrupt is enabled. the corresponding interrupt vector (see ?interrupts? on page 51) is executed when the ocf1a flag, located in tifr1, is set. bit 0 ? toie1: timer/counter1, overflow interrupt enable when this bit is written to one, and the i-flag in the status register is set (interrupts glo- bally enabled), the timer/counter1 overflow interrupt is enabled. the corresponding interrupt vector (see ?watchdog timer? on page 46.) is executed when the tov1 flag, located in tifr1, is set. timer/counter1 interrupt flag register ? tifr1 bit 7, 6 ? res: reserved bits these bits are unused bits in the atm ega48/88/168, and will alwa ys read as zero. bit 5 ? icf1: timer/count er1, input capture flag this flag is set when a capture event occurs on the icp1 pin. when the input capture register (icr1) is set by the wgm13:0 to be used as the top value, the icf1 flag is set when the counter reaches the top value. icf1 is automatically cleared when the input capture interrupt vector is executed. alter- natively, icf1 can be cleared by writing a logic one to its bit location. bit 4, 3 ? res: reserved bits these bits are unused bits in the atm ega48/88/168, and will alwa ys read as zero. bit 2 ? ocf1b: timer/counter1, output compare b match flag bit 76543210 ? ? icie1 ? ? ocie1b ocie1a toie1 timsk1 read/write r r r/w r r r/w r/w r/w initial value00000000 bit 76543210 ? ? icf1 ? ? ocf1b ocf1a tov1 tifr1 read/write r r r/w r r r/w r/w r/w initial value00000000
131 atmega48/88/168 2545d?avr?07/04 this flag is set in the timer clock cycle a fter the counter (tcnt1) value matches the out- put compare register b (ocr1b). note that a forced output compare (foc 1b) strobe will not set the ocf1b flag. ocf1b is automatically cleared when the output compare match b interrupt vector is executed. alternatively, ocf1b can be cleared by writing a logic one to its bit location. bit 1 ? ocf1a: timer/counter1, output compare a match flag this flag is set in the timer clock cycle a fter the counter (tcnt1) value matches the out- put compare register a (ocr1a). note that a forced output compare (foc 1a) strobe will not set the ocf1a flag. ocf1a is automatically cleared when the output compare match a interrupt vector is executed. alternatively, ocf1a can be cleared by writing a logic one to its bit location. bit 0 ? tov1: timer/counter1, overflow flag the setting of this flag is dependent of the wgm13:0 bits setting. in normal and ctc modes, the tov1 flag is set when the timer overflows. refer to table 57 on page 127 for the tov1 flag behavior when using another wgm13:0 bit setting. tov1 is automatically cleared when the timer/counter1 overflow interrupt vector is executed. alternatively, tov1 can be cleared by writing a logic one to its bit location.
132 atmega48/88/168 2545d?avr?07/04 8-bit timer/counter2 with pwm and asynchronous operation timer/counter2 is a general purpose, single channel, 8-bit timer/counter module. the main features are: single channel counter clear timer on compar e match (auto reload) glitch-free, phase correct pu lse width modulator (pwm) frequency generator 10-bit clock prescaler overflow and compare match interr upt sources (tov2, ocf2a and ocf2b) allows clocking from external 32 khz wa tch crystal independent of the i/o clock overview a simplified block diagram of the 8-bit time r/counter is shown in figure 54. for the actual placement of i/o pins, refer to ?pinout atmega48/88/168? on page 2. cpu acces- sible i/o registers, including i/o bits and i/o pins, are shown in bold. the device- specific i/o register and bit locations are listed in the ?8-bit timer/counter register description? on page 143. the prtim2 bit in ?power reduction register - prr? on page 37 must be written to zero to enable timer/counter2 module. figure 54. 8-bit timer/counter block diagram registers the timer/counter (tcnt2) and output compare register (ocr2a and ocr2b) are 8-bit registers. interrupt request (shorten as int.req.) signals are all visible in the timer interrupt flag register (tifr2). all interrupts are individually masked with the timer interrupt mask register (timsk2). tifr2 and timsk2 are not shown in the figure. the timer/counter can be clocked internally, via the prescaler, or asynchronously clocked from the tosc1/2 pins, as detailed later in this section. the asynchronous timer/counter data b u s ocrna ocrnb = = tcntn waveform generation waveform generation ocna ocnb = fixed top value control logic = 0 top bottom count clear direction tovn (int.req.) ocna (int.req.) ocnb (int.req.) tccrna tccrnb clk tn prescaler t/c oscillator clk i/o tosc1 tosc2
133 atmega48/88/168 2545d?avr?07/04 operation is controlled by the asynchronous status register (assr). the clock select logic block controls which clock source he timer/counter uses to increment (or decre- ment) its value. the timer/counter is inactive when no clock source is selected. the output from the clock select logic is referred to as the timer clock (clk t2 ). the double buffered output compare register (ocr2a and ocr2b) are compared with the timer/counter value at all times. the result of the compare can be used by the waveform generator to generate a pwm or variable frequency output on the output compare pins (oc2a and oc2b). see ?output compare unit? on page 134. for details. the compare match event will also set the co mpare flag (ocf2a or ocf2b) which can be used to generate an output compare interrupt request. definitions many register and bit references in this document are written in general form. a lower case ?n? replaces the timer/counter number, in this case 2. however, when using the register or bit defines in a program, the precise form must be used, i.e., tcnt2 for accessing timer/counter2 co unter value and so on. the definitions in table 59 are also used extensively throughout the section. timer/counter clock sources the timer/counter can be clocked by an internal synchronous or an external asynchro- nous clock source. t he clock source clk t2 is by default equal to the mcu clock, clk i/o . when the as2 bit in the assr re gister is written to logic one , the clock source is taken from the timer/counter oscillator connec ted to tosc1 and tosc2. for details on asynchronous operation, see ?asynchronous status regist er ? assr? on page 150. for details on clock sources and prescaler, see ?timer/counter prescaler? on page 152. counter unit the main part of the 8-bit timer/counter is the programmable bi-directional counter unit. figure 55 shows a block diagram of the counter and its surrounding environment. figure 55. counter unit block diagram signal description (internal signals): count increment or decrement tcnt2 by 1. direction selects between increment and decrement. table 59. definitions bottom the counter reaches the bottom when it becomes zero (0x00). max the counter reaches its maximum when it becomes 0xff (decimal 255). top the counter reaches the top when it becomes equal to the highest value in the count sequence. the top value can be assigned to be the fixed value 0xff (max) or the value stored in the ocr2a register. the assignment is dependent on the mode of operation. data b u s tcntn control logic count tovn (int.req.) top bottom direction clear tosc1 t/c oscillator tosc2 prescaler clk i/o clk tn
134 atmega48/88/168 2545d?avr?07/04 clear clear tcnt2 (set all bits to zero). clk tn timer/counter clock, referred to as clk t2 in the following. top signalizes that tcnt2 has reached maximum value. bottom signalizes that tcnt2 has reached minimum value (zero). depending on the mode of operation used, the counter is cleared, incremented, or dec- remented at each timer clock (clk t2 ). clk t2 can be generated from an external or internal clock source, selected by the clock select bits (cs22:0). when no clock source is selected (cs22:0 = 0) the timer is stopped. however, the tcnt2 value can be accessed by the cpu, regardless of whether clk t2 is present or not. a cpu write overrides (has priority over) all counter clear or count operations. the counting sequence is determined by the setting of the wgm21 and wgm20 bits located in the timer/ counter control regist er (tccr2a) and the wgm22 located in the timer/counter control register b (tcc r2b). there are close connections between how the counter behaves (counts) and how waveforms are generated on the output compare outputs oc2a and oc2b. for more details about advanced counting sequences and waveform generation, see ?modes of operation? on page 137. the timer/counter overflow flag (tov2) is set according to the mode of operation selected by the wgm22:0 bits. tov2 can be used for generating a cpu interrupt. output compare unit the 8-bit comparator continuously compares tcnt2 with the output compare register (ocr2a and ocr2b). whenever tcnt2 equals ocr2a or ocr2b, the comparator signals a match. a match will set the output compare flag (ocf2a or ocf2b) at the next timer clock cycle. if the correspondi ng interrupt is enabled, the output compare flag generates an output compare interrupt. the output compare flag is automatically cleared when the interrupt is executed. alternatively, the output compare flag can be cleared by software by writing a logical one to its i/o bit location. the waveform gener- ator uses the match signal to generate an output according to operating mode set by the wgm22:0 bits and compare output mode (com2x1:0) bits. the max and bottom sig- nals are used by the waveform generator for handling the special cases of the extreme values in some modes of operation (?modes of operation? on page 137). figure 56 shows a block diagram of the output compare unit.
135 atmega48/88/168 2545d?avr?07/04 figure 56. output compare unit, block diagram the ocr2x register is double buffered when using any of the pulse width modulation (pwm) modes. for the normal and clear timer on compare (ctc) modes of operation, the double buffering is disabled. the double buffering synchronizes the update of the ocr2x compare register to either top or bottom of the counting sequence. the syn- chronization prevents the occurrence of odd-length, non-symmetrical pwm pulses, thereby making the output glitch-free. the ocr2x register access may seem comple x, but this is not case. when the double buffering is enabled, the cpu has access to the ocr2x buffer register, and if double buffering is disabled the cp u will access the ocr2x directly. force output compare in non-pwm waveform generation modes, the match output of the comparator can be forced by writing a one to the force output compare (foc2x) bit. forcing compare match will not set the ocf2x flag or reload /clear the timer, but the oc2x pin will be updated as if a real compare match had occurred (the com2x1:0 bits settings define whether the oc2x pin is set, cleared or toggled). compare match blocking by tcnt2 write all cpu write operations to the tcnt2 register will block any compare match that occurs in the next timer clock cycle, even when the timer is stopped. this feature allows ocr2x to be initialized to the same value as tcnt2 without triggering an interrupt when the timer/counter clock is enabled. using the output compare unit since writing tcnt2 in any mode of operat ion will block all compare matches for one timer clock cycle, there are risks involved when changing tcnt2 when using the output compare channel, independently of whether the timer/counter is running or not. if the value written to tcnt2 equals the ocr2x va lue, the compare match will be missed, resulting in incorrect waveform generation . similarly, do not write the tcnt2 value equal to bottom when the counter is downcounting. the setup of the oc2x should be performed before setting the data direction register for the port pin to output. the easiest way of setting the oc2x value is to use the force output compare (foc2x) strobe bit in normal mode. the oc2x register keeps its value even when changing between waveform generation modes. ocfn x (int.req.) = (8-bit comparator ) ocrnx ocnx data b u s tcntn wgmn1:0 waveform generator top focn comnx1:0 bottom
136 atmega48/88/168 2545d?avr?07/04 be aware that the com2x1:0 bits are not double buffered together with the compare value. changing the com2x1:0 bi ts will take effect immediately. compare match output unit the compare output mode (com2x1:0) bits have two functions. the waveform gener- ator uses the com2x1:0 bits for defining the output compare (oc2x) state at the next compare match. also, the com2x1:0 bits control the oc2x pin output source. figure 57 shows a simplified schematic of the logic affected by the com2x1:0 bit setting. the i/o registers, i/o bits, and i/o pins in the figur e are shown in bold. only the parts of the general i/o port cont rol registers (ddr and port) that are affected by the com2x1:0 bits are shown. when referring to the oc2x state, the reference is for the internal oc2x register, not the oc2x pin. figure 57. compare match output unit, schematic the general i/o port function is overridden by the output compare (oc2x) from the waveform generator if either of the com2x1:0 bits are set. however, the oc2x pin direction (input or output) is still controlled by the data dir ection register (ddr) for the port pin. the data direction register bit for the oc2x pin (ddr_oc2x) must be set as output before the oc2x value is visible on the pin. the port override function is indepen- dent of the waveform generation mode. the design of the output compare pin logic allows initialization of the oc2x state before the output is enabled. note that some com2x1:0 bit settings are reserved for certain modes of operation. see ?8-bit timer/counter register description? on page 143. compare output mode and waveform generation the waveform generator uses the com2x1:0 bits differently in normal, ctc, and pwm modes. for all modes, setting the com2x1:0 = 0 tells the waveform generator that no action on the oc2x register is to be performed on the next compare match. for com- pare output actions in the non-pwm modes refer to table 63 on page 144. for fast pwm mode, refer to table 64 on page 144, and for phase correct pwm refer to table 65 on page 145. port ddr dq dq ocnx pin ocnx dq waveform generator comnx1 comnx0 0 1 data b u s focnx clk i/o
137 atmega48/88/168 2545d?avr?07/04 a change of the com2x1:0 bits state will have effect at the first co mpare match after the bits are written. for non-pwm modes, the action can be forced to have immediate effect by using the foc2x strobe bits. modes of operation the mode of operation, i.e., the behavior of the timer/counter and the output compare pins, is defined by the combination of the waveform generation mode (wgm22:0) and compare output mode (com2x1:0) bits. the compare output mode bits do not affect the counting sequence, while the waveform generation mode bits do. the com2x1:0 bits control whether the pwm output generated should be inverted or not (inverted or non-inverted pwm). for non-pwm modes the com2x1:0 bits control whether the out- put should be set, cleared, or toggled at a compare match (see ?compare match output unit? on page 136.). for detailed timing information refer to ?timer/counter timing diagrams? on page 141. normal mode the simplest mode of operation is the normal mode (wgm22:0 = 0). in this mode the counting direction is always up (incrementing), and no counter clear is performed. the counter simply overruns when it passes its maximum 8-bit value (top = 0xff) and then restarts from the bottom (0x00). in normal operation the timer/counter overflow flag (tov2) will be set in the same timer cloc k cycle as the tcnt2 becomes zero. the tov2 flag in this case behaves like a ninth bit, except that it is only set, not cleared. however, combined with the timer overflow interrupt that automatically clears the tov2 flag, the timer resolution can be increased by software. there are no special cases to consider in the normal mode, a new counter value can be written anytime. the output compare unit can be used to generate interrupts at some given time. using the output compare to generate waveforms in normal mode is not recommended, since this will occupy too much of the cpu time. clear timer on compare match (ctc) mode in clear timer on compare or ctc mode (wgm22:0 = 2), the ocr2a register is used to manipulate the counter resolution. in ctc mode the counter is cleared to zero when the counter value (tcnt2) matches the ocr2 a. the ocr2a defines the top value for the counter, hence also its resolution. this mode allows greater control of the compare match output frequency. it also simplifies th e operation of counting external events. the timing diagram for the ctc mode is shown in figure 58. the counter value (tcnt2) increases until a compare match occurs between tcnt2 and ocr2a, and then counter (tcnt2) is cleared. figure 58. ctc mode, timing diagram an interrupt can be generated each time the counter value reaches the top value by using the ocf2a flag. if the interrupt is enabled, the interrupt handler routine can be tcntn ocnx (toggle) ocnx interrupt flag set 1 4 period 2 3 (comnx1:0 = 1)
138 atmega48/88/168 2545d?avr?07/04 used for updating the top value. however, changing top to a value close to bottom when the counter is running with none or a low prescaler value must be done with care since the ctc mode does not have the double buffering feature. if the new value written to ocr2a is lower than the current value of tcnt2, the counter will miss the compare match. the counter will then have to count to its maxi mum value (0xff) and wrap around starting at 0x00 before the compare match can occur. for generating a waveform output in ctc mode, the oc2a output can be set to toggle its logical level on each compare match by setting the compare output mode bits to tog- gle mode (com2a1:0 = 1). the oc2a value w ill not be visible on the port pin unless the data direction for the pin is set to outp ut. the waveform gene rated will have a maximum frequency of f oc2a = f clk_i/o /2 when ocr2a is set to zero (0x00). the waveform fre- quency is defined by the following equation: the n variable represents the prescale factor (1, 8, 32, 64, 128, 256, or 1024). as for the normal mode of operation, the tov2 flag is set in the same timer clock cycle that the counter counts from max to 0x00. fast pwm mode the fast pulse width modulation or fast pwm mode (wgm22:0 = 3 or 7) provides a high frequency pwm waveform generation option. the fast pwm differs from the other pwm option by its single-slope operation. the counter counts from bottom to top then restarts from bottom. top is defined as 0xff when wgm2:0 = 3, and ocr2a when mgm2:0 = 7. in non-inverting compare output mode, the output compare (oc2x) is cleared on the compare match between tcnt2 and ocr2x, and set at bottom. in inverting compare output mode, the output is set on compare match and cleared at bottom. due to the single-slope operation, the operating frequency of the fast pwm mode can be twice as high as the phase corr ect pwm mode that uses dual-slope oper- ation. this high frequency makes the fast pwm mode well suited for power regulation, rectification, and dac applications. high fr equency allows physicall y small sized exter- nal components (coils, capacitors), and therefore reduces total system cost. in fast pwm mode, the counter is incremented until the counter value matches the top value. the counter is then cleared at the following timer clock cycle. the timing diagram for the fast pwm mode is shown in figure 59. the tcnt2 value is in the timing diagram shown as a histogram for illustrating the single-slope operation. the diagram includes non-inverted and inverted pwm outputs. the small horizontal line marks on the tcnt2 slopes represent compare matches between ocr2x and tcnt2. f ocnx f clk_i/o 2 n 1 ocrnx + () ?? ------------------------------------------------- - =
139 atmega48/88/168 2545d?avr?07/04 figure 59. fast pwm mode, timing diagram the timer/counter overflow flag (tov2) is set each time the counter reaches top. if the interrupt is enabled, the interrupt handler routine can be used for updating the com- pare value. in fast pwm mode, the compare unit allows generation of pwm waveforms on the oc2x pin. setting the com2x1:0 bits to two will produce a non-inverted pwm and an inverted pwm output can be generated by setting the com2x1:0 to three. top is defined as 0xff when wgm2:0 = 3, and ocr2a when mgm2:0 = 7. (see table 61 on page 143). the actual oc2x value will only be vi sible on the port pin if the data direction for the port pin is set as output. the pwm waveform is generated by setting (or clearing) the oc2x register at the compare match between ocr2x and tcnt2, and clearing (or setting) the oc2x register at the timer clo ck cycle the counter is cleared (changes from top to bottom). the pwm frequency for the output can be calculated by the following equation: the n variable represents the prescale factor (1, 8, 32, 64, 128, 256, or 1024). the extreme values for the ocr2a register represent special cases when generating a pwm waveform output in the fast pwm mode. if the ocr2a is set equal to bottom, the output will be a narro w spike for each max+1 timer cl ock cycle. setting the ocr2a equal to max will result in a constantly high or low output (depending on t he polarity of the output set by the com2a1:0 bits.) a frequency (with 50% duty cycle) waveform output in fast pwm mode can be achieved by setting oc2x to toggle its logical level on each compare match (com2x1:0 = 1). the waveform generated will have a maximum frequency of f oc2 = f clk_i/o /2 when ocr2a is set to zero. this feature is similar to t he oc2a toggle in ctc mode, except the double buffer feature of the output compare unit is enabled in the fast pwm mode. tcntn ocrnx update and tovn interrupt flag set 1 period 2 3 ocnx ocnx (comnx1:0 = 2) (comnx1:0 = 3) ocrnx interrupt flag set 4 5 6 7 f ocnxpwm f clk_i/o n 256 ? ------------------ =
140 atmega48/88/168 2545d?avr?07/04 phase correct pwm mode the phase correct pwm mode (wgm22:0 = 1 or 5) provides a high resolution phase correct pwm waveform generation option. the phase correct pwm mode is based on a dual-slope operation. the counter counts repeatedly from bottom to top and then from top to bottom. top is defined as 0xff when wgm2:0 = 3, and ocr2a when mgm2:0 = 7. in non-inverting compare output mode, the output compare (oc2x) is cleared on the compare match between tcnt2 and ocr2x while upcounting, and set on the compare match while downcounting. in inverting output compare mode, the operation is inverted. the dual-slope operation has lower maximum operation frequency than single slope operation. however, due to the symmetric feature of the dual-slope pwm modes, these modes are preferred for motor control applications. in phase correct pwm mode the counter is incremented until the counter value matches top. when the counter reaches top, it changes the count direction. the tcnt2 value will be equal to top for one timer clock cycle. the timing diagram for the phase correct pwm mode is shown on figure 60. the tcnt2 value is in the timing diagram shown as a histogram for illustrating the dual-slope operation. the diagra m includes non-inverted and inverted pwm outputs. the small horizontal line marks on the tcnt2 slopes repre- sent compare matches between ocr2x and tcnt2. figure 60. phase correct pwm mode, timing diagram the timer/counter overflow flag (tov2) is set each time the counter reaches bot- tom. the interrupt flag can be used to generate an interrupt each time the counter reaches the bottom value. in phase correct pwm mode, the compare unit allows generation of pwm waveforms on the oc2x pin. setting the co m2x1:0 bits to two will produ ce a non-inverted pwm. an inverted pwm output can be generated by setting the com2x1:0 to three. top is defined as 0xff when wgm2:0 = 3, and oc r2a when mgm2:0 = 7 (see table 62 on page 144). the actual oc2x value will only be vi sible on the port pin if the data direction for the port pin is set as output. the pwm waveform is generated by clearing (or setting) the oc2x register at the compare match between ocr2x and tcnt2 when the counter increments, and setting (or clearing) the oc2x register at compare match tovn interrupt flag set ocnx interrupt flag set 1 2 3 tcntn period ocnx ocnx (comnx1:0 = 2) (comnx1:0 = 3) ocrnx update
141 atmega48/88/168 2545d?avr?07/04 between ocr2x and tcnt2 when the counter decrements. the pwm frequency for the output when using phase correct pwm can be calculated by the following equation: the n variable represents the prescale factor (1, 8, 32, 64, 128, 256, or 1024). the extreme values for the ocr2a register represent special cases when generating a pwm waveform output in the phase correct pwm mode. if the ocr2a is set equal to bottom, the output will be cont inuously low and if set e qual to max the output will be continuously high for non-inverted pwm mode. for inverted pw m the output will have the opposite logic values. at the very start of period 2 in figure 60 ocnx has a transition from high to low even though there is no compare match. the point of this transition is to guarantee symmetry around bottom. there are two cases that give a transition without compare match. ocr2a changes its value from max, like in figure 60. when the ocr2a value is max the ocn pin value is the same as the result of a down-counting compare match. to ensure symmetry around bottom the ocn value at max must correspond to the result of an up-counting compare match. the timer starts counting from a value higher than the one in ocr2a, and for that reason misses the compare match and hence the ocn change that would have happened on the way up. timer/counter timing diagrams the following figures show the timer/counter in synchronous mode, and the timer clock (clk t2 ) is therefore shown as a clock enable signal. in asynchronous mode, clk i/o should be replaced by the timer/counter oscillato r clock. the figures include information on when interrupt flags are set. figure 61 c ontains timing data for basic timer/counter operation. the figure shows the count sequence close to the max value in all modes other than phase correct pwm mode. figure 61. timer/counter timing diagram, no prescaling figure 62 shows the same timing data, but with the prescaler enabled. f ocnxpcpwm f clk_i/o n 510 ? ------------------ = clk tn (clk i/o /1) tovn clk i/o tcntn max - 1 max bottom bottom + 1
142 atmega48/88/168 2545d?avr?07/04 figure 62. timer/counter timing diagram, with prescaler (f clk_i/o /8) figure 63 shows the setting of ocf2a in all modes except ctc mode. figure 63. timer/counter timing diagram, setting of ocf2a, with prescaler (f clk_i/o /8) figure 64 shows the setting of ocf2a and the clearing of tcnt2 in ctc mode. figure 64. timer/counter timing diagram, clear timer on compare match mode, with prescaler (f clk_i/o /8) tovn tcntn max - 1 max bottom bottom + 1 clk i/o clk tn (clk i/o /8) ocfnx ocrnx tcntn ocrnx value ocrnx - 1 ocrnx ocrnx + 1 ocrnx + 2 clk i/o clk tn (clk i/o /8) ocfnx ocrnx tcntn (ctc) top top - 1 top bottom bottom + 1 clk i/o clk tn (clk i/o /8)
143 atmega48/88/168 2545d?avr?07/04 8-bit timer/counter register description timer/counter control register a ? tccr2a bits 7:6 ? com2a1:0: compare match output a mode these bits control the output compare pi n (oc2a) behavior. if one or both of the com2a1:0 bits are set, the oc2a output overrides the normal port functionality of the i/o pin it is connected to. ho wever, note that t he data direction register (ddr) bit cor- responding to the oc2a pin must be set in order to enable the output driver. when oc2a is connected to the pin, the func tion of the com2a1:0 bits depends on the wgm22:0 bit setting. table 60 shows the com2a1:0 bit functionality when the wgm22:0 bits are set to a normal or ctc mode (non-pwm). table 61 shows the com2a1:0 bit functionality when the wgm21:0 bits are set to fast pwm mode. note: 1. a special case occurs when ocr2a equ als top and com2a1 is set. in this case, the compare match is ignored, but the set or clear is done at top. see ?fast pwm mode? on page 138 for more details. table 62 shows the com2a1:0 bit functionality when the wgm22:0 bits are set to phase correct pwm mode. bit 7 6 5 4 3 2 1 0 com2a1 com2a0 com2b1 com2b0 ? ? wgm21 wgm20 tccr2a read/write r/w r/w r/w r/w r r r/w r/w initial value 0 0 0 0 0 0 0 0 table 60. compare output mode, non-pwm mode com2a1 com2a0 description 0 0 normal port operation, oc0a disconnected. 0 1 toggle oc2a on compare match 1 0 clear oc2a on compare match 1 1 set oc2a on compare match table 61. compare output mode, fast pwm mode (1) com2a1 com2a0 description 0 0 normal port operation, oc2a disconnected. 0 1 wgm22 = 0: normal port operation, oc0a disconnected. wgm22 = 1: toggle oc2a on compare match. 1 0 clear oc2a on compare match, set oc2a at top 1 1 set oc2a on compare match, clear oc2a at top
144 atmega48/88/168 2545d?avr?07/04 note: 1. a special case occurs when ocr2a equ als top and com2a1 is set. in this case, the compare match is ignored, but the set or clear is done at top. see ?phase cor- rect pwm mode? on page 140 for more details. bits 5:4 ? com2b1:0: compare match output b mode these bits control the output compare pi n (oc2b) behavior. if one or both of the com2b1:0 bits are set, the oc2b output overrides the normal port functionality of the i/o pin it is connected to. ho wever, note that t he data direction register (ddr) bit cor- responding to the oc2b pin must be set in order to enable the output driver. when oc2b is connected to the pin, the func tion of the com2b1:0 bits depends on the wgm22:0 bit setting. table 63 shows the com2b1:0 bit functionality when the wgm22:0 bits are set to a normal or ctc mode (non-pwm). table 64 shows the com2b1:0 bit functionality when the wgm22:0 bits are set to fast pwm mode. note: 1. a special case occurs when ocr2b equ als top and com2b1 is set. in this case, the compare match is ignored, but the set or clear is done at top. see ?phase cor- rect pwm mode? on page 140 for more details. table 62. compare output mode, phase correct pwm mode (1) com2a1 com2a0 description 0 0 normal port operation, oc2a disconnected. 0 1 wgm22 = 0: normal port operation, oc2a disconnected. wgm22 = 1: toggle oc2a on compare match. 1 0 clear oc2a on compare match when up-counting. set oc2a on compare match when down-counting. 1 1 set oc2a on compare match when up-counting. clear oc2a on compare match when down-counting. table 63. compare output mode, non-pwm mode com2b1 com2b0 description 0 0 normal port operation, oc2b disconnected. 0 1 toggle oc2b on compare match 1 0 clear oc2b on compare match 1 1 set oc2b on compare match table 64. compare output mode, fast pwm mode (1) com2b1 com2b0 description 0 0 normal port operation, oc2b disconnected. 01reserved 1 0 clear oc2b on compare match, set oc2b at top 1 1 set oc2b on compare match, clear oc2b at top
145 atmega48/88/168 2545d?avr?07/04 table 65 shows the com2b1:0 bit functionality when the wgm22:0 bits are set to phase correct pwm mode. note: 1. a special case occurs when ocr2b equ als top and com2b1 is set. in this case, the compare match is ignored, but the set or clear is done at top. see ?phase cor- rect pwm mode? on page 140 for more details. bits 3, 2 ? res: reserved bits these bits are reserved bits in the atme ga48/88/168 and will al ways read as zero. bits 1:0 ? wgm21:0: waveform generation mode combined with the wgm22 bit found in the tccr2b register, these bits control the counting sequence of the counter, the source for maximum (top) counter value, and what type of waveform generation to be used, see table 66. modes of operation sup- ported by the timer/counter unit are: normal mode (counter), clear timer on compare match (ctc) mode, and two types of pulse width modulation (pwm) modes (see ?modes of operation? on page 137). notes: 1. max= 0xff 2. bottom= 0x00 table 65. compare output mode, phase correct pwm mode (1) com2b1 com2b0 description 0 0 normal port operation, oc2b disconnected. 01reserved 1 0 clear oc2b on compare match when up-counting. set oc2b on compare match when down-counting. 1 1 set oc2b on compare match when up-counting. clear oc2b on compare match when down-counting. table 66. waveform generation mode bit description mode wgm2 wgm1 wgm0 timer/counter mode of operation top update of ocrx at tov flag set on (1)(2) 0 0 0 0 normal 0xff immediate max 1 0 0 1 pwm, phase correct 0xff top bottom 2 0 1 0 ctc ocra immediate max 3 0 1 1 fast pwm 0xff top max 41 0 0reserved ? ? ? 5 1 0 1 pwm, phase correct ocra top bottom 61 1 0reserved ? ? ? 7 1 1 1 fast pwm ocra top top
146 atmega48/88/168 2545d?avr?07/04 timer/counter control register b ? tccr2b bit 7 ? foc2a: force output compare a the foc2a bit is only active when the wgm bits specify a non-pwm mode. however, for ensuring compatib ility with future devices, this bit must be set to zero when tccr2b is written when operating in pw m mode. when writing a logical one to the foc2a bit, an immediate compare match is forced on the waveform generation unit. the oc2a output is changed according to its com2a1:0 bits setting. note that the foc2a bit is implemented as a strobe. therefore it is the value present in the com2a1:0 bits that determines the effect of the forced compare. a foc2a strobe will not generate any interrup t, nor will it clear the timer in ctc mode using ocr2a as top. the foc2a bit is always read as zero. bit 6 ? foc2b: force output compare b the foc2b bit is only active when the wgm bits specify a non-pwm mode. however, for ensuring compatib ility with future devices, this bit must be set to zero when tccr2b is written when operating in pw m mode. when writing a logical one to the foc2b bit, an immediate compare match is forced on the waveform generation unit. the oc2b output is changed according to its com2b1:0 bits setting. note that the foc2b bit is implemented as a strobe. therefore it is the value present in the com2b1:0 bits that determines the effect of the forced compare. a foc2b strobe will not generate any interrup t, nor will it clear the timer in ctc mode using ocr2b as top. the foc2b bit is always read as zero. bits 5:4 ? res: reserved bits these bits are reserved bits in the atme ga48/88/168 and will al ways read as zero. bit 3 ? wgm22: waveform generation mode see the description in the ?timer/counter control register a ? tccr2a? on page 143. bit 2:0 ? cs22:0: clock select the three clock select bits select the clock source to be used by the timer/counter, see table 67. bit 7 6 5 4 3 2 1 0 foc2a foc2b ? ? wgm22 cs22 cs21 cs20 tccr2b read/write w w r r r r r/w r/w initial value 0 0 0 0 0 0 0 0
147 atmega48/88/168 2545d?avr?07/04 if external pin modes are used for the timer/counter0, transitions on the t0 pin will clock the counter even if the pin is configured as an output. this feature allows software control of the counting. timer/counter register ? tcnt2 the timer/counter register gives direct access, both for read and write operations, to the timer/counter unit 8-bit counter. writing to the tcnt2 register blocks (removes) the compare match on the following timer cl ock. modifying the counter (tcnt2) while the counter is running, introduces a risk of missing a compare match between tcnt2 and the ocr2x registers. output compare register a ? ocr2a the output compare register a contains an 8-bit value that is continuously compared with the counter value (tcnt2). a match can be used to generate an output compare interrupt, or to generate a waveform output on the oc2a pin. output compare register b ? ocr2b the output compare register b contains an 8-bit value that is continuously compared with the counter value (tcnt2). a match can be used to generate an output compare interrupt, or to generate a waveform output on the oc2b pin. table 67. clock select bit description cs22 cs21 cs20 description 0 0 0 no clock source (timer/counter stopped). 001clk t2s /(no prescaling) 010clk t2s /8 (from prescaler) 011clk t2s /32 (from prescaler) 100clk t2s /64 (from prescaler) 101clk t2s /128 (from prescaler) 110clk t 2 s /256 (from prescaler) 111clk t 2 s /1024 (from prescaler) bit 76543210 tcnt2[7:0] tcnt2 read/write r/w r/w r/w r/w r/w r/w r/w r/w initial value00000000 bit 76543210 ocr2a[7:0] ocr2a read/write r/w r/w r/w r/w r/w r/w r/w r/w initial value00000000 bit 76543210 ocr2b[7:0] ocr2b read/write r/w r/w r/w r/w r/w r/w r/w r/w initial value00000000
148 atmega48/88/168 2545d?avr?07/04 timer/counter2 interrupt mask register ? timsk2 bit 2 ? ocie2b: timer/counter2 output compare match b interrupt enable when the ocie2b bit is written to one and the i-bit in the status register is set (one), the timer/counter2 compare match b interrupt is enabled. the corresponding interrupt is executed if a compare match in timer/counter2 occurs, i.e., when the ocf2b bit is set in the timer/counter 2 interrupt flag register ? tifr2. bit 1 ? ocie2a: timer/counter2 output compare match a interrupt enable when the ocie2a bit is written to one and the i-bit in the status register is set (one), the timer/counter2 compare match a interrupt is enabled. the corresponding interrupt is executed if a compare match in timer/counter2 occurs, i.e., when the ocf2a bit is set in the timer/counter 2 interrupt flag register ? tifr2. bit 0 ? toie2: timer/counter2 overflow interrupt enable when the toie2 bit is written to one and the i- bit in the status register is set (one), the timer/counter2 overflow interrupt is enable d. the corresponding interrupt is executed if an overflow in timer/counter2 occurs, i.e., when the tov2 bit is set in the timer/counter2 interrupt flag register ? tifr2. timer/counter2 interrupt flag register ? tifr2 bit 2 ? ocf2b: output compare flag 2 b the ocf2b bit is set (one) when a compare match occurs between the timer/counter2 and the data in ocr2b ? output compare register2. ocf2b is cleared by hardware when executing the corresponding interrupt handling vector. alternatively, ocf2b is cleared by writing a logic one to the flag. when the i-bit in sreg, ocie2b (timer/counter2 compare match interrupt enable), and ocf2b are set (one), the timer/counter2 compare match interrupt is executed. bit 1 ? ocf2a: output compare flag 2 a the ocf2a bit is set (one) when a compare match occurs between the timer/counter2 and the data in ocr2a ? output compare register2. ocf2a is cleared by hardware when executing the corresponding interrupt handling vector. alternatively, ocf2a is cleared by writing a logic one to the flag. when the i-bit in sreg, ocie2a (timer/counter2 compare match interrupt enable), and ocf2a are set (one), the timer/counter2 compare match interrupt is executed. bit 0 ? tov2: timer/counter2 overflow flag the tov2 bit is set (one) when an overflow occurs in timer/counter2. tov2 is cleared by hardware when executing the corresponding interrupt handling vector. alternatively, tov2 is cleared by writing a logic one to the flag. when the sreg i-bit, toie2a (timer/counter2 overflow interrupt enable), and tov2 are set (one), the timer/counter2 overflow interrupt is executed. in pwm mode, this bit is set when timer/counter2 changes counting direction at 0x00. bit 76543 2 1 0 ? ? ? ? ? ocie2b ocie2a toie2 timsk2 read/write r r r r r r/w r/w r/w initial value 0 0 0 0 0 0 0 0 bit 76543210 ?????ocf2bocf2atov2tifr2 read/writerrrrrr/wr/wr/w initial value00000000
149 atmega48/88/168 2545d?avr?07/04 asynchronous operation of the timer/counter asynchronous operation of timer/counter2 when timer/counter2 operates asynchronously, some considerations must be taken. warning: when switching between asynchronous and synchronous clocking of timer/counter2, the timer registers tcnt2, ocr2x, and tccr2x might be corrupted. a safe procedure for switching clock source is: 1. disable the timer/counter2 interrupts by clearing ocie2x and toie2. 2. select clock source by setting as2 as appropriate. 3. write new values to tcnt2, ocr2x, and tccr2x. 4. to switch to asynchronous operation: wait for tcn2xub, ocr2xub, and tcr2xub. 5. clear the timer/counter2 interrupt flags. 6. enable interrupts, if needed. the cpu main clock frequency must be more than four times the oscillator frequency. when writing to one of the registers tcnt2, ocr2x, or tccr2x, the value is transferred to a temporary register, and latched after two positive edges on tosc1. the user should not write a new value before the contents of the temporary register have been transferred to its destination. each of the five mentioned registers have their individual temporary register, which means that e.g. writing to tcnt2 does not disturb an ocr2x write in progress. to detect that a transfer to the destination register has taken place, the asynchronous status register ? assr has been implemented. when entering power-save or adc noise reduction mode after having written to tcnt2, ocr2x, or tccr2x, the user must wait until the written register has been updated if timer/ counter2 is used to wake up th e device. otherwise, the mcu will enter sleep mode before the changes are effective. this is particularly important if any of the output compare2 interrupt is used to wake up the device, since the output compare function is disabled during writing to ocr2x or tcnt2. if the write cycle is not finished, and the mcu enters sleep mode before the corresponding ocr2xub bit returns to zero, the devi ce will never receive a compare match interrupt, and the mcu will not wake up. if timer/counter2 is used to wake the device up from power-save or adc noise reduction mode, precautions must be taken if the user wants to re-enter one of these modes: the interrupt logic needs one tosc1 cycle to be reset. if the time between wake-up and re-entering sleep mode is less than one tosc1 cycle, the interrupt will not occur, and the device will fa il to wake up. if the user is in doubt whether the time before re-entering power-save or adc noise reduction mode is sufficient, the following algorithm can be used to ensure that one tosc1 cycle has elapsed: 1. write a value to tccr2x, tcnt2, or ocr2x. 2. wait until the corresponding update busy flag in assr returns to zero. 3. enter power-save or adc noise reduction mode. when the asynchronous operation is se lected, the 32.768 khz oscillator for timer/counter2 is always running, except in power-down and standby modes. after a power-up reset or wake-up from power-down or standby mode, the user should be aware of the fact that this oscillator mi ght take as long as one second to stabilize. the user is advised to wait for at least one second before using timer/counter2 after power-up or wake-up from power-down or standby mode. the contents of all
150 atmega48/88/168 2545d?avr?07/04 timer/counter2 registers must be considered lost after a wake-up from power- down or standby mode due to unstable clock signal upon start-up, no matter whether the oscillator is in use or a cl ock signal is applied to the tosc1 pin. description of wake up from power-save or adc noise reduction mode when the timer is clocked asynchronously: when the interrupt condition is met, the wake up process is started on the following cycle of the timer clock, that is, the timer is always advanced by at least one before the processor can read the counter value. after wake-up, the mcu is halted for four cycles, it executes the interrupt routine, and resumes execution from the instruction following sleep. reading of the tcnt2 register shortly after wake-up from power-save may give an incorrect result. since tcnt2 is clocked on the asynchronous tosc clock, reading tcnt2 must be done through a register synchronized to the internal i/o clock domain. synchronization takes place for every rising tosc1 edge. when waking up from power-save mode, and the i/o clock (clk i/o ) again becomes active, tcnt2 will read as the previous value (before entering sleep) until the next rising tosc1 edge. the phase of the tosc clock after waking up from power-save mode is essentially unpredictable, as it depends on the wake-up time. the recommended procedure for reading tcnt2 is thus as follows: 1. write any value to either of the registers ocr2x or tccr2x. 2. wait for the corresponding update busy flag to be cleared. 3. read tcnt2. during asynchronous operation, the synchronization of the interrupt flags for the asyn- chronous timer takes 3 processor cycles plus one timer cycle. the timer is therefore advanced by at least one before the processor can read the timer value causing the set- ting of the interrupt flag. the output compare pin is changed on the timer clock and is not synchronized to the processor clock. asynchronous status register ? assr bit 6 ? exclk: enable external clock input when exclk is written to one, and asynchronous clock is selected, the external clock input buffer is enabled and an external clock can be input on timer oscillator 1 (tosc1) pin instead of a 32 khz crystal. writing to exclk should be done before asynchronous operation is selected. note th at the crystal oscillator will only run when this bit is zero. bit 5 ? as2: asynchronous timer/counter2 when as2 is written to zero, timer/count er2 is clocked from the i/o clock, clk i/o . when as2 is written to one, timer/counter2 is cl ocked from a crystal oscillator connected to the timer oscillator 1 (tosc1) pin. when th e value of as2 is ch anged, the contents of tcnt2, ocr2a, ocr2b, tccr2a and tccr2b might be corrupted. bit 4 ? tcn2ub: timer/counter2 update busy when timer/counter2 operates asynchronous ly and tcnt2 is written, this bit becomes set. when tcnt2 has been updated from the temporary storage register, this bit is cleared by hardware. a logica l zero in this bit indicate s that tcnt2 is ready to be updated with a new value. bit 7 6 5 4 3 2 1 0 ? exclk as2 tcn2ub ocr2aub ocr2bub tcr2aub tcr2bub assr read/write r r/w r/w r r r r r initial value 0 0 0 0 0 0 0 0
151 atmega48/88/168 2545d?avr?07/04 bit 3 ? ocr2aub: output co mpare register2 update busy when timer/counter2 operates asynchronous ly and ocr2a is written, this bit becomes set. when ocr2a has been updated from the temporary storage register, this bit is cleared by hardware. a logical zero in this bit indicates that ocr2a is ready to be updated with a new value. bit 2 ? ocr2bub: output compare register2 update busy when timer/counter2 operates asynchronous ly and ocr2b is written, this bit becomes set. when ocr2b has been updated from the temporary storage register, this bit is cleared by hardware. a logical zero in this bit indicates that ocr2b is ready to be updated with a new value. bit 1 ? tcr2aub: timer/counter control register2 update busy when timer/counter2 operates asynchronously and tccr2a is written, this bit becomes set. when tccr2a has been updated from the temporary storage register, this bit is cleared by hardware. a logical zero in this bit indicate s that tccr2a is ready to be updated with a new value. bit 0 ? tcr2bub: timer/counter control register2 update busy when timer/counter2 operates asynchronously and tccr2b is written, this bit becomes set. when tccr2b has been updated from the temporary storage register, this bit is cleared by hardware. a logical zero in this bit indicate s that tccr2b is ready to be updated with a new value. if a write is performed to any of the five timer/counter2 registers while its update busy flag is set, the updated value might get corrupted and cause an unintentional interrupt to occur. the mechanisms for reading tcnt2, ocr2 a, ocr2b, tccr2a and tccr2b are dif- ferent. when reading tcnt2, the actual timer value is read. when reading ocr2a, ocr2b, tccr2a and tccr2b the value in the temporary storage register is read.
152 atmega48/88/168 2545d?avr?07/04 timer/counter prescaler figure 65. prescaler for timer/counter2 the clock source for timer/counter2 is named clk t2s . clk t2s is by default connected to the main system i/o clock clk i o . by setting the as2 bit in assr , timer/counter2 is asyn- chronously clocked from the tosc1 pin. th is enables use of timer/counter2 as a real time counter (rtc). when as2 is set, pins tosc1 and tosc2 are disconnected from port c. a crystal can then be connected between the tosc1 and tosc2 pins to serve as an independent clock source for timer/counter2. the oscillator is optimized for use with a 32.768 khz crystal. applying an ex ternal clock source to tosc1 is not recommended. for timer/counter2, the possible prescaled selections are: clk t2s /8, clk t2s /32, clk t2s /64, clk t2s /128, clk t2s /256, and clk t2s /1024. additionally, clk t2s as well as 0 (stop) may be selected. setting the psrasy bit in gtccr resets the prescaler. this allows the user to operate with a predictable prescaler. general timer/counter control register ? gtccr bit 1 ? psrasy: prescaler reset timer/counter2 when this bit is one, the timer/counter2 pr escaler will be reset. this bit is normally cleared immediately by hardware. if the bit is written when timer/counter2 is operating in asynchronous mode, the bit will remain one until the pr escaler has been reset. the bit will not be cleared by hardware if the tsm bit is set. refer to the description of the ?bit 7 ? tsm: timer/counter synchronization mo de? on page 103 for a description of the timer/counter synchronization mode. 10-bit t/c prescaler timer/counter2 clock source clk i/o clk t2s tosc1 as2 cs20 cs21 cs22 clk t2s /8 clk t2s /64 clk t2s /128 clk t2s /1024 clk t2s /256 clk t2s /32 0 psrasy clear clk t2 bit 7 6 5 4 3 2 1 0 tsm ? ? ? ? ?psrasy psrsync gtccr read/write r/w r r r r r r/w r/w initial value 0 0 0 0 0 0 0 0
153 atmega48/88/168 2545d?avr?07/04 serial peripheral interface ? spi the serial peripheral interface (spi) allows high-speed synchronous data transfer between the atmega48/88/168 and peripheral devices or between several avr devices. the atmega48/88/168 spi includes the following features: full-duplex, three-wire synchronous data transfer master or slave operation lsb first or msb first data transfer seven programmable bit rates end of transmission interrupt flag write collision flag protection wake-up from idle mode double speed (ck/2) master spi mode the usart can also be used in master spi mode, see ?usart in spi mode? on page 189. the prspi bit in ?power reduction register - prr? on page 37 must be written to zero to enable spi module. figure 66. spi block diagram (1) note: 1. refer to figure 1 on page 2, and table 33 on page 69 for spi pin placement. the interconnection between master and slav e cpus with spi is shown in figure 67. the system consists of two shift registers, and a master clock generator. the spi mas- ter initiates the communication cycle when pulling low the slave select ss pin of the desired slave. master and slave prepare the data to be sent in their respective shift registers, and the master generates the required clock pulses on the sck line to inter- change data. data is always shifted from master to slave on the master out ? slave in, mosi, line, and from slave to master on the master in ? slave out, miso, line. after spi2x spi2x divider /2/4/8/16/32/64/128
154 atmega48/88/168 2545d?avr?07/04 each data packet, the master will synchronize the slave by pulling high the slave select, ss , line. when configured as a master, the spi interface has no automatic control of the ss line. this must be handled by user software before communication can start. when this is done, writing a byte to the spi data register starts the spi clock generator, and the hardware shifts the eight bits into the slave. after shifting one byte, the spi clock gener- ator stops, setting the end of transmission fl ag (spif). if the spi interrupt enable bit (spie) in the spcr register is set, an interrupt is requested. the master may continue to shift the next byte by writing it into spdr, or signal the end of packet by pulling high the slave select, ss line. the last incoming byte will be kept in the buffer register for later use. when configured as a slave, the spi interf ace will remain sleeping with miso tri-stated as long as the ss pin is driven high. in this state, software may update the contents of the spi data register, spdr, but the data will not be shifted out by incoming clock pulses on the sck pin until the ss pin is driven low. as one byte has been completely shifted, the end of transmission flag, spif is set. if the spi interrupt enable bit, spie, in the spcr register is set, an interrupt is requested. the slave may continue to place new data to be sent into spdr before reading the incoming data. the last incoming byte will be kept in the buffer register for later use. figure 67. spi master-slave interconnection the system is single buffered in the transmit direction and double buffered in the receive direction. this means that bytes to be tr ansmitted cannot be written to the spi data register before the entire shift cycle is completed. when receiving data, however, a received character must be read from the spi data register before the next character has been completely shifted in. otherwise, the first byte is lost. in spi slave mode, the control logic will sample the incoming signal of the sck pin. to ensure correct sampling of the clock signal, the frequency of the spi clock should never exceed f osc /4. when the spi is enabled, the data direction of the mosi, miso, sck, and ss pins is overridden according to table 68 on page 155. for more details on automatic port over- rides, refer to ?alternate port functions? on page 67. shift enable
155 atmega48/88/168 2545d?avr?07/04 note: 1. see ?alternate functions of port b? on page 69 for a detailed description of how to define the direction of the user defined spi pins. the following code examples show how to initialize the spi as a master and how to per- form a simple transmission. ddr_spi in the ex amples must be replaced by the actual data direction register controlling the spi pins. dd_mosi, dd_miso and dd_sck must be replaced by the actual data direction bits for these pins. e.g. if mosi is placed on pin pb5, replace dd_mosi with ddb5 and ddr_spi with ddrb. table 68. spi pin overrides (1) pin direction, master spi direction, slave spi mosi user defined input miso input user defined sck user defined input ss user defined input
156 atmega48/88/168 2545d?avr?07/04 note: 1. the example code assumes that the pa rt specific header file is included. assembly code example (1) spi_masterinit: ; set mosi and sck output, all others input ldi r17,(1< 157 atmega48/88/168 2545d?avr?07/04 the following code examples show how to in itialize the spi as a slave and how to per- form a simple reception. note: 1. the example code assumes that the pa rt specific header file is included. assembly code example (1) spi_slaveinit: ; set miso output, all others input ldi r17,(1< 158 atmega48/88/168 2545d?avr?07/04 ss pin functionality slave mode when the spi is configured as a slave, the slave select (ss) pin is always input. when ss is held low, the spi is activated, and miso becomes an output if configured so by the user. all other pins are inputs. when ss is driven high, all pins are inputs, and the spi is passive, which means that it will no t receive incoming data. note that the spi logic will be reset once the ss pin is driven high. the ss pin is useful for packet/byte synchroniza tion to keep the slave bit counter syn- chronous with the master clock generator. when the ss pin is driven high, the spi slave will immediately reset the send and receive logi c, and drop any partially received data in the shift register. master mode when the spi is configured as a master (mstr in spcr is set), the user can determine the direction of the ss pin. if ss is configured as an output, the pin is a general output pin which does not affect the spi system. typically, the pin will be driving the ss pin of the spi slave. if ss is configured as an input, it must be held high to ensure master spi operation. if the ss pin is driven low by peripheral circuitry when the spi is configured as a master with the ss pin defined as an input, the spi system interprets this as another master selecting the spi as a slave and starting to send data to it. to avoid bus contention, the spi system takes the following actions: 1. the mstr bit in spcr is cleared and the spi system becomes a slave. as a result of the spi becoming a slave, the mosi and sck pins become inputs. 2. the spif flag in spsr is set, and if the spi interrupt is enab led, and the i-bit in sreg is set, the interrup t routine will be executed. thus, when interrupt-driven spi transmission is used in master mode, and there exists a possibility that ss is driven low, the interrupt shoul d always check that the mstr bit is still set. if the mstr bit has been cleared by a slave select, it must be set by the user to re-enable spi master mode. spi control register ? spcr bit 7 ? spie: spi interrupt enable this bit causes the spi interrupt to be execut ed if spif bit in the spsr register is set and the if the global interrupt enable bit in sreg is set. bit 6 ? spe: spi enable when the spe bit is written to one, the spi is enabled. this bit must be set to enable any spi operations. bit 5 ? dord: data order when the dord bit is written to one, the lsb of the data word is transmitted first. when the dord bit is written to zero, the msb of the data word is transmitted first. bit 4 ? mstr: master/slave select this bit selects master spi mode when written to one, and slave spi mode when written logic zero. if ss is configured as an i nput and is driven low while mstr is set, mstr will bit 76543210 spie spe dord mstr cpol cpha spr1 spr0 spcr read/write r/w r/w r/w r/w r/w r/w r/w r/w initial value00000000
159 atmega48/88/168 2545d?avr?07/04 be cleared, and spif in spsr will become set. the user will then have to set mstr to re-enable spi master mode. bit 3 ? cpol: clock polarity when this bit is written to one, sck is high when idle. when cpol is written to zero, sck is low when idle. refer to figure 68 and figure 69 for an example. the cpol func- tionality is summarized below: bit 2 ? cpha: clock phase the settings of the clock phase bit (cpha) determine if data is sampled on the leading (first) or trailing (last) edge of sck. refer to figure 68 and figure 69 for an example. the cpol functionality is summarized below: bits 1, 0 ? spr1, spr0: spi clock rate select 1 and 0 these two bits control the sck rate of the device configured as a master. spr1 and spr0 have no effect on the slave. the relationship between sck and the oscillator clock frequency f osc is shown in the following table: table 69. cpol functionality cpol leading edge trailing edge 0 rising falling 1 falling rising table 70. cpha functionality cpha leading edge trailing edge 0 sample setup 1 setup sample table 71. relationship between sck an d the oscillator frequency spi2x spr1 spr0 sck frequency 000 f osc / 4 001 f osc / 16 010 f osc / 64 011 f osc / 128 100 f osc / 2 101 f osc / 8 110 f osc / 32 111 f osc / 64
160 atmega48/88/168 2545d?avr?07/04 spi status register ? spsr bit 7 ? spif: spi interrupt flag when a serial transfer is complete, the spif flag is set. an interrupt is generated if spie in spcr is set and global interrupts are enabled. if ss is an input and is driven low when the spi is in master mo de, this will also set the sp if flag. spif is cleared by hardware when executing the corresponding in terrupt handling vector. alternatively, the spif bit is cleared by first reading the spi status register with spif set, then accessing the spi data register (spdr). bit 6 ? wcol: write collision flag the wcol bit is set if the spi data regist er (spdr) is written during a data transfer. the wcol bit (and the spif bit) are clear ed by first reading the spi status register with wcol set, and then accessing the spi data register. bit 5..1 ? res: reserved bits these bits are reserved bits in the atme ga48/88/168 and will al ways read as zero. bit 0 ? spi2x: double spi speed bit when this bit is written logic one the spi speed (sck frequen cy) will be doubled when the spi is in master mode (see table 71). this means that the minimum sck period will be two cpu clock periods. when the spi is configured as slave, the spi is only guaran- teed to work at f osc /4 or lower. the spi interface on the atmega48/88/168 is also used for program memory and eeprom downloading or uplo ading. see page 286 for serial programming and verification. spi data register ? spdr the spi data register is a read/write register used for data transfer between the regis- ter file and the spi shift register. writing to the register initiates data transmission. reading the register causes the shift register receive buffer to be read. data modes there are four combinations of sck phase and polarity with respect to serial data, which are determined by control bits cpha and cpol. the spi data transfer formats are shown in figure 68 and figure 69. data bits are shifted out and latched in on oppo- site edges of the sck signal, en suring sufficient time for dat a signals to stabilize. this is clearly seen by summarizing table 69 and table 70, as done below. bit 76543210 spif wcol ? ? ? ? ? spi2x spsr read/writerrrrrrrr/w initial value00000000 bit 76543210 msb lsb spdr read/write r/w r/w r/w r/w r/w r/w r/w r/w initial value x x x x x x x x undefined
161 atmega48/88/168 2545d?avr?07/04 figure 68. spi transfer format with cpha = 0 figure 69. spi transfer format with cpha = 1 table 72. cpol functionality leading edge trailing edge spi mode cpol=0, cpha=0 sample (rising) setup (falling) 0 cpol=0, cpha=1 setup (rising) sample (falling) 1 cpol=1, cpha=0 sample (falling) setup (rising) 2 cpol=1, cpha=1 setup (falling) sample (rising) 3 bit 1 bit 6 lsb msb sck (cpol = 0) mode 0 sample i mosi/miso change 0 mosi pin change 0 miso pin sck (cpol = 1) mode 2 ss msb lsb bit 6 bit 1 bit 5 bit 2 bit 4 bit 3 bit 3 bit 4 bit 2 bit 5 msb first (dord = 0) lsb first (dord = 1) sck (cpol = 0) mode 1 sample i mosi/miso change 0 mosi pin change 0 miso pin sck (cpol = 1) mode 3 ss msb lsb bit 6 bit 1 bit 5 bit 2 bit 4 bit 3 bit 3 bit 4 bit 2 bit 5 bit 1 bit 6 lsb msb msb first (dord = 0) lsb first (dord = 1)
162 atmega48/88/168 2545d?avr?07/04 usart0 the universal synchronous and asynchronous serial receiver and transmitter (usart) is a highly flexible serial communication device. the main features are: full duplex operation (i ndependent serial receive and transmit registers) asynchronous or synchronous operation master or slave clocked synchronous operation high resolution baud rate generator supports serial frames with 5, 6, 7, 8, or 9 data bits and 1 or 2 stop bits odd or even parity generation and parity check supported by hardware data overrun detection framing error detection noise filtering includes false start bit detection and digital low pass filter three separate interrupts on tx complete , tx data register empty and rx complete multi-processor communication mode double speed asynchronous communication mode the usart can also be used in master spi mode, see ?usart in spi mode? on page 189. the power reduction usart bit, prus art0, in ?power reduction register - prr? on page 37 must be disabled by writing a logical zero to it. overview a simplified block diagram of the usart tran smitter is shown in figure 70. cpu acces- sible i/o registers and i/o pins are shown in bold. figure 70. usart block diagram (1) note: 1. refer to figure 1 on page 2 and table 39 on page 75 for usart0 pin placement. parity generator ubrrn [h:l] udr n(transmit) ucsrna ucsrnb ucsrnc baud rate generator transmit shift register receive shift register rxdn txdn pin control udrn (receive) pin control xckn data recovery clock recovery pin control tx control rx control parity checker data bus osc sync logic clock generator transmitter receiver
163 atmega48/88/168 2545d?avr?07/04 the dashed boxes in the block diagram separate the three main parts of the usart (listed from the top): clock generator, transmitter and receiver. control registers are shared by all units. the clock generation l ogic consists of sync hronization logic for external clock input used by synchronous slave operation, and the baud rate generator. the xckn (transfer clock) pin is only used by synchronous transfer mode. the trans- mitter consists of a single write buffer, a serial shift register, parity generator and control logic for handling different serial frame formats. the write buffer allows a contin- uous transfer of data without any delay between frames. the receiver is the most complex part of the usart module due to its clock and data recovery units. the recov- ery units are used for asynchronous data reception. in addition to the recovery units, the receiver includes a parity checker, control logic, a shift register and a two level receive buffer (udrn). the receiver supports the same frame formats as the transmit- ter, and can detect frame error, data overrun and parity errors. clock generation the clock generation logic generates the bas e clock for the transmitter and receiver. the usart supports four modes of clock operation: normal asynchronous, double speed asynchronous, master synchronous and slave synchronous mode. the umseln bit in usart control and status register c (ucsrnc) selects between asynchronous and synchronous operation. double speed (asynchronous mode only) is controlled by the u2xn found in the ucsrna register. when using synchronous mode (umseln = 1), the data direction register for the xckn pin (ddr_xckn) co ntrols whether the clock source is internal (master mode) or external (slave mode). the xckn pin is only active when using synchronous mode. figure 71 shows a block diagram of the clock generation logic. figure 71. clock generation logic, block diagram signal description: txclk transmitter clock (internal signal). rxclk receiver base clock (internal signal). xcki input from xck pin (internal signal). used for synchronous slave operation. xcko clock output to xck pin (internal signal). used for synchronous master operation. fosc xtal pin frequency (system clock). prescaling down-counter /2 ubrrn /4 /2 foscn ubrrn+1 sync register osc xckn pin txclk u2xn umseln ddr_xckn 0 1 0 1 xcki xcko ddr_xckn rxclk 0 1 1 0 edge detector ucpoln
164 atmega48/88/168 2545d?avr?07/04 internal clock generation ? the baud rate generator internal clock generation is used for the asynchronous and the synchronous master modes of operation. the description in this section refers to figure 71. the usart baud rate register (ubrrn) and the down-counter connected to it func- tion as a programmable prescaler or baud rate generator. the down-counter, running at system clock (f osc ), is loaded with the ubrrn value each time the counter has counted down to zero or when the ubrrnl register is written. a clock is generated each time the counter reaches zero. this clock is the baud rate generator clock output (= f osc /(ubrrn+1)). the transmitter divides the baud rate generator clock output by 2, 8 or 16 depending on mode. the baud rate generator output is used directly by the receiver?s clock and data recovery units. however, the recovery units use a state machine that uses 2, 8 or 16 states dependi ng on mode set by the state of the umseln, u2xn and ddr_xckn bits. table 73 contains equations for calculating the baud rate (in bits per second) and for calculating the ubrrn value for each mode of operation using an internally generated clock source. note: 1. the baud rate is defined to be the transfer rate in bit per second (bps) baud baud rate (in bits per second, bps) f osc system oscillator clock frequency ubrrn contents of the ubrrnh and ubrrnl registers, (0-4095) some examples of ubrrn values for some system clock frequencies are found in table 81 (see page 185). table 73. equations for calculating baud rate register setting operating mode equation for calculating baud rate (1) equation for calculating ubrrn value asynchronous normal mode (u2xn = 0) asynchronous double speed mode (u2xn = 1) synchronous master mode baud f osc 16 ubrr n 1 + () ----------------------------------------- - = ubrr n f osc 16 baud ----------------------- - 1 ? = baud f osc 8 ubrr n 1 + () -------------------------------------- - = ubrr n f osc 8 baud -------------------- 1 ? = baud f osc 2 ubrr n 1 + () -------------------------------------- - = ubrr n f osc 2 baud -------------------- 1 ? =
165 atmega48/88/168 2545d?avr?07/04 double speed operation (u2xn) the transfer rate can be doubled by setting the u2xn bit in ucsrna. setting this bit only has effect for the asynchronous operation. set this bit to zero when using synchro- nous operation. setting this bit will reduce the divisor of the baud rate di vider from 16 to 8, effectively doubling the transfer rate for asynchronous communication. note however that the receiver will in this case only use half the number of samples (reduced from 16 to 8) for data sampling and clock recovery, and therefore a more accurate baud rate setting and system clock are required when this mode is used. for the transmitter, there are no downsides. external clock external clocking is used by the synchron ous slave modes of operation. the description in this section refers to figure 71 for details. external clock input from the xckn pin is sa mpled by a synchronization register to mini- mize the chance of meta-stability. the output from the synchronization register must then pass through an edge detector before it can be used by the transmitter and receiver. this process introduces a two cp u clock period delay and therefore the max- imum external xckn clock frequency is limited by the following equation: note that f osc depends on the stability of the system clock source. it is therefore recom- mended to add some margin to avoid possible loss of data due to frequency variations. synchronous clock operation when synchronous mode is used (umseln = 1), the xckn pin will be used as either clock input (slave) or cl ock output (master). the dependency between the clock edges and data sampling or data change is the same. the basic principle is that data input (on rxdn) is sampled at the opposite xckn cl ock edge of the edge the data output (txdn) is changed. figure 72. synchronous mode xckn timing. the ucpoln bit ucrsc selects which xckn cl ock edge is used for data sampling and which is used for data change. as figure 72 shows, when ucpoln is zero the data will be changed at rising xckn edge and sampled at falling xckn edge. if ucpoln is set, the data will be changed at falling xckn edge and sampled at rising xckn edge. f xck f osc 4 ----------- < rxd / txd xck rxd / txd xck ucpol = 0 ucpol = 1 sample sample
166 atmega48/88/168 2545d?avr?07/04 frame formats a serial frame is defined to be one character of data bits with synchronization bits (start and stop bits), and optionally a parity bit fo r error checking. the usart accepts all 30 combinations of the following as valid frame formats: 1 start bit 5, 6, 7, 8, or 9 data bits no, even or odd parity bit 1 or 2 stop bits a frame starts with the start bit followed by the least significant data bit. then the next data bits, up to a total of nine, are succeeding, ending with the most significant bit. if enabled, the parity bit is inserted after the data bits, before the stop bits. when a com- plete frame is transmitted, it can be dire ctly followed by a new frame, or the communication line can be set to an idle (high) state. figure 73 illustrates the possible combinations of the frame formats. bits inside brackets are optional. figure 73. frame formats st start bit, always low. (n) data bits (0 to 8). p parity bit. can be odd or even. sp stop bit, always high. idle no transfers on the communication line (rxdn or txdn). an idle line must be high. the frame format used by the usart is set by the ucszn2:0, upmn1:0 and usbsn bits in ucsrnb and ucsrnc. the receiv er and transmitter use the same setting. note that changing the setting of any of these bits will corrupt all ongoing communica- tion for both the receiver and transmitter. the usart character size (ucszn2:0) bits select the number of data bits in the frame. the usart parity mode (upmn1:0) bits enable and set the type of parity bit. the selec- tion between one or two stop bits is done by the usart stop bit select (usbsn) bit. the receiver ignores the second stop bit. an fe (frame error) will therefore only be detected in the cases where the first stop bit is zero. parity bit calculation the parity bit is calculated by doing an exclus ive-or of all the data bits. if odd parity is used, the result of the exclusive or is inverted. the relation between the parity bit and data bits is as follows: p even parity bit using even parity p odd parity bit using odd parity d n data bit n of the character 1 0 2 3 4 [5] [6] [7] [8] [p] st sp1 [sp2] (st / idle) (idle) frame p even d n 1 ? d 3 d 2 d 1 d 0 0 p odd d n 1 ? d 3 d 2 d 1 d 0 1 = =
167 atmega48/88/168 2545d?avr?07/04 if used, the parity bit is located between the last data bit and first stop bit of a serial frame. usart initialization the usart has to be initialized before any communication can take place. the initial- ization process normally consists of setting the baud rate, setting frame format and enabling the transmitter or the receiver depending on the usage. for interrupt driven usart operation, the global interrupt flag should be cleared (and interrupts globally disabled) when doing the initialization. before doing a re-initialization with changed baud rate or frame format, be sure that there are no ongoing transmissions during th e period the registers are changed. the txcn flag can be used to check that the transmitter has completed all transfers, and the rxc flag can be used to check that there are no unread data in the receive buffer. note that the txcn flag must be cleared before each transmission (before udrn is written) if it is us ed for this purpose. the following simple usart initialization code examples show one assembly and one c function that are equal in functionality. the examples assume asynchronous opera- tion using polling (no interrupts enabled) and a fixed frame format. the baud rate is given as a function parameter. for the assembly code, the baud rate parameter is assumed to be stored in the r17:r16 registers. note: 1. the example code assumes that th e part specific header file is included. for i/o registers located in extended i/o map, ?in?, ?out?, ?s bis?, ?sbic?, ?cbi?, and ?sbi? instructions must be replaced with instructions that allow access to assembly code example (1) usart_init: ; set baud rate out ubrrnh, r17 out ubrrnl, r16 ; enable receiver and transmitter ldi r16, (1<>8); ubrrnl = ( unsigned char )baud; /* enable receiver and transmitter */ ucsrnb = (1< 168 atmega48/88/168 2545d?avr?07/04 extended i/o. typically ?lds? and ?sts? combined with ?sbrs?, ?sbrc?, ?sbr?, and ?cbr?. more advanced initialization routines can be made that include frame format as parame- ters, disable interrupts and so on. however, many applications use a fixed setting of the baud and control registers, and for these types of applications the initialization code can be placed directly in the main routine, or be combined with initialization code for other i/o modules. data transmission ? the usart transmitter the usart transmitter is enabled by setting the transmit enable (txen) bit in the ucsrnb register. when the transmitter is enabled, the normal port operation of the txdn pin is overridden by the usart and gi ven the function as the transmitter?s serial output. the baud rate, mode of operation and frame format must be set up once before doing any transmissions. if synchronous opera tion is used, the clock on the xckn pin will be overridden and used as transmission clock. sending frames with 5 to 8 data bit a data transmission is initiated by loading the transmit buffer with the data to be trans- mitted. the cpu can load the transmit buffer by writing to the udrn i/o location. the buffered data in the transmit buffer will be moved to the shift register when the shift register is ready to send a new frame. the shift register is loaded with new data if it is in idle state (no ongoing transmission) or immediately after the last stop bit of the previ- ous frame is transmitted. when the shift register is loaded with new data, it will transfer one complete frame at the rate given by the baud register, u2xn bit or by xckn depending on mode of operation. the following code examples show a simple usart transmit func tion based on polling of the data register empty (udren) flag. when using frames with less than eight bits, the most significant bits written to the udrn are ignored. the usar t has to be initial- ized before the function can be used. for the assembly code, the data to be sent is assumed to be stored in register r16 note: 1. the example code assumes that th e part specific header file is included. for i/o registers located in extended i/o map, ?in?, ?out?, ?s bis?, ?sbic?, ?cbi?, assembly code example (1) usart_transmit: ; wait for empty transmit buffer sbis ucsrna,udren rjmp usart_transmit ; put data (r16) into buffer, sends the data out udrn,r16 ret c code example (1) void usart_transmit( unsigned char data ) { /* wait for empty transmit buffer */ while ( !( ucsrna & (1< 169 atmega48/88/168 2545d?avr?07/04 and ?sbi? instructions must be replaced with instructions that allow access to extended i/o. typically ?lds? and ?sts? combined with ?sbrs?, ?sbrc?, ?sbr?, and ?cbr?. the function simply waits for the transmit buffer to be empty by checking the udren flag, before loading it with new data to be transmitted. if the data register empty inter- rupt is utilized, the inte rrupt routine writes the data into the buffer. sending frames with 9 data bit if 9-bit characters are used (ucszn = 7), the ninth bit must be written to the txb8 bit in ucsrnb before the low byte of the characte r is written to udrn. the following code examples show a transmit fu nction that handles 9-bit characters. for the assembly code, the data to be sent is assumed to be stored in registers r17:r16. notes: 1. these transmit functions are written to be general functions. they can be optimized if the contents of the ucsrnb is static. for example, only the txb8 bit of the ucsrnb register is used after initialization. 2. the example code assumes that the part specific header file is included. for i/o registers located in extended i/o map, ?in?, ?out?, ?s bis?, ?sbic?, ?cbi?, and ?sbi? instructions must be replaced with instructions that allow access to extended i/o. typically ?lds? and ?sts? combined with ?sbrs?, ?sbrc?, ?sbr?, and ?cbr?. the ninth bit can be used for indicating an address frame when using multi processor communication mode or for other protocol handling as for example synchronization. assembly code example (1)(2) usart_transmit: ; wait for empty transmit buffer sbis ucsrna,udren rjmp usart_transmit ; copy 9th bit from r17 to txb8 cbi ucsrnb,txb8 sbrc r17,0 sbi ucsrnb,txb8 ; put lsb data (r16) into buffer, sends the data out udrn,r16 ret c code example (1)(2) void usart_transmit( unsigned int data ) { /* wait for empty transmit buffer */ while ( !( ucsrna & (1< 170 atmega48/88/168 2545d?avr?07/04 transmitter flags and interrupts the usart transmitter has two flags that indicate its state: usart data register empty (udren) and transmit complete (txcn). both flags can be used for generating interrupts. the data register empty (udren) flag indicates whether the transmit buffer is ready to receive new data. this bit is set when the transmit buffer is empty, and cleared when the transmit buffer contains data to be transmitted that has not yet been moved into the shift register. for compatibility with fu ture devices, always write this bit to zero when writing the ucsrna register. when the data register empty interrupt en able (udrien) bit in ucsrnb is written to one, the usart data register empty interr upt will be executed as long as udren is set (provided that global interrupts are enabled). udren is cleared by writing udrn. when interrupt-driven data transmission is used, the data register empty interrupt rou- tine must either write new data to udrn in order to clear udren or disable the data register empty interrupt, otherwise a new inte rrupt will occur once t he interrupt routine terminates. the transmit complete (txcn) flag bit is set one when the entire frame in the transmit shift register has been shifted out and there are no new data currently present in the transmit buffer. the txcn flag bit is automatically cleared when a transmit complete interrupt is executed, or it can be cleared by writing a one to its bit location. the txcn flag is useful in half-duplex communication interfaces (like the rs-485 standard), where a transmitting application must enter receive mode and free the communication bus immediately after completing the transmission. when the transmit compete interrupt enable (txcien) bit in ucsrnb is set, the usart transmit complete interrupt will be ex ecuted when the txcn flag becomes set (provided that global interrupts are enabled). when the transmit complete interrupt is used, the interrupt handling routine does not have to clear the txcn flag, this is done automatically when the interrupt is executed. parity generator the parity generator calculates the parity bit for the serial frame data. when parity bit is enabled (upmn1 = 1), the transmitter control logic inserts the parity bit between the last data bit and the first stop bit of the frame that is sent. disabling the transmitter the disabling of the transmitter (setting the txen to zero) will not become effective until ongoing and pending transmissions are completed, i.e., when the transmit shift register and transmit buffer register do not contain data to be transmitted. when dis- abled, the transmitter will no longer override the txdn pin.
171 atmega48/88/168 2545d?avr?07/04 data reception ? the usart receiver the usart receiver is enabled by writi ng the receive enable (rxenn) bit in the ucsrnb register to one. when the receiver is enabled, the normal pin operation of the rxdn pin is overridden by the usart and given the function as the receiver?s serial input. the baud rate, mode of operation and frame format must be set up once before any serial reception can be done. if synchronous operation is used, the clock on the xckn pin will be used as transfer clock. receiving frames with 5 to 8 data bits the receiver starts data reception when it detects a valid start bit. each bit that follows the start bit will be sampled at the baud rate or xckn clock, and shifted into the receive shift register until the first stop bit of a frame is received. a second stop bit will be ignored by the receiver. when the first stop bit is received, i.e., a complete serial frame is present in the receive shift register, the contents of the shift register will be moved into the receive buffer. the receive buffer can then be read by reading the udrn i/o location. the following code example shows a simple usart receive function based on polling of the receive complete (rxcn) flag. when using frames with less than eight bits the most significant bits of the data read fr om the udrn will be masked to zero. the usart has to be initialized before the function can be used. note: 1. the example code assumes that the pa rt specific header file is included. for i/o registers located in extended i/o map, ?in?, ?out?, ?s bis?, ?sbic?, ?cbi?, and ?sbi? instructions must be replaced with instructions that allow access to extended i/o. typically ?lds? and ?sts? combined with ?sbrs?, ?sbrc?, ?sbr?, and ?cbr?. the function simply waits for data to be pres ent in the receive buffer by checking the rxcn flag, before reading the buffer and returning the value. assembly code example (1) usart_receive: ; wait for data to be received sbis ucsrna, rxcn rjmp usart_receive ; get and return received data from buffer in r16, udrn ret c code example (1) unsigned char usart_receive( void ) { /* wait for data to be received */ while ( !(ucsrna & (1< 172 atmega48/88/168 2545d?avr?07/04 receiving frames with 9 data bits if 9-bit characters are used (ucszn=7) the ninth bit must be read from the rxb8n bit in ucsrnb before reading the low bits from the udrn. this rule applies to the fen, dorn and upen status flags as well. read status from ucsrna, then data from udrn. reading the udrn i/o location will change the state of the receive buffer fifo and consequently the txb8n, fen, dorn and upen bits, which all are stored in the fifo, will change. the following code example shows a simple u sart receive function that handles both nine bit characters and the status bits.
173 atmega48/88/168 2545d?avr?07/04 note: 1. the example code assumes that the pa rt specific header file is included. for i/o registers located in extended i/o map, ?in?, ?out?, ?s bis?, ?sbic?, ?cbi?, and ?sbi? instructions must be replaced with instructions that allow access to extended i/o. typically ?lds? and ?sts? combined with ?sbrs?, ?sbrc?, ?sbr?, and ?cbr?. assembly code example (1) usart_receive: ; wait for data to be received sbis ucsrna, rxcn rjmp usart_receive ; get status and 9th bit, then data from buffer in r18, ucsrna in r17, ucsrnb in r16, udrn ; if error, return -1 andi r18,(1<> 1) & 0x01; return ((resh << 8) | resl); }
174 atmega48/88/168 2545d?avr?07/04 the receive function example reads all the i/o registers into the register file before any computation is done. this gives an opti mal receive buffer utilization since the buffer location read will be free to accept new data as early as possible. receive compete flag and interrupt the usart receiver has one flag that indicates the receiver state. the receive complete (rxcn) flag indicate s if there are unread data present in the receive buffer. this flag is one when unread data exist in the receive buffer, and zero when the receive buffer is empty (i.e., does not contain any unread data). if the receiver is disabled (rxenn = 0), the receive buff er will be flushed and consequently the rxcn bit will become zero. when the receive complete interrupt enable (rxcien) in ucsrnb is set, the usart receive complete inte rrupt will be executed as long as the rxcn flag is set (provided that global interrupts are enabled). when interrupt-driven data reception is used, the receive complete routine must read the received data from udrn in order to clear the rxcn flag, otherwise a new interrupt will o ccur once the interrupt routine terminates. receiver error flags the usart receiver has three error flags: frame error (fen), data overrun (dorn) and parity error (upen). all can be accessed by reading ucsrna. common for the error flags is that they are located in the receive buffer together with the frame for which they indicate the error status. due to the buffering of the error flags, the ucsrna must be read before the receive buffer (udrn), since reading the udrn i/o location changes the buffer read location. another equality for the error flags is that they can not be altered by software doing a write to the flag location. however, all flags must be set to zero when the ucsrna is wri tten for upward compatibility of future usart implementa- tions. none of the error flags can generate interrupts. the frame error (fen) flag indicates the state of the first stop bit of the next readable frame stored in the receive buffer. the fen flag is zero when the stop bit was correctly read (as one), and the fen flag will be one when the stop bit was incorre ct (zero). this flag can be used for detecting out-of-sync c onditions, detecting break conditions and protocol handling. the fen flag is not affected by the setting of the usbsn bit in ucs- rnc since the receiver ignores all, except for the first, stop bits. for compatibility with future devices, always set this bit to zero when writing to ucsrna. the data overrun (dorn) flag indicates data loss due to a receiver buffer full condi- tion. a data overrun occurs when the receive buffer is full (two char acters), it is a new character waiting in the receive shift register, and a new start bit is detected. if the dorn flag is set there was one or more serial frame lost between the frame last read from udrn, and the next frame read from udrn. for compatibility with future devices, always write this bit to zero when writing to ucsrna. t he dorn flag is cleared when the frame received was successfully moved from the shift register to the receive buffer. the parity error (upen) flag indicates that the next frame in the receive buffer had a parity error when received. if parity chec k is not enabled the upen bit will always be read zero. for compatibility with future devices, always set this bit to zero when writing to ucsrna. for more details see ?parity bit calculation? on page 166 and ?parity checker? on page 175.
175 atmega48/88/168 2545d?avr?07/04 parity checker the parity checker is active when the hi gh usart parity mode (upmn1) bit is set. type of parity check to be performed (odd or even) is selected by the upmn0 bit. when enabled, the parity checker calculates the par ity of the data bits in incoming frames and compares the result with the parity bit from the serial frame. the result of the check is stored in the receive buffer together with the received data and stop bits. the parity error (upen) flag can then be read by software to check if the frame had a parity error. the upen bit is set if the next character that can be read from the receive buffer had a parity error when received and the parity checking was enabled at that point (upmn1 = 1). this bit is valid until the re ceive buffer (udrn) is read. disabling the receiver in contrast to the transmitter, disabling of the receiver will be immediate. data from ongoing receptions will therefor e be lost. when disabled (i.e., the rxenn is set to zero) the receiver will no longer override the nor mal function of the rxdn port pin. the receiver buffer fifo will be flushed when the receiver is disabled. remaining data in the buffer will be lost flushing the receive buffer the receiver buffer fifo will be flushed when the receiver is disabled, i.e., the buffer will be emptied of its contents. unread data will be lost. if the buffer has to be flushed during normal operation, due to for instance an error condition, read the udrn i/o loca- tion until the rxcn flag is cleared. the following code example shows how to flush the receive buffer. note: 1. the example code assumes that the pa rt specific header file is included. for i/o registers located in extended i/o map, ?in?, ?out?, ?s bis?, ?sbic?, ?cbi?, and ?sbi? instructions must be replaced with instructions that allow access to extended i/o. typically ?lds? and ?sts? combined with ?sbrs?, ?sbrc?, ?sbr?, and ?cbr?. asynchronous data reception the usart includes a clock recovery and a data recovery unit for handling asynchro- nous data reception. the clock recovery logi c is used for synchronizing the internally generated baud rate clock to the incoming asynchronous serial frames at the rxdn pin. the data recovery logic samples and low pass filters each incoming bit, thereby improv- ing the noise immunity of the receiver. the asynchronous reception operational range depends on the accuracy of the internal baud rate clock, the rate of the incoming frames, and the frame size in number of bits. assembly code example (1) usart_flush: sbis ucsrna, rxcn ret in r16, udrn rjmp usart_flush c code example (1) void usart_flush( void ) { unsigned char dummy; while ( ucsrna & (1< 176 atmega48/88/168 2545d?avr?07/04 asynchronous clock recovery the clock recovery logic synchronizes intern al clock to the incomi ng serial frames. fig- ure 74 illustrates the sampling pr ocess of the start bit of an incoming frame. the sample rate is 16 times the baud rate for normal mode, and eight times the baud rate for double speed mode. the horizontal arrows illustrate the synchronization variation due to the sampling process. note the larger time variation when using the double speed mode (u2xn = 1) of operation. samples denoted zero are samples done when the rxdn line is idle (i.e., no communication activity). figure 74. start bit sampling when the clock recovery logic detects a high (idle) to low (start) transition on the rxdn line, the start bit detection sequence is initiated. let sample 1 denote the first zero-sam- ple as shown in the figure. the clock recovery logic then uses samples 8, 9, and 10 for normal mode, and samples 4, 5, and 6 for double speed mode (indicated with sample numbers inside boxes on the figure), to decide if a valid start bit is received. if two or more of these three samples have logical high levels (the majority wins), the start bit is rejected as a noise spike and the receiver starts looking for the next high to low-transi- tion. if however, a valid start bit is detected, the clock recovery logic is synchronized and the data recovery can begin. the synchronization process is repeated for each start bit. asynchronous data recovery when the receiver clock is synchronized to the start bit, the data recovery can begin. the data recovery unit uses a state machine that has 16 states for each bit in normal mode and eight states for each bit in double speed mode. figure 75 shows the sam- pling of the data bits and the parity bit. each of the samples is given a number that is equal to the state of the recovery unit. figure 75. sampling of data and parity bit the decision of the logic level of the received bit is taken by doing a majority voting of the logic value to the three samples in the center of the received bit. the center samples are emphasized on the figure by having the sample number inside boxes. the majority voting process is done as follows: if two or all three samples have high levels, the received bit is registered to be a logic 1. if two or all three samples have low levels, the received bit is registered to be a logic 0. this majority voting process acts as a low pass filter for the incoming signal on the rxdn pin. the recovery process is then repeated until a complete frame is received. including the first stop bit. note that the receiver only uses the first stop bit of a frame. 1234567 8 9 10 11 12 13 14 15 16 12 start idle 0 0 bit 0 3 123 4 5 678 12 0 rxd sample (u2x = 0) sample (u2x = 1) 1234567 8 9 10 11 12 13 14 15 16 1 bit n 123 4 5 678 1 rxd sample (u2x = 0) sample (u2x = 1)
177 atmega48/88/168 2545d?avr?07/04 figure 76 shows the sampling of the stop bit and the earliest possible beginning of the start bit of the next frame. figure 76. stop bit sampling and ne xt start bit sampling the same majority voting is done to the stop bit as done for the other bits in the frame. if the stop bit is registered to have a logic 0 value, the frame error (fen) flag will be set. a new high to low transition indicating the start bit of a new frame can come right after the last of the bits used for majority voting. for normal speed mode, the first low level sample can be at point marked (a) in figur e 76. for double speed mode the first low level must be delayed to (b). (c) marks a stop bit of full length. the early start bit detec- tion influences the operational range of the receiver. asynchronous operational range the operational range of the receiver is dependent on the mismatch between the received bit rate and the internally generated baud rate. if the transmitter is sending frames at too fast or too slow bit rates, or the internally generated baud rate of the receiver does not have a similar (see tabl e 74) base frequency, the receiver will not be able to synchronize the frames to the start bit. the following equations can be used to calculate the ratio of the incoming data rate and internal receiver baud rate. d sum of character size and parity size (d = 5 to 10 bit) s samples per bit. s = 16 for normal speed mode and s = 8 for double speed mode. s f first sample number used for majority voting. s f = 8 for normal speed and s f = 4 for double speed mode. s m middle sample number used for majority voting. s m = 9 for normal speed and s m = 5 for double speed mode. r slow is the ratio of the slowest incoming data rate that can be accepted in relation to the receiver baud rate. r fast is the ratio of the fastest incoming data rate that can be accepted in relation to the receiver baud rate. table 74 and table 75 list the maximum receiver baud rate error that can be tolerated. note that normal speed mode has higher toleration of baud rate variations. 1234567 8 9 10 0/1 0/1 0/1 stop 1 123 4 5 6 0/1 rxd sample (u2x = 0) sample (u2x = 1) (a) (b) (c) r slow d 1 + () s s 1 ? ds ? s f ++ ------------------------------------------ - = r fast d 2 + () s d 1 + () ss m + ----------------------------------- =
178 atmega48/88/168 2545d?avr?07/04 the recommendations of the maximum receiver baud rate error was made under the assumption that the receiver and transmitter equally divides the maximum total error. there are two possible sources for the receivers baud rate error. the receiver?s system clock (xtal) will always have some minor in stability over the supp ly voltage range and the temperature range. when using a crystal to generate the system clock, this is rarely a problem, but for a resonator the system clock may differ more than 2% depending of the resonators tolerance. the second source for the error is more controllable. the baud rate generator can not always do an exact division of the system frequency to get the baud rate wanted. in this case an ubrrn value that gives an acceptable low error can be used if possible. table 74. recommended maximum receiver baud rate error for normal speed mode (u2xn = 0) d # (data+parity bit) r slow (%) r fast (%) max total error (%) recommended max receiver error (%) 5 93.20 106.67 +6.67/-6.8 3.0 6 94.12 105.79 +5.79/-5.88 2.5 7 94.81 105.11 +5.11/-5.19 2.0 8 95.36 104.58 +4.58/-4.54 2.0 9 95.81 104.14 +4.14/-4.19 1.5 10 96.17 103.78 +3.78/-3.83 1.5 table 75. recommended maximum receiver baud rate error for double speed mode (u2xn = 1) d # (data+parity bit) r slow (%) r fast (%) max total error (%) recommended max receiver error (%) 5 94.12 105.66 +5.66/-5.88 2.5 6 94.92 104.92 +4.92/-5.08 2.0 7 95.52 104,35 +4.35/-4.48 1.5 8 96.00 103.90 +3.90/-4.00 1.5 9 96.39 103.53 +3.53/-3.61 1.5 10 96.70 103.23 +3.23/-3.30 1.0
179 atmega48/88/168 2545d?avr?07/04 multi-processor communication mode setting the multi-processor communicati on mode (mpcmn) bit in ucsrna enables a filtering function of incoming frames received by the usart receiver. frames that do not contain address in formation will be ignored and not put into the receive buffer. this effectively reduces the number of incomi ng frames that has to be handled by the cpu, in a system with multiple mcus that comm unicate via the same serial bus. the trans- mitter is unaffected by the mpcmn setting, bu t has to be used differently when it is a part of a system utilizing the mu lti-processor communication mode. if the receiver is set up to receive frames that contain 5 to 8 data bits, then the first stop bit indicates if the frame contains data or address information. if the receiver is set up for frames with nine data bits, then the ninth bit (rxb8n) is used for identifying address and data frames. when the frame type bit (the first stop or the ninth bit) is one, the frame contains an address. when the frame type bit is zero the frame is a data frame. the multi-processor communication mode enables several slave mcus to receive data from a master mcu. this is done by first decoding an address frame to find out which mcu has been addressed. if a particular slav e mcu has been addressed, it will receive the following data frames as normal, while the ot her slave mcus will ignore the received frames until another address frame is received. using mpcmn for an mcu to act as a master mcu, it can use a 9-bit character frame format (ucszn = 7). the ninth bit (txb8n) must be set when an address frame (txb8n = 1) or cleared when a data frame (txb = 0) is being transmitted. the slave mcus must in this case be set to use a 9-bit character frame format. the following procedure should be used to exchange data in multi-processor communi- cation mode: 1. all slave mcus are in multi-proc essor communication mode (mpcmn in ucsrna is set). 2. the master mcu sends an address frame, and all slaves receive and read this frame. in the slave mcus, the rxcn fl ag in ucsrna will be set as normal. 3. each slave mcu reads the udrn register and determines if it has been selected. if so, it clears the mpcmn bit in ucsrna, otherwise it waits for the next address byte and keeps the mpcmn setting. 4. the addressed mcu will receive all data frames until a new address frame is received. the other slave mcus, which still have the mpcmn bit set, will ignore the data frames. 5. when the last data frame is received by the addressed mcu, the addressed mcu sets the mpcmn bit and waits for a new address frame from master. the process then repeats from 2. using any of the 5- to 8-bit character frame formats is possible, but impractical since the receiver must change between using n and n+1 character frame formats. this makes full-duplex operation difficult since the transmitter and receiver uses the same charac- ter size setting. if 5- to 8-bit character frames are used, the transmitter must be set to use two stop bit (usbsn = 1) since the first st op bit is used for indi cating the frame type. do not use read-modify-write instructions (sbi and cbi) to set or clear the mpcmn bit. the mpcmn bit shares the same i/o location as the txcn flag and this might acciden- tally be cleared when using sbi or cbi instructions.
180 atmega48/88/168 2545d?avr?07/04 usart register description usart i/o data register n? udrn the usart transmit data buffer register and usart receive data buffer registers share the same i/o address referred to as usart data register or udrn. the trans- mit data buffer register (txb) will be t he destination for data written to the udrn register location. reading the udrn register location will return the contents of the receive data buffer register (rxb). for 5-, 6-, or 7-bit characters the upper unus ed bits will be ignored by the transmitter and set to zero by the receiver. the transmit buffer can only be written w hen the udren flag in the ucsrna register is set. data written to udrn when the udren flag is not set, will be ignored by the usart transmitter. when data is written to the transmit buffer, and the transmitter is enabled, the transmitter will lo ad the data into the transm it shift register when the shift register is empty. then the data will be serially transmitted on the txdn pin. the receive buffer consists of a two level fifo. the fifo will change its state whenever the receive buffer is accessed. due to this behavior of the receive buffer, do not use read-modify-write instructions (sbi and cbi) on this location. be careful when using bit test instructions (sbic and sbis), since these also will ch ange the state of the fifo. usart control and status register n a ? ucsrna bit 7 ? rxcn: usart receive complete this flag bit is set when there are unread data in the receive buffer and cleared when the receive buffer is empty (i.e., does not contain any unread data). if the receiver is dis- abled, the receive buffer will be flushed and consequently the rxcn bit will become zero. the rxcn flag can be used to generate a receive complete interrupt (see description of the rxcien bit). bit 6 ? txcn: usart transmit complete this flag bit is set when the entire frame in the transmit shift register has been shifted out and there are no new data currently present in the transmit buffer (udrn). the txcn flag bit is automatically cleared when a transmit complete interrupt is executed, or it can be cleared by writing a one to its bit location. the txcn flag can generate a transmit complete interrupt (see description of the txcien bit). bit 76543210 rxb[7:0] udrn (read) txb[7:0] udrn (write) read/write r/wr/wr/wr/wr/wr/wr/wr/w initial value00000000 bit 76543210 rxcn txcn udren fen dorn upen u2xn mpcmn ucsrna read/write r r/w r r r r r/w r/w initial value00100000
181 atmega48/88/168 2545d?avr?07/04 bit 5 ? udren: usart data register empty the udren flag indicates if the transmit bu ffer (udrn) is ready to receive new data. if udren is one, the buffer is empty, and therefore ready to be written. the udren flag can generate a data register empty interrupt (see description of the udrien bit). udren is set after a reset to indicate that the transmitter is ready. bit 4 ? fen: frame error this bit is set if the next character in the receive buffer had a frame error when received. i.e., when the first stop bit of the next character in the receive buffer is zero. this bit is valid until the receive buffer (u drn) is read. the fen bit is zero when the stop bit of received data is one. always set th is bit to zero when writing to ucsrna. bit 3 ? dorn: data overrun this bit is set if a data overrun condition is detected. a data overrun occurs when the receive buffer is full (two char acters), it is a new characte r waiting in the receive shift register, and a new start bit is detected. this bit is valid until the receive buffer (udrn) is read. always set this bit to zero when writing to ucsrna. bit 2 ? upen: usart parity error this bit is set if the next character in the receive buffer had a parity error when received and the parity checking was enabled at that point (upmn1 = 1). this bit is valid until the receive buffer (udrn) is re ad. always set this bit to zero when writing to ucsrna. bit 1 ? u2xn: double the usart transmission speed this bit only has effect for the asynchronous operation. write this bit to zero when using synchronous operation. writing this bit to one will reduce the divisor of the baud rate divider from 16 to 8 effec- tively doubling the transfer rate for asynchronous communication. bit 0 ? mpcmn: multi-processor communication mode this bit enables the multi-processor communication mode. when the mpcmn bit is writ- ten to one, all the incoming frames received by the usart receiver that do not contain address information will be ignored. the transmitter is unaffected by the mpcmn set- ting. for more detailed information see ?multi-processor communication mode? on page 179.
182 atmega48/88/168 2545d?avr?07/04 usart control and status register n b ? ucsrnb bit 7 ? rxcien: rx complete interrupt enable n writing this bit to one enables interrupt on the rxcn flag. a usart receive complete interrupt will be generat ed only if the rxcien bit is wr itten to one, t he global interrupt flag in sreg is written to one an d the rxcn bit in ucsrna is set. bit 6 ? txcien: tx complete interrupt enable n writing this bit to one enables interrupt on the txcn flag. a usart transmit complete interrupt will be generated only if the txcien bit is written to one, the global interrupt flag in sreg is written to one and the txcn bit in ucsrna is set. bit 5 ? udrien: usart data register empty interrupt enable n writing this bit to one enables interrupt on the udren flag. a data register empty interrupt will be generated only if the udrien bit is written to one, the global interrupt flag in sreg is written to one and the udren bit in ucsrna is set. bit 4 ? rxenn: receiver enable n writing this bit to one enables the usart receiver. the receiver will override normal port operation for the rxdn pin when enabled. disabling the receiver will flush the receive buffer invalidating the fen, dorn, and upen flags. bit 3 ? txenn: transmitter enable n writing this bit to one enables the usart transmitter. the trans mitter will override nor- mal port operation for the txdn pin when enabled. the disabling of the transmitter (writing txenn to zero) will not become effective until ongoing and pending transmis- sions are completed, i.e., when the transmit shift register and transmit buffer register do not contain data to be tr ansmitted. when disabled, the transmitte r will no longer override the txdn port. bit 2 ? ucszn2: character size n the ucszn2 bits combined with the ucszn1:0 bit in ucsrnc sets the number of data bits (character size) in a frame the receiver and transmitter use. bit 1 ? rxb8n: receive data bit 8 n rxb8n is the ninth data bit of the received character when operating with serial frames with nine data bits. must be read before reading the low bits from udrn. bit 0 ? txb8n: transmit data bit 8 n txb8n is the ninth data bit in the character to be transmitted when operating with serial frames with nine data bits. must be written before writing the low bits to udrn. bit 76543210 rxcien txcien udrien rxenn txenn ucszn2 rxb8n txb8n ucsrnb read/write r/w r/w r/w r/w r/w r/w r r/w initial value 0 0 0 0 0 0 0 0
183 atmega48/88/168 2545d?avr?07/04 usart control and status register n c ? ucsrnc bits 7:6 ? umseln1:0 usart mode select these bits select the mode of operation of the usartn as shown in table 76. note: 1. see ?usart in spi mode? on page 189 for full description of the master spi mode (mspim) operation bits 5:4 ? upmn1:0: parity mode these bits enable and set type of parity generation and check. if enabled, the transmit- ter will automatically generate and send th e parity of the transmitted data bits within each frame. the receiver will generate a parity value for the incoming data and com- pare it to the upmn setting. if a mismatch is detected, the upen flag in ucsrna will be set. bit 3 ? usbsn: stop bit select this bit selects the number of stop bits to be inserted by the transmitter. the receiver ignores this setting. bit 7 6 5 4 3 2 1 0 umseln1 umseln0 upmn1 upmn0 usbsn ucszn1 ucszn0 ucpoln ucsrnc read/write r/w r/w r/w r/w r/w r/w r/w r/w initial value 0 0 0 0 0 1 1 0 table 76. umseln bits settings umseln1 umseln0 mode 0 0 asynchronous usart 0 1 synchronous usart 1 0 (reserved) 1 1 master spi (mspim) (1) table 77. upmn bits settings upmn1 upmn0 parity mode 0 0 disabled 01reserved 1 0 enabled, even parity 1 1 enabled, odd parity table 78. usbs bit settings usbsn stop bit(s) 01-bit 12-bit
184 atmega48/88/168 2545d?avr?07/04 bit 2:1 ? ucszn1:0: character size the ucszn1:0 bits combined with the ucszn2 bit in ucsrnb sets the number of data bits (character size) in a frame the receiver and transmitter use. bit 0 ? ucpoln: clock polarity this bit is used for synchronous mode only. write this bit to zero when asynchronous mode is used. the ucpoln bit sets the relationship between data output change and data input sample, and the synchronous clock (xckn). usart baud rate registers ? ubrrnl and ubrrnh bit 15:12 ? reserved bits these bits are reserved for future use. fo r compatibility with future devices, these bit must be written to zero when ubrrnh is written. bit 11:0 ? ubrr11:0: usart baud rate register this is a 12-bit register which contains the usart baud rate. the ubrrnh contains the four most significant bits, and the ubrrnl contains the eight least significant bits of the usart baud rate. on going transmissions by the tr ansmitter and receiver will be corrupted if the baud rate is changed. writing ubrrnl will trigger an im mediate update of the baud rate prescaler. table 79. ucszn bits settings ucszn2 ucszn1 ucszn0 character size 0005-bit 0016-bit 0107-bit 0118-bit 100reserved 101reserved 110reserved 1119-bit table 80. ucpoln bit settings ucpoln transmitted data changed (output of txdn pin) received data sampled (input on rxdn pin) 0 rising xckn edge falling xckn edge 1 falling xckn edge rising xckn edge bit 151413121110 9 8 ???? ubrrn[11:8] ubrrnh ubrrn[7:0] ubrrnl 76543210 read/write r r r r r/w r/w r/w r/w r/w r/w r/w r/w r/w r/w r/w r/w initial value00000000 00000000
185 atmega48/88/168 2545d?avr?07/04 examples of baud rate setting for standard crystal and resonator frequencies, the most commonly used baud rates for asynchronous operation can be generated by us ing the ubrrn settings in table 81. ubrrn values which yield an actual baud rate differing less than 0.5% from the target baud rate, are bold in the ta ble. higher error ratings are a cceptable, but the receiver will have less noise resistance when the error rati ngs are high, especially for large serial frames (see ?asynchronous operational range? on page 177). the error values are cal- culated using the following equation: note: 1. ubrrn = 0, error = 0.0% error[%] baudrate closest match baudrate -------------------------------------------------- 1 ? ?? ?? 100 % ? = table 81. examples of ubrrn settings for co mmonly used oscillator frequencies baud rate (bps) f osc = 1.0000 mhz f osc = 1.8432 mhz f osc = 2.0000 mhz u2xn = 0u2xn = 1u2xn = 0u2xn = 1u2xn = 0u2xn = 1 ubrrn error ubrrn error ubrrn error ubrrn error ubrrn error ubrrn error 2400 250.2%510.2%470.0%950.0%510.2%1030.2% 4800 120.2%250.2%230.0%470.0%250.2%510.2% 9600 6 -7.0% 12 0.2% 11 0.0% 23 0.0% 12 0.2% 25 0.2% 14.4k 3 8.5% 8 -3.5% 7 0.0% 15 0.0% 8 -3.5% 16 2.1% 19.2k 2 8.5% 6 -7.0% 5 0.0% 11 0.0% 6 -7.0% 12 0.2% 28.8k 1 8.5% 3 8.5% 3 0.0% 7 0.0% 3 8.5% 8 -3.5% 38.4k 1 -18.6% 2 8.5% 2 0.0% 5 0.0% 2 8.5% 6 -7.0% 57.6k 0 8.5% 1 8.5% 1 0.0% 3 0.0% 1 8.5% 3 8.5% 76.8k ? ? 1 -18.6% 1 -25.0% 2 0.0% 1 -18.6% 2 8.5% 115.2k ? ? 0 8.5% 0 0.0% 1 0.0% 0 8.5% 1 8.5% 230.4k??????00.0%???? 250k??????????00.0% max. (1) 62.5 kbps 125 kbps 115.2 kbps 2 30.4 kbps 125 kbps 250 kbps
186 atmega48/88/168 2545d?avr?07/04 table 82. examples of ubrrn settings for commonly used oscillator frequencies (continued) baud rate (bps) f osc = 3.6864 mhz f osc = 4.0000 mhz f osc = 7.3728 mhz u2xn = 0u2xn = 1u2xn = 0u2xn = 1u2xn = 0u2xn = 1 ubrrn error ubrrn error ubrrn error ubrrn error ubrrn error ubrrn error 2400 95 0.0% 191 0.0% 103 0.2% 207 0.2% 191 0.0% 383 0.0% 4800 47 0.0% 95 0.0% 51 0.2% 103 0.2% 95 0.0% 191 0.0% 9600 230.0%470.0%250.2%510.2%470.0%950.0% 14.4k 15 0.0% 31 0.0% 16 2.1% 34 -0.8% 31 0.0% 63 0.0% 19.2k 11 0.0% 23 0.0% 12 0.2% 25 0.2% 23 0.0% 47 0.0% 28.8k 7 0.0% 15 0.0% 8 -3.5% 16 2.1% 15 0.0% 31 0.0% 38.4k 5 0.0% 11 0.0% 6 -7.0% 12 0.2% 11 0.0% 23 0.0% 57.6k 3 0.0% 7 0.0% 3 8.5% 8 -3.5% 7 0.0% 15 0.0% 76.8k 2 0.0% 5 0.0% 2 8.5% 6 -7.0% 5 0.0% 11 0.0% 115.2k 1 0.0% 3 0.0% 1 8.5% 3 8.5% 3 0.0% 7 0.0% 230.4k 0 0.0% 1 0.0% 0 8.5% 1 8.5% 1 0.0% 3 0.0% 250k 0 -7.8% 1 -7.8% 0 0.0% 1 0.0% 1 -7.8% 3 -7.8% 0.5m ? ? 0 -7.8% ? ? 0 0.0% 0 -7.8% 1 -7.8% 1m ??????????0-7.8% max. (1) 230.4 kbps 460.8 kbps 250 kbps 0.5 mbps 460.8 kbps 921.6 kbps 1. ubrrn = 0, error = 0.0%
187 atmega48/88/168 2545d?avr?07/04 table 83. examples of ubrrn settings for commonly used oscillator frequencies (continued) baud rate (bps) f osc = 8.0000 mhz f osc = 11.0592 mhz f osc = 14.7456 mhz u2xn = 0u2xn = 1u2xn = 0u2xn = 1u2xn = 0u2xn = 1 ubrrn error ubrrn error ubrrn error ubrrn error ubrrn error ubrrn error 2400 207 0.2% 416 -0.1% 287 0.0% 575 0.0% 383 0.0% 767 0.0% 4800 103 0.2% 207 0.2% 143 0.0% 287 0.0% 191 0.0% 383 0.0% 9600 51 0.2% 103 0.2% 71 0.0% 143 0.0% 95 0.0% 191 0.0% 14.4k 34 -0.8% 68 0.6% 47 0.0% 95 0.0% 63 0.0% 127 0.0% 19.2k 25 0.2% 51 0.2% 35 0.0% 71 0.0% 47 0.0% 95 0.0% 28.8k 16 2.1% 34 -0.8% 23 0.0% 47 0.0% 31 0.0% 63 0.0% 38.4k 12 0.2% 25 0.2% 17 0.0% 35 0.0% 23 0.0% 47 0.0% 57.6k 8 -3.5% 16 2.1% 11 0.0% 23 0.0% 15 0.0% 31 0.0% 76.8k 6 -7.0% 12 0.2% 8 0.0% 17 0.0% 11 0.0% 23 0.0% 115.2k 3 8.5% 8 -3.5% 5 0.0% 11 0.0% 7 0.0% 15 0.0% 230.4k 1 8.5% 3 8.5% 2 0.0% 5 0.0% 3 0.0% 7 0.0% 250k 1 0.0% 3 0.0% 2 -7.8% 5 -7.8% 3 -7.8% 6 5.3% 0.5m 0 0.0% 1 0.0% ? ? 2 -7.8% 1 -7.8% 3 -7.8% 1m ? ? 0 0.0% ? ? ? ? 0 -7.8% 1 -7.8% max. (1) 0.5 mbps 1 mbps 691.2 kbps 1.3824 mbps 921.6 kbps 1.8432 mbps 1. ubrrn = 0, error = 0.0%
188 atmega48/88/168 2545d?avr?07/04 table 84. examples of ubrrn settings for commonly used oscilla tor frequencies (continued) baud rate (bps) f osc = 16.0000 mhz f osc = 18.4320 mhz f osc = 20.0000 mhz u2xn = 0u2xn = 1u2xn = 0u2xn = 1u2xn = 0u2xn = 1 ubrrn error ubrrn error ubrrn error ubrrn error ubrrn error ubrrn error 2400 416 -0.1% 832 0.0% 479 0.0% 959 0.0% 520 0.0% 1041 0.0% 4800 207 0.2% 416 -0.1% 239 0.0% 479 0.0% 259 0.2% 520 0.0% 9600 103 0.2% 207 0.2% 119 0.0% 239 0.0% 129 0.2% 259 0.2% 14.4k 68 0.6% 138 -0.1% 79 0.0% 159 0.0% 86 -0.2% 173 -0.2% 19.2k 51 0.2% 103 0.2% 59 0.0% 119 0.0% 64 0.2% 129 0.2% 28.8k 34 -0.8% 68 0.6% 39 0.0% 79 0.0% 42 0.9% 86 -0.2% 38.4k 25 0.2% 51 0.2% 29 0.0% 59 0.0% 32 -1.4% 64 0.2% 57.6k 16 2.1% 34 -0.8% 19 0.0% 39 0.0% 21 -1.4% 42 0.9% 76.8k 12 0.2% 25 0.2% 14 0.0% 29 0.0% 15 1.7% 32 -1.4% 115.2k 8 -3.5% 16 2.1% 9 0.0% 19 0.0% 10 -1.4% 21 -1.4% 230.4k 3 8.5% 8 -3.5% 4 0.0% 9 0.0% 4 8.5% 10 -1.4% 250k 3 0.0% 7 0.0% 4 -7.8% 8 2.4% 4 0.0% 9 0.0% 0.5m 1 0.0% 3 0.0% ? ? 4 -7.8% ? ? 4 0.0% 1m 0 0.0% 1 0.0% ? ? ? ? ? ? ? ? max. (1) 1 mbps 2 mbps 1.152 mbps 2.3 04 mbps 1.25 mbps 2.5 mbps 1. ubrrn = 0, error = 0.0%
189 atmega48/88/168 2545d?avr?07/04 usart in spi mode the universal synchronous and asynchronous serial receiver and transmitter (usart) can be set to a master spi compliant mode of operation. the master spi mode (mspim) has the following features: full duplex, three-wire synchronous data transfer master operation supports all four spi modes of operation (mode 0, 1, 2, and 3) lsb first or msb first data tran sfer (configurable data order) queued operation (double buffered) high resolution baud rate generator high speed operatio n (fxckmax = fck/2) flexible interrupt generation overview setting both umseln1:0 bits to one enables the usart in mspim logic. in this mode of operation the spi master control logic takes direct control over the usart resources. these resources include the transmitter and receiver shift register and buffers, and the baud rate generator. the parity generator and checker, the data and clock recovery logic, and the rx and tx control logic is disabled. the usart rx and tx control logic is replaced by a common spi transfer control logic. however, the pin control logic and interrupt generation logic is identical in both modes of operation. the i/o register locations are the same in both modes. however, some of the functional- ity of the control registers changes when using mspim. clock generation the clock generation logic generates the bas e clock for the transmitter and receiver. for usart mspim mode of operation only internal clock generation (i.e. master opera- tion) is supported. the data direction re gister for the xckn pin (ddr_xckn) must therefore be set to one (i.e. as output) for the usart in mspim to operate correctly. preferably the ddr_xckn should be set up before the usart in mspim is enabled (i.e. txenn and rxenn bit set to one). the internal clock generation used in mspim mode is identical to the usart synchro- nous master mode. the baud rate or ubrrn setting can therefore be calculated using the same equations, see table 85: note: 1. the baud rate is defined to be the transfer rate in bit per second (bps) baud baud rate (in bits per second, bps) f osc system oscillator clock frequency ubrrn contents of the ubrrnh and ubrrnl registers, (0-4095) table 85. equations for calculating baud rate register setting operating mode equation for calculating baud rate (1) equation for calculating ubrrn value synchronous master mode baud f osc 2 ubrr n 1 + () -------------------------------------- - = ubrr n f osc 2 baud -------------------- 1 ? =
190 atmega48/88/168 2545d?avr?07/04 spi data modes and timing there are four combinations of xckn (sck) phase and polarity with respect to serial data, which are determined by control bits ucphan and ucpoln. the data transfer timing diagrams are shown in figure 77. data bits are shifted out and latched in on opposite edges of the xckn si gnal, ensuring sufficient time for data signals to stabilize. the ucpoln and ucphan functionality is summarized in table 86. note that changing the setting of any of these bits will corr upt all ongoing communi cation for both the receiver and transmitter. figure 77. ucphan and ucpoln data transfer timing diagrams. frame formats a serial frame for the mspim is defined to be one character of 8 data bits. the usart in mspim mode has two valid frame formats: 8-bit data with msb first 8-bit data with lsb first a frame starts with the least or most significant data bit. then the next data bits, up to a total of eight, are succeeding, ending with the most or least significant bit accordingly. when a complete frame is transmitted, a new frame can directly follow it, or the commu- nication line can be set to an idle (high) state. the udordn bit in ucsrnc sets the frame format used by the usart in mspim mode. the receiver and transmitter use the same setting. note that changing the set- ting of any of these bits will corrupt all on going communication for both the receiver and transmitter. 16-bit data transfer can be achieved by writing two data bytes to udrn. a uart trans- mit complete interrupt will th en signal that the 16-bit va lue has been shifted out. usart mspim initialization the usart in mspim mode has to be initialized before any communication can take place. the initialization process normally cons ists of setting the baud rate, setting mas- ter mode of operation (by setting ddr_xckn to one), setting frame format and enabling table 86. ucpoln and ucphan functionality- ucpoln ucphan spi mode leading edge trailing edge 0 0 0 sample (rising) setup (falling) 0 1 1 setup (rising) sample (falling) 1 0 2 sample (falling) setup (rising) 1 1 3 setup (falling) sample (rising) xck data setup (txd) data sample (rxd) xck data setup (txd) data sample (rxd) xck data setup (txd) data sample (rxd) xck data setup (txd) data sample (rxd) ucpol=0 ucpol=1 ucpha=0 ucpha=1
191 atmega48/88/168 2545d?avr?07/04 the transmitter and the receiver. only the transmitter can operate independently. for interrupt driven usart operation, the global interrupt flag should be cleared (and thus interrupts globally disabled) when doing the initialization. note: to ensure immediate initialization of th e xckn output the baud-rate register (ubrrn) must be zero at the time the transmitter is enabled. contrary to the normal mode usart operation the ubrrn must then be written to the desired value after the transmitter is enabled, but before the first transmission is started. setting ubrrn to zero before enabling the transmitter is not necessary if the initialization is done immediately after a reset since ubrrn is reset to zero. before doing a re-initialization with changed baud rate, data mode, or frame format, be sure that there is no ongoing transmissions during the period the registers are changed. the txcn flag can be used to check that the transmitter has completed all transfers, and the rxcn flag can be used to check that there are no unread data in the receive buffer. note that the txcn flag must be cleared before each transmission (before udrn is written) if it is used for this purpose. the following simple usart initialization code examples show one assembly and one c function that are equal in functionality. the examples assume polling (no interrupts enabled). the baud rate is given as a functi on parameter. for the assembly code, the baud rate parameter is assumed to be stored in the r17:r16 registers.
192 atmega48/88/168 2545d?avr?07/04 note: 1. the example code assumes that the part specific header file is included. for i/o reg- isters located in extended i/o map, "in", "out", "sbis", "sbic", "cbi", and "sbi" instructions must be replaced with instruct ions that allow access to extended i/o. typ- ically "lds" and "sts" combined with "sbrs", "sbrc", "sbr", and "cbr". data transfer using the usart in mspi mode requires the transmitter to be enabled, i.e. the txenn bit in the ucsrnb register is set to one. when the transmitter is enabled, the normal port operation of the txdn pin is overridden and given the function as the transmitter's serial output. enabling the receiver is optional and is done by setting the rxenn bit in the ucsrnb register to one. when the receiver is enabled, the normal pin operation of the rxdn pin is overridden and given the func tion as the receiver's serial input. the xckn will in both cases be used as the transfer clock. assembly code example (1) usart_init: clr r18 out ubrrnh,r18 out ubrrnl,r18 ; setting the xckn port pin as output, enables master mode. sbi xckn_ddr, xckn ; set mspi mode of operation and spi data mode 0. ldi r18, (1< 193 atmega48/88/168 2545d?avr?07/04 after initialization the usart is ready for doing data transfers. a data transfer is initiated by writing to the udrn i/o lo cation. this is the case for both sending and receiving data since the transmitter controls the transfer clock. the data written to udrn is moved from the transmit buffer to the shift register when the shift register is ready to send a new frame. note: to keep the input buffer in sync with the number of data bytes transmitted, the udrn reg- ister must be read once for each byte transmitted. the input buffer operation is identical to normal usart mode, i.e. if an overflow occu rs the character last received will be lost, not the first data in the buffer. this means that if four bytes are transferred, byte 1 first, then byte 2, 3, and 4, and the udrn is not read before all transfers are completed, then byte 3 to be received will be lost, and not byte 1. the following code examples show a simple usart in mspim mode transfer function based on polling of the data register empty (udren) flag and the receive complete (rxcn) flag. the usart has to be initialized before the function can be used. for the assembly code, the data to be sent is assu med to be stored in register r16 and the data received will be availa ble in the same register (r 16) after the function returns. the function simply waits for the transmit buffer to be empty by checking the udren flag, before loading it with new data to be transmitted. the function then waits for data to be present in the receive buffer by checki ng the rxcn flag, before reading the buffer and returning the value.
194 atmega48/88/168 2545d?avr?07/04 note: 1. the example code assumes that the part specific header file is included. for i/o reg- isters located in extended i/o map, "in", "out", "sbis", "sbic", "cbi", and "sbi" instructions must be replaced with instruct ions that allow access to extended i/o. typ- ically "lds" and "sts" combined with "sbrs", "sbrc", "sbr", and "cbr". transmitter and receiver flags and interrupts the rxcn, txcn, and udren flags and corres ponding interrupts in usart in mspim mode are identical in function to the normal usart operation. however, the receiver error status flags (fe, dor, and pe) are not in use and is always read as zero. disabling the transmitter or receiver the disabling of the transmitter or receiver in usart in mspim mode is identical in function to the normal usart operation. usart mspim register description the following section describes the registers used for spi operation using the usart. usart mspim i/o data register - udrn the function and bit description of the u sart data register (udrn) in mspi mode is identical to normal usart operation. see ?usart i/o data register n? udrn? on page 180. assembly code example (1) usart_mspim_transfer: ; wait for empty transmit buffer sbis ucsrna, udren rjmp usart_mspim_transfer ; put data (r16) into buffer, sends the data out udrn,r16 ; wait for data to be received usart_mspim_wait_rxcn: sbis ucsrna, rxcn rjmp usart_mspim_wait_rxcn ; get and return received data from buffer in r16, udrn ret c code example (1) unsigned char usart_receive( void ) { /* wait for empty transmit buffer */ while ( !( ucsrna & (1< 195 atmega48/88/168 2545d?avr?07/04 usart mspim control and status register n a - ucsrna bit 7 - rxcn: usart receive complete this flag bit is set when there are unread data in the receive buffer and cleared when the receive buffer is empty (i.e., does not contain any unread data). if the receiver is dis- abled, the receive buffer will be flushed and consequently the rxcn bit will become zero. the rxcn flag can be used to generate a receive complete interrupt (see description of the rxcien bit). bit 6 - txcn: usart transmit complete this flag bit is set when the entire frame in the transmit shift register has been shifted out and there are no new data currently present in the transmit buffer (udrn). the txcn flag bit is automatically cleared when a transmit complete interrupt is executed, or it can be cleared by writing a one to its bit location. the txcn flag can generate a transmit complete interrupt (see description of the txcien bit). bit 5 - udren: usart data register empty the udren flag indicates if the transmit bu ffer (udrn) is ready to receive new data. if udren is one, the buffer is empty, and therefore ready to be written. the udren flag can generate a data register empty interrupt (see description of the udrie bit). udren is set after a reset to indicate that the transmitter is ready. bit 4:0 - reserved bits in mspi mode when in mspi mode, these bits are reserved for future use. for compatibility with future devices, these bits must be written to zero when ucsrna is written. usart mspim control and status register n b - ucsrnb bit 7 - rxcien: rx complete interrupt enable writing this bit to one enables interrupt on the rxcn flag. a usart receive complete interrupt will be generat ed only if the rxcien bit is wr itten to one, t he global interrupt flag in sreg is written to one an d the rxcn bit in ucsrna is set. bit 6 - txcien: tx complete interrupt enable writing this bit to one enables interrupt on the txcn flag. a usart transmit complete interrupt will be generated only if the txcien bit is written to one, the global interrupt flag in sreg is written to one and the txcn bit in ucsrna is set. bit 5 - udrie: usart data re gister empty interrupt enable writing this bit to one enables interrupt on the udren flag. a data register empty interrupt will be genera ted only if the udrie bit is wr itten to one, the global interrupt flag in sreg is written to one and the udren bit in ucsrna is set. bit 4 - rxenn: receiver enable writing this bit to one enables the usart receiver in mspim mode. the receiver will override normal port operation for the rxdn pin when enabled. disabling the receiver will flush the receive buffer. only enabling the receiver in mspi mode (i.e. setting bit 7 6 5 4 3 2 1 0 rxcn txcn udren - - - - - ucsrna read/write r/w r/w r/w r r r r r initial value 0 0 0 0 0 1 1 0 bit 7 6543210 rxcien txcien udrie rxenn txenn - - - ucsrnb read/write r/w r/w r/w r/w r/w r r r initial value 0 0 0 0 0 1 1 0
196 atmega48/88/168 2545d?avr?07/04 rxenn=1 and txenn=0) has no meaning since it is the transmitter that controls the transfer clock and since only master mode is supported. bit 3 - txenn: transmitter enable writing this bit to one enables the usart transmitter. the trans mitter will override nor- mal port operation for the txdn pin when enabled. the disabling of the transmitter (writing txenn to zero) will not become effective until ongoing and pending transmis- sions are completed, i.e., when the transmit shift register and transmit buffer register do not contain data to be tr ansmitted. when disabled, the transmitte r will no longer override the txdn port. bit 2:0 - reserved bits in mspi mode when in mspi mode, these bits are reserved for future use. for compatibility with future devices, these bits must be written to zero when ucsrnb is written. usart mspim control and status register n c - ucsrnc bit 7:6 - umseln1:0: usart mode select these bits select the mode of operation of the usart as shown in table 87. see ?usart control and status register n c ? ucsrnc? on page 183 for full description of the normal usart operation. the mspim is enabled when both umseln bits are set to one. the udordn, ucphan, and ucpoln can be set in the same write operation where the mspim is enabled. bit 5:3 - reserved bits in mspi mode when in mspi mode, these bits are reserved for future use. for compatibility with future devices, these bits must be written to zero when ucsrnc is written. bit 2 - udordn: data order when set to one the lsb of the data word is transmitted first. when set to zero the msb of the data word is transmitted first. refer to the frame formats section page 4 for details. bit 1 - ucphan: clock phase the ucphan bit setting determine if data is sampled on the leasing edge (first) or tailing (last) edge of xckn. refer to the spi data modes and timing section page 4 for details. bit 0 - ucpoln: clock polarity the ucpoln bit sets the polarity of the xckn clock. the combination of the ucpoln and ucphan bit settings determine the timing of the data transfer. refer to the spi data modes and timing section page 4 for details. bit 7 6 5 4 3 2 1 0 umseln1 umseln0 - - - udordn ucphan ucpoln ucsrnc read/write r/w r/w r r r r/w r/w r/w initial value 0 0 0 0 0 1 1 0 table 87. umseln bits settings umseln1 umseln0 mode 0 0 asynchronous usart 0 1 synchronous usart 1 0 (reserved) 1 1 master spi (mspim)
197 atmega48/88/168 2545d?avr?07/04 usart mspim baud rate registers - ubrrnl and ubrrnh the function and bit description of the baud rate registers in mspi mode is identical to normal usart operation. see ?usart baud rate registers ? ubrrnl and ubrrnh? on page 184. avr usart mspim vs. avr spi the usart in mspim mode is fully co mpatible with the avr spi regarding: master mode timing diagram. the ucpoln bit functionality is identical to the spi cpol bit. the ucphan bit functionality is identical to the spi cpha bit. the udordn bit functionality is identical to the spi dord bit. however, since the usart in mspim mode reuses the usart resources, the use of the usart in mspim mode is somewhat different compared to the spi. in addition to differences of the control register bits, and that only master operation is supported by the usart in mspim mode, the following features differ between the two modules: the usart in mspim mode includes (double) buffering of the transmitter. the spi has no buffer. the usart in mspim mode receiver includes an additional buffer level. the spi wcol (write collision) bit is not included in u sart in mspim mode. the spi double speed mode (spi2x) bit is not included. however, the same effect is achieved by setting ubrrn accordingly. interrupt timing is not compatible. pin control differs due to the master only operation of the usart in mspim mode. a comparison of the usart in mspim mode and the spi pins is shown in table 88 on page 197. table 88. comparison of usart in mspim mode and spi pins. usart_mspim spi comment txdn mosi master out only rxdn miso master in only xckn sck (functionally identical) (n/a) ss not supported by usart in mspim
198 atmega48/88/168 2545d?avr?07/04 2-wire serial interface features simple yet powerful and flexible communi cation interface, only two bus lines needed both master and slave operation supported device can operate as transmitter or receiver 7-bit address space allows up to 128 different slave addresses multi-master arbitration support up to 400 khz data transfer speed slew-rate limited output drivers noise suppression circuitry rejects spikes on bus lines fully programmable slave address with general call support address recognition causes wake-up when avr is in sleep mode 2-wire serial interface bus definition the 2-wire serial interface (twi) is ideally suited for typical microcontroller applications. the twi protocol allows the systems desig ner to interconnect up to 128 different devices using only two bi-directional bus lines, one for clock (scl) and one for data (sda). the only external hardware needed to implement the bus is a single pull-up resistor for each of the twi bus lines. a ll devices connected to the bus have individual addresses, and mechanisms for resolving bus contention are inherent in the twi protocol. figure 78. twi bus interconnection twi terminology the following definitions are frequently encountered in this section. the prtwi bit in ?power reduction register - prr? on page 37 must be written to zero to enable the 2-wire serial interface. device 1 device 2 device 3 device n sda scl ........ r1 r2 v cc table 89. twi terminology term description master the device that initiates and terminates a transmission. the master also generates the scl clock. slave the device addressed by a master. transmitter the device placing data on the bus. receiver the device reading data from the bus.
199 atmega48/88/168 2545d?avr?07/04 electrical interconnection as depicted in figure 78, both bus lines are connected to the positive supply voltage through pull-up resistors. the bus drivers of all twi-compliant devices are open-drain or open-collector. this implements a wired-and function which is essential to the opera- tion of the interface. a low level on a twi bus line is generated when one or more twi devices output a zero. a high level is output when all twi devices tri-state their outputs, allowing the pull-up resistors to pull the line high. note that all avr devices connected to the twi bus must be powered in order to allow any bus operation. the number of devices that can be connect ed to the bus is only limited by the bus capacitance limit of 400 pf and the 7-bit sl ave address space. a detailed specification of the electrical characteristics of the twi is given in ?2-wire serial interface characteris- tics? on page 294. two different sets of specifications are presented there, one relevant for bus speeds below 100 khz, and one valid for bus speeds up to 400 khz. data transfer and frame format transferring bits each data bit transferred on the twi bus is accompanied by a pulse on the clock line. the level of the data line must be stable when the clock line is high. the only exception to this rule is for generating start and stop conditions. figure 79. data validity start and stop conditions the master initiates and terminates a data transmission. the transmission is initiated when the master issues a start condition on the bus, and it is terminated when the master issues a stop condition. between a start and a stop condition, the bus is considered busy, and no other master should tr y to seize control of the bus. a special case occurs when a new start condition is issued between a start and stop con- dition. this is referred to as a repe ated start condition, and is used when the master wishes to initiate a new transfer without relinquishing control of the bus. after a repeated start, the bus is considered busy until the next stop. this is identical to the start behavior, and therefore start is used to describe both start and repeated start for the rema inder of this dat asheet, unless otherwise noted. as depicted below, start and stop conditions are signalled by changing the level of the sda line when the scl line is high. sda scl data stable data stable data change
200 atmega48/88/168 2545d?avr?07/04 figure 80. start, repeated start and stop conditions address packet format all address packets transmitted on the twi bus are 9 bits long, consisting of 7 address bits, one read/write control bit and an acknowledge bit. if the read/write bit is set, a read operation is to be performed, otherwise a write operation should be per- formed. when a slave recognizes that it is being addressed, it should acknowledge by pulling sda low in the ninth scl (ack) cycle. if the addressed slave is busy, or for some other reason can not service the master?s request, the sda line should be left high in the ack clock cycle. the master c an then transmit a st op condition, or a repeated start condition to initiate a ne w transmission. an address packet consist- ing of a slave address and a read or a write bit is called sla+r or sla+w, respectively. the msb of the address byte is transmitted fi rst. slave addresses can freely be allo- cated by the designer, but the address 0000 000 is reserved for a general call. when a general call is issued, all slaves should respond by pulling the sda line low in the ack cycle. a general call is used when a master wishes to transmit the same mes- sage to several slaves in the system. when the general call addres s followed by a write bit is transmitted on the bus, all slaves set up to acknowledge the general call will pull the sda line low in the ack cycle. the followi ng data packets will then be received by all the slaves that acknowledged the general call. note that transmitting the general call address followed by a read bit is meaningless , as this would cause contention if several slaves started transmitting different data. all addresses of the format 1111 xxx should be reserved for future purposes. figure 81. address packet format sda scl start stop repeated start stop start sda scl start 12 78 9 addr msb addr lsb r/w ack
201 atmega48/88/168 2545d?avr?07/04 data packet format all data packets transmitted on the twi bus are nine bits long, consisting of one data byte and an acknowledge bit. during a data transfer, the master generates the clock and the start and stop conditions, while the re ceiver is responsible for acknowledging the reception. an acknowledge (ack) is si gnalled by the receiver pulling the sda line low during the ninth scl cycle. if the receiv er leaves the sda line high, a nack is sig- nalled. when the receiver has received the last byte, or for some reason cannot receive any more bytes, it should inform the transmitter by sending a nack after the final byte. the msb of the data byte is transmitted first. figure 82. data packet format combining address and data packets into a transmission a transmission basically consists of a star t condition, a sla+r/w, one or more data packets and a stop condition. an empty me ssage, consisting of a start followed by a stop condition, is illegal. no te that the wired-anding of the scl line can be used to implement handshaking between the master and the slave. the slave can extend the scl low period by pulling the scl line low. this is useful if the clock speed set up by the master is too fast for the slave, or t he slave needs extra time for processing between the data transmissions. the sl ave extending the scl low period will not affect the scl high period, which is determined by the master. as a consequence, the slave can reduce the twi data transfer speed by prolonging the scl duty cycle. figure 83 shows a typical data transmission. note that several data bytes can be trans- mitted between the sla+r/w and the stop condition, depending on the software protocol implemented by the application software. figure 83. typical data transmission 12 78 9 data msb data lsb ack aggregate sda sda from transmitter sda from receiver scl from master sla+r/w data byte stop, repeated start or next data byte 12 78 9 data byte data msb data lsb ack sda scl start 12 78 9 addr msb addr lsb r/w ack sla+r/w stop
202 atmega48/88/168 2545d?avr?07/04 multi-master bus systems, arbitration and synchronization the twi protocol allows bus systems with several masters. special concerns have been taken in order to ensure that transmissi ons will proceed as normal, even if two or more masters initiate a transm ission at the same time. two problems arise in multi-mas- ter systems: an algorithm must be implemented allowing only one of the masters to complete the transmission. all other masters should cease transmission when they discover that they have lost the selection process. this selection process is called arbitration. when a contending master discovers that it has lost the arbitration process, it should immediately switch to slave mode to check whether it is being addressed by the winning master. the fact that multiple masters have started transmission at the same time should not be detectable to the slaves, i.e. the data being transferred on the bus must not be corrupted. different masters may use different scl frequencies. a scheme must be devised to synchronize the serial clocks from all masters, in order to let the transmission proceed in a lockstep fashion. this will facilitate the arbitration process. the wired-anding of the bus lines is used to solve both these problems. the serial clocks from all masters will be wired-anded, yielding a combined clock with a high period equal to the one from the master with the shortest high period. the low period of the combined clock is equal to the low period of the master with the longest low period. note that all masters listen to the scl line, effectively starting to count their scl high and low time-out periods when the combined scl line goes high or low, respectively. figure 84. scl synchronization between multiple masters arbitration is carried out by all masters continuously monitoring the sda line after out- putting data. if the value read from the sda line does not match the value the master had output, it has lost the arbitration. note that a master can only lose arbitration when it outputs a high sda value while another master outputs a low value. the losing master should immediately go to slave mode, checki ng if it is being addressed by the winning master. the sda line should be left high, but losing masters are allowed to generate a clock signal until the end of the current data or address packet. arbitration will continue until only one master remains, and this may take many bits. if several masters are trying to address the same slave, arbitrat ion will continue into the data packet. ta low ta high scl from master a scl from master b scl bus line tb low tb high masters start counting low period masters start counting high period
203 atmega48/88/168 2545d?avr?07/04 figure 85. arbitration between two masters note that arbitration is not allowed between: a repeated start cond ition and a data bit. a stop condition and a data bit. a repeated start and a stop condition. it is the user software?s re sponsibility to ensure that th ese illegal arbitration conditions never occur. this implies that in multi-master systems, all data transfers must use the same composition of sla+r/w and data pa ckets. in other words: all transmissions must contain the same number of data packets, otherwise the result of the arbitration is undefined. sda from master a sda from master b sda line synchronized scl line start master a loses arbitration, sda a sda
204 atmega48/88/168 2545d?avr?07/04 overview of the twi module the twi module is comprised of several submodules, as shown in figure 86. all regis- ters drawn in a thick line are accessible through the avr data bus. figure 86. overview of the twi module scl and sda pins these pins interface the avr twi with the rest of the mcu system. the output drivers contain a slew-rate limiter in order to conform to the twi specification. the input stages contain a spike suppression unit removing spikes shorter than 50 ns. note that the inter- nal pull-ups in the avr pads can be enabled by setting the port bits corresponding to the scl and sda pins, as explained in the i/o port section. the internal pull-ups can in some systems eliminate the need for external ones. bit rate generator unit this unit controls the period of scl when operating in a master mode. the scl period is controlled by settings in the twi bit rate register (twbr) and the prescaler bits in the twi status register (twsr). slave operati on does not depend on bit rate or pres- caler settings, but the cpu clock frequency in the slave must be at least 16 times higher than the scl frequency. note that slaves may prolong the scl low period, thereby reducing the average twi bus clock period. the scl frequency is generated according to the following equation: twi unit address register (twar) address match unit address comparator control unit control register (twcr) status register (twsr) state machine and status control scl slew-rate control spike filter sda slew-rate control spike filter bit rate generator bit rate register (twbr) prescaler bus interface unit start / stop control arbitration detection ack spike suppression address/data shift register (twdr) scl frequency cpu clock frequency 16 2 (twbr) prescalervalue () ? + ---------------------------------------------------------------------------------------- - =
205 atmega48/88/168 2545d?avr?07/04 twbr = value of the twi bit rate register. prescalervalue = value of the prescaler, see table 90 on page 208. note: twbr should be 10 or higher if the twi o perates in master mode. if twbr is lower than 10, the master may produce an incorrect outp ut on sda and scl for the reminder of the byte. the problem occurs when operating the twi in master mode, sending start + sla + r/w to a slave (a slave does not need to be connected to the bus for the condition to happen). bus interface unit this unit contains the data and address shift register (twdr), a start/stop con- troller and arbitration detection hardware. the twdr contains the address or data bytes to be transmitted, or the address or data bytes received. in addition to the 8-bit twdr, the bus interface unit also contains a register containing the (n)ack bit to be transmitted or received. this (n)ack register is not directly accessible by the applica- tion software. however, when receiving, it can be set or cleared by manipulating the twi control register (twcr). when in transmitter mode, the value of the received (n)ack bit can be determined by the value in the twsr. the start/stop controller is responsi ble for generation and detection of start, repeated start, and stop conditions. the start/stop controller is able to detect start and stop conditions even when the avr mcu is in one of the sleep modes, enabling the mcu to wake up if addressed by a master. if the twi has initiated a transmission as master, the arbitration detection hardware continuously monitors the transmission trying to determine if arbitration is in process. if the twi has lost an arbitration, the control unit is informed. correct action can then be taken and appropriate status codes generated. address match unit the address match unit checks if received address bytes match the seven-bit address in the twi address register (twar). if the twi general call recognition enable (twgce) bit in the twar is written to one, all incoming address bits will also be com- pared against the general call address. upon an address match, the control unit is informed, allowing correct action to be taken. the twi may or may not acknowledge its address, depending on settings in the twcr. the address match unit is able to com- pare addresses even when the avr mcu is in sleep mode, enabling the mcu to wake up if addressed by a master. if another inte rrupt (e.g., int0) occurs during twi power- down address match and wakes up the cpu, the twi aborts operation and return to it?s idle state. if this cause any problems, ensure that twi address match is the only enabled interrupt when entering power-down. control unit the control unit monitors the twi bus and generates responses corresponding to set- tings in the twi control register (twcr). when an event requiring the attention of the application occurs on the twi bus, the twi in terrupt flag (twint) is asserted. in the next clock cycle, the twi status register (tws r) is updated with a status code identify- ing the event. the twsr only contains re levant status information when the twi interrupt flag is asserted. at all other times, the twsr contains a special status code indicating that no relevant status information is available. as long as the twint flag is set, the scl line is held low. this allows the application software to complete its tasks before allowing the twi transmission to continue. the twint flag is set in the following situations: after the twi has transmitted a start/repeated start condition. after the twi has transmitted sla+r/w. after the twi has transmitted an address byte. after the twi has lost arbitration.
206 atmega48/88/168 2545d?avr?07/04 after the twi has been addressed by own slave address or general call. after the twi has received a data byte. after a stop or repeated start has been received while still addressed as a slave. when a bus error has occurred due to an illegal start or stop condition. twi register description twi bit rate register ? twbr bits 7..0 ? twi bit rate register twbr selects the division factor for the bit rate generator. the bit rate generator is a frequency divider which generates the scl clock frequency in the master modes. see ?bit rate generator unit? on page 204 for calculating bit rates. twi control register ? twcr the twcr is used to control the operation of the twi. it is used to enable the twi, to initiate a master access by applying a start condition to the bus, to generate a receiver acknowledge, to generate a stop condition, and to control halting of the bus while the data to be written to the bus are written to the twdr. it also indicates a write collision if data is attempted written to twdr while the register is inaccessible. bit 7 ? twint: twi interrupt flag this bit is set by hardware when the twi has finished its current job and expects appli- cation software resp onse. if the i-bit in sreg and twie in twcr are set, the mcu will jump to the twi interrupt vector. while the twint flag is set, the scl low period is stretched. the twint flag must be cleared by software by writing a logic one to it. note that this flag is not automatically cleared by hardware when executing the interrupt rou- tine. also note that clearing this flag starts the operation of the twi, so all accesses to the twi address register (twar), twi stat us register (twsr), and twi data regis- ter (twdr) must be complete before clearing this flag. bit 6 ? twea: twi enable acknowledge bit the twea bit controls the generation of the acknowledge pulse. if the twea bit is writ- ten to one, the ack pulse is generated on the twi bus if the following conditions are met: 1. the device?s own slave address has been received. 2. a general call has been received, while the twgce bit in the twar is set. 3. a data byte has been received in master receiver or slave receiver mode. by writing the twea bit to zero, the device can be virtually disconnected from the 2-wire serial bus temporarily. address recognition can then be resumed by writing the twea bit to one again. bit 76543210 twbr7 twbr6 twbr5 twbr4 twbr3 twbr2 twbr1 twbr0 twbr read/write r/w r/w r/w r/w r/w r/w r/w r/w initial value00000000 bit 76543210 twint twea twsta twsto twwc twen ? twie twcr read/write r/w r/w r/w r/w r r/w r r/w initial value00000000
207 atmega48/88/168 2545d?avr?07/04 bit 5 ? twsta: twi start condition bit the application writes the twsta bit to one when it desires to become a master on the 2-wire serial bus. the twi hardware checks if the bus is available, and generates a start condition on the bus if it is free. however, if the bus is not free, the twi waits until a stop condition is detected, and then generates a new start condition to claim the bus master status. twsta must be cleared by software when the start condition has been transmitted. bit 4 ? twsto: twi stop condition bit writing the twsto bit to one in master mode will generate a stop condition on the 2- wire serial bus. when the stop conditi on is executed on the bus, the twsto bit is cleared automatically. in slave mode, setting the twsto bit can be used to recover from an error conditi on. this will not generate a stop condition, but the twi returns to a well-defined unaddressed slave mode and releases the scl and sda lines to a high impedance state. bit 3 ? twwc: twi write collision flag the twwc bit is set when attempting to wr ite to the twi data register ? twdr when twint is low. this flag is cleared by writ ing the twdr register when twint is high. bit 2 ? twen: twi enable bit the twen bit enables twi operation and ac tivates the twi interface. when twen is written to one, the twi takes control over the i/o pins connected to the scl and sda pins, enabling the slew-rate limiters and spike filters. if this bit is written to zero, the twi is switched off and all twi transmissions are terminated, regardless of any ongoing operation. bit 1 ? res: reserved bit this bit is a reserved bit an d will always read as zero. bit 0 ? twie: twi interrupt enable when this bit is written to one, and the i-bi t in sreg is set, the twi interrupt request will be activated for as long as the twint flag is high. twi status register ? twsr bits 7..3 ? tws: twi status these 5 bits reflect the status of the twi lo gic and the 2-wire serial bus. the different status codes are described later in this se ction. note that the value read from twsr contains both the 5-bit status value and the 2-bit prescaler value. the application designer should mask the prescaler bits to zero when checking the status bits. this makes status checking independent of prescaler setting. this approach is used in this datasheet, unless otherwise noted. bit 2 ? res: reserved bit this bit is reserved and will always read as zero. bits 1..0 ? twps: twi prescaler bits these bits can be read and written, and control the bit rate prescaler. bit 76543210 tws7 tws6 tws5 tws4 tws3 ? twps1 twps0 twsr read/write r r r r r r r/w r/w initial value11111000
208 atmega48/88/168 2545d?avr?07/04 to calculate bit rates, see ?bit rate generator unit? on page 204. the value of twps1..0 is used in the equation. twi data register ? twdr in transmit mode, twdr contains the next byte to be transmitted. in receive mode, the twdr contains the last byte received. it is writable while the twi is not in the process of shifting a byte. this occurs when the twi interrupt flag (twint) is set by hardware. note that the data register cannot be initialized by the user before the first interrupt occurs. the data in twdr remains stable as long as twint is set. while data is shifted out, data on the bus is simultaneously shifted in. twdr always contains the last byte present on the bus, except after a wake up from a sleep mode by the twi interrupt. in this case, the contents of twdr is undefined. in the case of a lost bus arbitration, no data is lost in the transition from master to slave. handling of the ack bit is controlled automatically by the twi logic, the cp u cannot access the ack bit directly. bits 7..0 ? twd: twi data register these eight bits constitute th e next data byte to be transmitted, or the latest data byte received on the 2-wire serial bus. twi (slave) address register ? twar the twar should be loaded with the 7-bit slave address (in the seven most significant bits of twar) to which the twi will respon d when programmed as a slave transmitter or receiver, and not needed in the master modes. in multi master systems, twar must be set in masters which can be addressed as slaves by other masters. the lsb of twar is used to enable recognition of the general call address (0x00). there is an associated address comparator that looks for the slave address (or general call address if enabled) in the received serial address. if a match is found, an interrupt request is generated. bits 7..1 ? twa: twi (slave) address register these seven bits constitute the slave address of the twi unit. bit 0 ? twgce: twi general call recognition enable bit if set, this bit enables the recognition of a general call given over the 2-wire serial bus. table 90. twi bit rate prescaler twps1 twps0 prescaler value 001 014 1016 1164 bit 76543210 twd7 twd6 twd5 twd4 twd3 twd2 twd1 twd0 twdr read/write r/w r/w r/w r/w r/w r/w r/w r/w initial value11111111 bit 76543210 twa6 twa5 twa4 twa3 twa2 twa1 twa0 twgce twar read/write r/w r/w r/w r/w r/w r/w r/w r/w initial value11111110
209 atmega48/88/168 2545d?avr?07/04 twi (slave) address mask register ? twamr bits 7..1 ? twam: twi address mask the twamr can be loaded with a 7-bit salve address mask. each of the bits in twamr can mask (disable) the corresponding address bits in the twi address regis- ter (twar). if the mask bit is set to one then the address match logic ignores the compare between the incoming address bit and the corresponding bit in twar. figure 87 shown the address match logic in detail. figure 87. twi address match logic, block diagram bit 0 ? res: reserved bit this bit is an unused bit in the atmega4 8/88/168, and will always read as zero. using the twi the avr twi is byte-oriented and interrupt based. interrupts are issued after all bus events, like reception of a byte or transmis sion of a start condition. because the twi is interrupt-based, the application software is free to carry on other operations during a twi byte transfer. note that the twi interrupt enable (twie) bit in twcr together with the global interrupt enable bit in sreg a llow the application to decide whether or not assertion of the twint flag should generate an interrupt request. if the twie bit is cleared, the application must poll the twint fl ag in order to detect actions on the twi bus. when the twint flag is asserted, the twi has finished an operation and awaits appli- cation response. in this case, the twi st atus register (twsr) contains a value indicating the current state of the twi bus. the application software can then decide how the twi should behave in the next tw i bus cycle by manipulating the twcr and twdr registers. figure 88 is a simple example of how the ap plication can interface to the twi hardware. in this example, a master wishes to transmit a single data byte to a slave. this descrip- tion is quite abstract, a more detailed explanation follows later in this section. a simple code example implementing the de sired behavior is also presented. bit 76543210 twam[6:0] ? twamr read/write r/w r/w r/w r/w r/w r/w r/w r initial value00000000 address match address bit comparator 0 address bit comparator 6..1 twar0 twamr0 address bit 0
210 atmega48/88/168 2545d?avr?07/04 figure 88. interfacing the application to the twi in a typical transmission 1. the first step in a twi transmission is to transmit a start condition. this is done by writing a specific value into twcr, instructing the twi hardware to transmit a start condition. which value to write is described later on. however, it is important that the twint bit is se t in the value written. writing a one to twint clears the flag. the twi will not start any operation as long as the twint bit in twcr is set. immediatel y after the application has cleared twint, the twi will initiate transmission of the start condition. 2. when the start condition has been transmitted, the twint flag in twcr is set, and twsr is updated with a status code indicating that the start condition has successfully been sent. 3. the application software should now examine the value of twsr, to make sure that the start condition was successfully transmitted. if twsr indicates other- wise, the application software might ta ke some special action, like calling an error routine. assuming that the status code is as expected, the application must load sla+w into twdr. remember that twdr is used both for address and data. after twdr has been loaded with the desired sla+w, a specific value must be written to twcr, instructing the twi hardware to transmit the sla+w present in twdr. which value to write is described later on. however, it is important that the twint bit is set in the value written. writing a one to twint clears the flag. the twi will not start any operation as long as the twint bit in twcr is set. immediately after the ap plication has cleared twint, the twi will initiate transmission of the address packet. 4. when the address packet has been transmitted, the twint flag in twcr is set, and twsr is updated with a status code indicating that the address packet has successfully been sent. the status co de will also reflect whether a slave acknowledged the packet or not. 5. the application software should now examine the value of twsr, to make sure that the address packet was successfully transmitted, and that the value of the ack bit was as expected. if twsr indicates otherwise, the application software might take some special action, like callin g an error routine. assuming that the status code is as expected, the application must load a data packet into twdr. subsequently, a specific value must be written to twcr, instructing the twi hardware to transmit the data packet present in twdr. which value to write is start sla+w a data a stop 1. application writes to twcr to initiate transmission of start 2. twint set. status code indicates start condition sent 4. twint set. status code indicates sla+w sent, ack received 6. twint set. status code indicates data sent, ack received 3. check twsr to see if start was sent. application loads sla+w into twdr, and loads appropriate control signals into twcr, makin sure that twint is written to one, and twsta is written to zero. 5. check twsr to see if sla+w was sent and ack received. application loads data into twdr, and loads appropriate control signals into twcr, making sure that twint is written to one 7. check twsr to see if data was sent and ack received. application loads appropriate control signals to send stop into twcr, making sure that twint is written to one twi bus indicates twint set application action twi hardware action
211 atmega48/88/168 2545d?avr?07/04 described later on. however, it is important that the twint bit is set in the value written. writing a one to twint clears the flag. the twi will not start any opera- tion as long as the twint bit in twcr is set. immediately after the application has cleared twint, the twi will initia te transmission of the data packet. 6. when the data packet has been transmitted, the twint flag in twcr is set, and twsr is updated with a status code indicating that the data packet has suc- cessfully been sent. the status co de will also reflect whether a slave acknowledged the packet or not. 7. the application software should now examine the value of twsr, to make sure that the data packet was successfully transmitted, and that the value of the ack bit was as expected. if twsr indicates otherwise, the application software might take some special action, like calling an error routine. assu ming that the status code is as expected, the application must write a specific value to twcr, instructing the twi hardware to transmit a stop condition. which value to write is described later on. however, it is im portant that the twint bit is set in the value written. writing a one to twint clears the flag. the twi will not start any operation as long as the twint bit in twcr is set. immediately after the appli- cation has cleared twint, the twi will initiate transmission of the stop condition. note that twint is not set after a stop condition has been sent. even though this example is simple, it shows th e principles involved in all twi transmis- sions. these can be summarized as follows: when the twi has finished an operation and expects application response, the twint flag is set. the scl line is pulled low until twint is cleared. when the twint flag is set, the user must update all twi registers with the value relevant for the next twi bus cycle. as an example, twdr must be loaded with the value to be transmitted in the next bus cycle. after all twi register updates and other pending application software tasks have been completed, twcr is written. when writing twcr, the tw int bit should be set. writing a one to twint clears the fl ag. the twi will then commence executing whatever operation was specified by the twcr setting. in the following an assembly and c implement ation of the example is given. note that the code below assumes that several definitions have been made, for example by using include-files.
212 atmega48/88/168 2545d?avr?07/04 assembly code example c example comments 1 ldi r16, (1< 213 atmega48/88/168 2545d?avr?07/04 transmission modes the twi can operate in one of four major modes. these are named master transmitter (mt), master receiver (mr), slave transmitter (st) and slave receiver (sr). several of these modes can be used in the same appl ication. as an example, the twi can use mt mode to write data into a twi eeprom, mr mode to read the data back from the eeprom. if other masters are present in the sy stem, some of these might tran smit data to the twi, and then sr mode would be used. it is the application software that decides which modes are legal. the following sections describe each of these modes. possible status codes are described along with figures detailing data tran smission in each of the modes. these fig- ures contain the following abbreviations: s: start condition rs: repeated start condition r: read bit (high level at sda) w: write bit (low level at sda) a: acknowledge bit (low level at sda) a : not acknowledge bit (high level at sda) data: 8-bit data byte p: stop condition sla: slave address in figure 90 to figure 96, circles are used to indicate that the twint flag is set. the numbers in the circles show the status code held in twsr, with the prescaler bits masked to zero. at these points, actions must be taken by the application to continue or complete the twi transfer. the twi transfer is suspended until the twint flag is cleared by software. when the twint flag is set, the status code in twsr is used to determine the appro- priate software action. for each status code, the required software action and details of the following serial transfer are given in table 91 to table 94. note that the prescaler bits are masked to zero in these tables. master transmitter mode in the master transmitter mode, a number of data bytes are transmitted to a slave receiver (see figure 89). in order to enter a master mode, a start condition must be transmitted. the format of the following address packet determines whether master transmitter or master receiver mode is to be entered. if sla+w is transmitted, mt mode is entered, if sla+r is transmitted, mr mode is entered. all the status codes mentioned in this section assume that the prescaler bits are zero or are masked to zero.
214 atmega48/88/168 2545d?avr?07/04 figure 89. data transfer in ma ster transmitter mode a start condition is sent by wr iting the following value to twcr: twen must be set to enable the 2-wire serial interface, twsta must be written to one to transmit a start condition and twint must be written to one to clear the twint flag. the twi will then test the 2-wire seri al bus and generate a start condition as soon as the bus becomes free. after a start condition has been transmitted, the twint flag is set by hardwa re, and the status code in twsr will be 0x08 (see table 91). in order to enter mt mode, sla+w must be transmitted. this is done by writing sla+w to twdr. thereafter the twint bit should be cleared (by writing it to one) to continue the transfer. this is accomplishe d by writing the following value to twcr: when sla+w have been transmitted and an acknowledgement bit has been received, twint is set again and a number of status codes in twsr are possible. possible sta- tus codes in master mode are 0x18, 0x20, or 0x38. the appropriate action to be taken for each of these status codes is detailed in table 91. when sla+w has been successfully transmitt ed, a data packet should be transmitted. this is done by writing the data byte to twdr. twdr must only be written when twint is high. if not, the access will be discarded, and the writ e collision bit (twwc) will be set in the twcr register. afte r updating twdr, the twint bit should be cleared (by writing it to one) to continue the transfer. this is acco mplished by writing the following value to twcr: this scheme is repeated until the last byte has been sent and the transfer is ended by generating a stop condition or a repeated start condition. a stop condition is gen- erated by writing the following value to twcr: a repeated start condition is generated by writing the following value to twcr: twcr twint twea twsta twsto twwc twen ? twie value 1 x10 x1 0 x twcr twint twea twsta twsto twwc twen ? twie value 1 x00 x1 0 x twcr twint twea twsta twsto twwc twen ? twie value 1 x00 x1 0 x twcr twint twea twsta twsto twwc twen ? twie value 1 x01 x1 0 x twcr twint twea twsta twsto twwc twen ? twie value 1 x10 x1 0 x device 1 master transmitter device 2 slave receiver device 3 device n sda scl ........ r1 r2 v cc
215 atmega48/88/168 2545d?avr?07/04 after a repeated start condition (state 0x10) the 2-wire serial interface can access the same slave again, or a new slave without transmitting a stop condition. repeated start enables the master to switch between slaves, master transmitter mode and master receiver mode without losing control of the bus. table 91. status codes for master transmitter mode status code (twsr) prescaler bits are 0 status of the 2-wire serial bus and 2-wire serial interface hard- ware application software response next action taken by twi hardware to/from twdr to twcr sta sto twint twea 0x08 a start condition has been transmitted load sla+w 0 0 1 x sla+w will be transmitted; ack or not ack will be received 0x10 a repeated start condition has been transmitted load sla+w or load sla+r 0 0 0 0 1 1 x x sla+w will be transmitted; ack or not ack will be received sla+r will be transmitted; logic will switch to master receiver mode 0x18 sla+w has been transmitted; ack has been received load data byte or no twdr action or no twdr action or no twdr action 0 1 0 1 0 0 1 1 1 1 1 1 x x x x data byte will be transmitted and ack or not ack will be received repeated start will be transmitted stop condition will be transmitted and twsto flag will be reset stop condition followed by a start condition will be transmitted and twsto flag will be reset 0x20 sla+w has been transmitted; not ack has been received load data byte or no twdr action or no twdr action or no twdr action 0 1 0 1 0 0 1 1 1 1 1 1 x x x x data byte will be transmitted and ack or not ack will be received repeated start will be transmitted stop condition will be transmitted and twsto flag will be reset stop condition followed by a start condition will be transmitted and twsto flag will be reset 0x28 data byte has been transmitted; ack has been received load data byte or no twdr action or no twdr action or no twdr action 0 1 0 1 0 0 1 1 1 1 1 1 x x x x data byte will be transmitted and ack or not ack will be received repeated start will be transmitted stop condition will be transmitted and twsto flag will be reset stop condition followed by a start condition will be transmitted and twsto flag will be reset 0x30 data byte has been transmitted; not ack has been received load data byte or no twdr action or no twdr action or no twdr action 0 1 0 1 0 0 1 1 1 1 1 1 x x x x data byte will be transmitted and ack or not ack will be received repeated start will be transmitted stop condition will be transmitted and twsto flag will be reset stop condition followed by a start condition will be transmitted and twsto flag will be reset 0x38 arbitration lost in sla+w or data bytes no twdr action or no twdr action 0 1 0 0 1 1 x x 2-wire serial bus will be released and not addressed slave mode entered a start condition will be transmitted when the bus be- comes free
216 atmega48/88/168 2545d?avr?07/04 figure 90. formats and states in the master transmitter mode s sla w a data a p $08 $18 $28 r sla w $10 ap $20 p $30 a or a $38 a other master continues a or a $38 other master continues r a $68 other master continues $78 $b0 to corresponding states in slave mode mt m r successfull transmission to a slave receiver next transfer started with a repeated start condition not acknowledge received after the slave address not acknowledge received after a data byte arbitration lost in slave address or data byte arbitration lost and addressed as slave data a n from master to slave from slave to master any number of data bytes and their associated acknowledge bits this number (contained in twsr) corresponds to a defined state of the 2-wire serial bus. the p rescaler bits are zero or masked to zero s
217 atmega48/88/168 2545d?avr?07/04 master receiver mode in the master receiver mode, a number of data bytes are received from a slave trans- mitter (slave see figure 91). in order to enter a master mode, a start condition must be transmitted. the format of the following address packet determines whether master transmitter or master receiver mode is to be entered. if sla+w is transmitted, mt mode is entered, if sla+r is transmitted, mr mode is entered. all the status codes mentioned in this section assume that the prescaler bits are zero or are masked to zero. figure 91. data transfer in master receiver mode a start condition is sent by wr iting the following value to twcr: twen must be written to one to enable the 2-wire serial interface, twsta must be written to one to transmit a start condition and twint must be set to clear the twint flag. the twi will then test the 2-wire seri al bus and generate a start condition as soon as the bus becomes free. after a start condition has been transmitted, the twint flag is set by hardware, and the status code in twsr will be 0x08 (see table 91). in order to enter mr mode, sla+r must be transmitted. this is done by writing sla+r to twdr. thereafter the twint bit should be cleared (by writing it to one) to continue the transfer. this is accomplishe d by writing the following value to twcr: when sla+r have been transmitted and an acknowledgement bit has been received, twint is set again and a number of status codes in twsr are possible. possible sta- tus codes in master mode are 0x38, 0x40, or 0x48. the appropriate action to be taken for each of these status codes is detailed in table 92. received data can be read from the twdr register when the twint flag is set high by hardware. this scheme is repeated until the last byte has been received. after the last byte has been received, the mr should inform the st by sending a nack after the last received data byte. the transfer is ended by generating a stop c ondition or a repeated start condition. a stop condition is generated by wr iting the following value to twcr: a repeated start condition is generated by writing the following value to twcr: twcr twint twea twsta twsto twwc twen ? twie value 1 x10 x1 0 x twcr twint twea twsta twsto twwc twen ? twie value 1 x00 x1 0 x twcr twint twea twsta twsto twwc twen ? twie value 1 x01 x1 0 x twcr twint twea twsta twsto twwc twen ? twie value 1 x10 x1 0 x device 1 master receiver device 2 slave transmitter device 3 device n sda scl ........ r1 r2 v cc
218 atmega48/88/168 2545d?avr?07/04 after a repeated start condition (state 0x10) the 2-wire serial interface can access the same slave again, or a new slave without transmitting a stop condition. repeated start enables the master to switch between slaves, master transmitter mode and master receiver mode without losing control over the bus. table 92. status codes for master receiver mode status code (twsr) prescaler bits are 0 status of the 2-wire serial bus and 2-wire serial interface hard- ware application software response next action taken by twi hardware to/from twdr to twcr sta sto twint twea 0x08 a start condition has been transmitted load sla+r 0 0 1 x sla+r will be transmitted ack or not ack will be received 0x10 a repeated start condition has been transmitted load sla+r or load sla+w 0 0 0 0 1 1 x x sla+r will be transmitted ack or not ack will be received sla+w will be transmitted logic will switch to master transmitter mode 0x38 arbitration lost in sla+r or not ack bit no twdr action or no twdr action 0 1 0 0 1 1 x x 2-wire serial bus will be released and not addressed slave mode will be entered a start condition will be transmitted when the bus becomes free 0x40 sla+r has been transmitted; ack has been received no twdr action or no twdr action 0 0 0 0 1 1 0 1 data byte will be received and not ack will be returned data byte will be received and ack will be returned 0x48 sla+r has been transmitted; not ack has been received no twdr action or no twdr action or no twdr action 1 0 1 0 1 1 1 1 1 x x x repeated start will be transmitted stop condition will be transmitted and twsto flag will be reset stop condition followed by a start condition will be transmitted and twsto flag will be reset 0x50 data byte has been received; ack has been returned read data byte or read data byte 0 0 0 0 1 1 0 1 data byte will be received and not ack will be returned data byte will be received and ack will be returned 0x58 data byte has been received; not ack has been returned read data byte or read data byte or read data byte 1 0 1 0 1 1 1 1 1 x x x repeated start will be transmitted stop condition will be transmitted and twsto flag will be reset stop condition followed by a start condition will be transmitted and twsto flag will be reset
219 atmega48/88/168 2545d?avr?07/04 figure 92. formats and states in the master receiver mode slave receiver mode in the slave receiver mode, a number of data bytes are received from a master trans- mitter (see figure 93). all the status codes mentioned in this section assume that the prescaler bits are zero or are masked to zero. figure 93. data transfer in slave receiver mode to initiate the slave receiver mode, twar and twcr must be initialized as follows: s sla r a data a $08 $40 $50 sla r $10 ap $48 a or a $38 other master continues $38 other master continues w a $68 other master continues $78 $b0 to corresponding states in slave mode mr m t successfull reception from a slave receiver next transfer started with a repeated start condition not acknowledge received after the slave address arbitration lost in slave address or data byte arbitration lost and addressed as slave data a n from master to slave from slave to master any number of data bytes and their associated acknowledge bits this number (contained in twsr) corresponds to a defined state of the 2-wire serial bus. the p rescaler bits are zero or masked to zero p data a $58 a r s twar twa6 twa5 twa4 twa3 twa2 twa1 twa0 twgce value device?s own slave address device 3 device n sda scl ........ r1 r2 v cc device 2 master transmitter device 1 slave receiver
220 atmega48/88/168 2545d?avr?07/04 the upper 7 bits are the address to which the 2-wire serial interfac e will respond when addressed by a master. if the lsb is set, the twi will respond to the general call address (0x00), otherwise it will ignore the general call address. twen must be written to one to enable the twi. the twea bit must be written to one to enable the acknowledgement of the device?s own slave address or the general call address. twsta and twsto must be written to zero. when twar and twcr have been initialized, th e twi waits until it is addressed by its own slave address (or the general call address if enabled) followed by the data direction bit. if the direction bit is ?0? (write), the twi will operate in sr mode, otherwise st mode is entered. after its own slave address and the write bit have been received, the twint flag is set and a valid status code can be read from twsr. the status code is used to determine the appropriate software action. the appropriate action to be taken for each status code is detailed in table 93. the slave receiver mode may also be entered if arbitration is lost while the twi is in the master mode (see states 0x68 and 0x78). if the twea bit is reset during a transfer, the twi will return a ?not acknowledge? (?1?) to sda after the next received data byte. this can be used to indicate that the slave is not able to receive any more bytes. while twea is zero, the twi does not acknowledge its own slave address. however, the 2-wire se rial bus is still monitored and address rec- ognition may resume at any time by setting twea. this implies that the twea bit may be used to temporarily isolate the twi from the 2-wire serial bus. in all sleep modes other than idle mode, the clock system to the twi is turned off. if the twea bit is set, the interfac e can still acknowledge its own slave address or the general call address by using the 2-wire serial bus clock as a clock source. the part will then wake up from sleep and the twi will hold the scl clock low during the wake up and until the twint flag is cleared (by writing it to one). furt her data recept ion will be car- ried out as normal, with the avr clocks running as normal. observe that if the avr is set up with a long start-up time, the scl line may be held low for a long time, blocking other data transmissions. note that the 2-wire serial interface data register ? twdr does not reflect the last byte present on the bus when waking up from these sleep modes. twcr twint twea twsta twsto twwc twen ? twie value 0 100 01 0 x
221 atmega48/88/168 2545d?avr?07/04 table 93. status codes for slave receiver mode status code (twsr) prescaler bits are 0 status of the 2-wire serial bus and 2-wire serial interface hardware application software response next action taken by twi hardware to/from twdr to twcr sta sto twint twea 0x60 own sla+w has been received; ack has been returned no twdr action or no twdr action x x 0 0 1 1 0 1 data byte will be received and not ack will be returned data byte will be received and ack will be returned 0x68 arbitration lost in sla+r/w as master; own sla+w has been received; ack has been returned no twdr action or no twdr action x x 0 0 1 1 0 1 data byte will be received and not ack will be returned data byte will be received and ack will be returned 0x70 general call address has been received; ack has been returned no twdr action or no twdr action x x 0 0 1 1 0 1 data byte will be received and not ack will be returned data byte will be received and ack will be returned 0x78 arbitration lost in sla+r/w as master; general call address has been received; ack has been returned no twdr action or no twdr action x x 0 0 1 1 0 1 data byte will be received and not ack will be returned data byte will be received and ack will be returned 0x80 previously addressed with own sla+w; data has been received; ack has been returned read data byte or read data byte x x 0 0 1 1 0 1 data byte will be received and not ack will be returned data byte will be received and ack will be returned 0x88 previously addressed with own sla+w; data has been received; not ack has been returned read data byte or read data byte or read data byte or read data byte 0 0 1 1 0 0 0 0 1 1 1 1 0 1 0 1 switched to the not addressed slave mode; no recognition of own sla or gca switched to the not addressed slave mode; own sla will be recognized; gca will be recognized if twgce = ?1? switched to the not addressed slave mode; no recognition of own sla or gca; a start condition will be transmitted when the bus becomes free switched to the not addressed slave mode; own sla will be recognized; gca will be recognized if twgce = ?1?; a start condition will be transmitted when the bus becomes free 0x90 previously addressed with general call; data has been re- ceived; ack has been returned read data byte or read data byte x x 0 0 1 1 0 1 data byte will be received and not ack will be returned data byte will be received and ack will be returned 0x98 previously addressed with general call; data has been received; not ack has been returned read data byte or read data byte or read data byte or read data byte 0 0 1 1 0 0 0 0 1 1 1 1 0 1 0 1 switched to the not addressed slave mode; no recognition of own sla or gca switched to the not addressed slave mode; own sla will be recognized; gca will be recognized if twgce = ?1? switched to the not addressed slave mode; no recognition of own sla or gca; a start condition will be transmitted when the bus becomes free switched to the not addressed slave mode; own sla will be recognized; gca will be recognized if twgce = ?1?; a start condition will be transmitted when the bus becomes free 0xa0 a stop condition or repeated start condition has been received while still addressed as slave no action 0 0 1 1 0 0 0 0 1 1 1 1 0 1 0 1 switched to the not addressed slave mode; no recognition of own sla or gca switched to the not addressed slave mode; own sla will be recognized; gca will be recognized if twgce = ?1? switched to the not addressed slave mode; no recognition of own sla or gca; a start condition will be transmitted when the bus becomes free switched to the not addressed slave mode; own sla will be recognized; gca will be recognized if twgce = ?1?; a start condition will be transmitted when the bus becomes free
222 atmega48/88/168 2545d?avr?07/04 figure 94. formats and states in the slave receiver mode s sla w a data a $60 $80 $88 a $68 reception of the own slave address and one or more data bytes. all are acknowledged last data byte received is not acknowledged arbitration lost as master and addressed as slave reception of the general call address and one or more data bytes last data byte received is not acknowledged n from master to slave from slave to master any number of data bytes and their associated acknowledge bits this number (contained in twsr) corresponds to a defined state of the 2-wire serial bus. the prescaler bits are zero or masked to zero p or s data a $80 $a0 p or s a a data a $70 $ 9 0 $ 9 8 a $78 p or s data a $ 9 0 $a0 p or s a general call arbitration lost as master and addressed as slave by general call data a
223 atmega48/88/168 2545d?avr?07/04 slave transmitter mode in the slave transmitter mode, a number of data bytes are transmitted to a master receiver (see figure 95). all the status codes mentioned in this section assume that the prescaler bits are zero or are masked to zero. figure 95. data transfer in slave transmitter mode to initiate the slave transm itter mode, twar and twcr mu st be initialized as follows: the upper seven bits are th e address to which the 2-wire serial interface will respond when addressed by a master. if the lsb is set, the twi will respond to the general call address (0x00), otherwise it will ignore the general call address. twen must be written to one to enable the twi. the twea bit must be written to one to enable the acknowledgement of the device?s own slave address or the general call address. twsta and twsto must be written to zero. when twar and twcr have been initialized, th e twi waits until it is addressed by its own slave address (or the general call address if enabled) followed by the data direction bit. if the direction bit is ?1? (read), the twi will operate in st mo de, otherwise sr mode is entered. after its own slave address and the write bit have been received, the twint flag is set and a valid status code can be read from twsr. the status code is used to determine the appropriate software action. the appropriate action to be taken for each status code is detailed in table 94. the slave transmitter mode may also be entered if arbitration is lost while the twi is in the master mode (see state 0xb0). if the twea bit is written to zero during a tr ansfer, the twi will tran smit the last byte of the transfer. state 0xc0 or state 0xc8 will be entered, depending on wh ether the master receiver transmits a nack or ack after the final byte. the twi is switched to the not addressed slave mode, and will ignore the master if it continues the transfer. thus the master receiver receives all ?1? as serial data. state 0xc8 is entered if the master demands additional data bytes (by transmitting ack), even though the slave has trans- mitted the last byte (twea zero and expecting nack from the master). while twea is zero, the twi does not respond to its own slave address. however, the 2-wire serial bus is still monitored and address recognition may resume at any time by setting twea. this implies that the twea bit may be used to temporarily isolate the twi from the 2-wire serial bus. twar twa6 twa5 twa4 twa3 twa2 twa1 twa0 twgce value device?s own slave address twcr twint twea twsta twsto twwc twen ? twie value 0 100 01 0 x device 3 device n sda scl ........ r1 r2 v cc device 2 master receiver device 1 slave transmitter
224 atmega48/88/168 2545d?avr?07/04 in all sleep modes other than idle mode, the clock system to the twi is turned off. if the twea bit is set, the interfac e can still acknowledge its own slave address or the general call address by using the 2-wire serial bus clock as a clock source. the part will then wake up from sleep and the twi will hold t he scl clock will low during the wake up and until the twint flag is cleared (by writing it to one). further data transmission will be carried out as normal, with the avr clocks running as normal. observe that if the avr is set up with a long start-up time, the scl line may be held low for a long time, blocking other data transmissions. note that the 2-wire serial interface data register ? twdr does not reflect the last byte present on the bus when waking up from these sleep modes. table 94. status codes for slave transmitter mode status code (twsr) prescaler bits are 0 status of the 2-wire serial bus and 2-wire serial interface hardware application software response next action taken by twi hardware to/from twdr to twcr sta sto twint twea 0xa8 own sla+r has been received; ack has been returned load data byte or load data byte x x 0 0 1 1 0 1 last data byte will be transmitted and not ack should be received data byte will be transmitted and ack should be re- ceived 0xb0 arbitration lost in sla+r/w as master; own sla+r has been received; ack has been returned load data byte or load data byte x x 0 0 1 1 0 1 last data byte will be transmitted and not ack should be received data byte will be transmitted and ack should be re- ceived 0xb8 data byte in twdr has been transmitted; ack has been received load data byte or load data byte x x 0 0 1 1 0 1 last data byte will be transmitted and not ack should be received data byte will be transmitted and ack should be re- ceived 0xc0 data byte in twdr has been transmitted; not ack has been received no twdr action or no twdr action or no twdr action or no twdr action 0 0 1 1 0 0 0 0 1 1 1 1 0 1 0 1 switched to the not addressed slave mode; no recognition of own sla or gca switched to the not addressed slave mode; own sla will be recognized; gca will be recognized if twgce = ?1? switched to the not addressed slave mode; no recognition of own sla or gca; a start condition will be transmitted when the bus becomes free switched to the not addressed slave mode; own sla will be recognized; gca will be recognized if twgce = ?1?; a start condition will be transmitted when the bus becomes free 0xc8 last data byte in twdr has been transmitted (twea = ?0?); ack has been received no twdr action or no twdr action or no twdr action or no twdr action 0 0 1 1 0 0 0 0 1 1 1 1 0 1 0 1 switched to the not addressed slave mode; no recognition of own sla or gca switched to the not addressed slave mode; own sla will be recognized; gca will be recognized if twgce = ?1? switched to the not addressed slave mode; no recognition of own sla or gca; a start condition will be transmitted when the bus becomes free switched to the not addressed slave mode; own sla will be recognized; gca will be recognized if twgce = ?1?; a start condition will be transmitted when the bus becomes free
225 atmega48/88/168 2545d?avr?07/04 figure 96. formats and states in the slave transmitter mode miscellaneous states there are two status codes that do not correspond to a defined twi state, see table 95. status 0xf8 indicates that no relevant info rmation is available because the twint flag is not set. this occurs between other states, and when the twi is not involved in a serial transfer. status 0x00 indicates that a bus error has occurred during a 2-wire serial bus transfer. a bus error occurs when a start or stop co ndition occurs at an illegal position in the format frame. examples of such illegal positions are during the serial transfer of an address byte, a data byte, or an acknowledge bit. when a bus error occurs, twint is set. to recover from a bus error, the twsto flag must set and twint must be cleared by writing a logic one to it. this causes the twi to enter the not addressed slave mode and to clear the twsto flag (no other bits in twcr are affected). the sda and scl lines are released, and no stop condition is transmitted. s sla r a data a $a8 $b8 a $b0 reception of the own slave address and one or more data bytes last data byte transmitted. switched to not addressed slave (twea = '0') arbitration lost as master and addressed as slave n from master to slave from slave to master any number of data bytes and their associated acknowledge bits this number (contained in twsr) corresponds to a defined state of the 2-wire serial bus. the prescaler bits are zero or masked to zero p or s data $c0 data a a $c8 p or s all 1's a table 95. miscellaneous states status code (twsr) prescaler bits are 0 status of the 2-wire serial bus and 2-wire serial interface hard- ware application software response next action taken by twi hardware to/from twdr to twcr sta sto twint twea 0xf8 no relevant state information available; twint = ?0? no twdr action no twcr action wait or proceed current transfer 0x00 bus error due to an illegal start or stop condition no twdr action 0 1 1 x only the internal hardware is affected, no stop condi- tion is sent on the bus. in all cases, the bus is released and twsto is cleared.
226 atmega48/88/168 2545d?avr?07/04 combining several twi modes in some cases, several twi modes must be combined in or der to complete the desired action. consider for example reading data from a serial eeprom. typically, such a transfer involves the following steps: 1. the transfer must be initiated. 2. the eeprom must be instructed what location should be read. 3. the reading must be performed. 4. the transfer must be finished. note that data is transmitted both from master to slave and vice versa. the master must instruct the slave what location it wants to read, requiring the use of the mt mode. sub- sequently, data must be read from the slave, implying the use of the mr mode. thus, the transfer direction must be changed. the master must keep control of the bus during all these steps, and the steps should be carried out as an atomical operation. if this prin- ciple is violated in a multi master system, another master can alter the data pointer in the eeprom between st eps 2 and 3, and the master will read the wrong data location. such a change in transfer direction is accomplished by transmitting a repeated start between the transmission of the address byte and reception of the data. after a repeated start, the master keeps ownership of the bus. the following figure shows the flow in this transfer. figure 97. combining several twi modes to access a serial eeprom multi-master systems and arbitration if multiple masters are connected to the sa me bus, transmissions may be initiated simul- taneously by one or more of them. the twi standard ensures that such situations are handled in such a way that one of the master s will be allowed to pr oceed with the trans- fer, and that no data will be lost in the process. an example of an arbitration situation is depicted below, where two masters are trying to transmit data to a slave receiver. figure 98. an arbitration example several different scenarios may arise during arbitration, as described below: master transmitter master receiver s = start rs = repeated start p = stop transmitted from master to slave transmitted from slave to master s sla+w a address a rs sla+r a data a p device 1 master transmitter device 2 master transmitter device 3 slave receiver device n sda scl ........ r1 r2 v cc
227 atmega48/88/168 2545d?avr?07/04 two or more masters are performing identical communication with the same slave. in this case, neither the slave nor an y of the masters will know about the bus contention. two or more masters are accessing the same slave with different data or direction bit. in this case, arbitration will occur, eith er in the read/write bit or in the data bits. the masters trying to output a one on sda while another master outputs a zero will lose the arbitration. losing masters will switch to not addre ssed slave mode or wait until the bus is free and transmit a new start condition, depending on application software action. two or more masters are accessing differ ent slaves. in this case, arbitration will occur in the sla bits. masters trying to output a one on sda while another master outputs a zero will lose the arbitration. masters losing ar bitration in sla will switch to slave mode to check if they are being addressed by the winning master. if addressed, they will switch to sr or st mode, depending on the value of the read/write bit. if they are not being add ressed, they will switch to not addressed slave mode or wait until the bus is free and transmit a new start condition, depending on application software action. this is summarized in figure 99. possibl e status values are given in circles. figure 99. possible status codes caused by arbitration own address / general call received arbitration lost in sla twi bus will be released and not addressed slave mode will be entered a start condition will be transmitted when the bus becomes free no arbitration lost in data direction ye s write data byte will be received and not ack will be returned data byte will be received and ack will be returned last data byte will be transmitted and not ack should be received data byte will be transmitted and ack should be received read b0 68/78 38 sla start data stop
228 atmega48/88/168 2545d?avr?07/04 analog comparator the analog comparator compares the input values on the positive pin ain0 and nega- tive pin ain1. when the voltage on the positive pin ain0 is higher than the voltage on the negative pin ain1, the analog comparator output, aco, is set. the comparator?s output can be set to trigger the timer/counter1 input capture function. in addition, the comparator can trigger a separate interrupt, exclusive to the analog comparator. the user can select interrupt triggering on comparator output rise, fall or toggle. a block dia- gram of the comparator and its surrounding logic is shown in figure 100 . the power reduction adc bit, pradc, in ?power reduction register - prr? on page 37 must be disabled by writing a logical zero to be able to use the adc input mux. figure 100. analog comparator block diagram (2) notes: 1. see table 97 on page 230. 2. refer to figure 1 on page 2 and table 39 on page 75 for analog comparator pin placement. adc control and status register b ? adcsrb bit 6 ? acme: analog comparator multiplexer enable when this bit is written logic one and the adc is switched off (aden in adcsra is zero), the adc multiplexer selects the negative input to the analog comparator. when this bit is written logic zero, ain1 is applied to the negative input of the analog compar- ator. for a detailed description of this bit, see ?analog comparator multiplexed input? on page 230. analog comparator control and status register ? acsr bit 7 ? acd: analog comparator disable when this bit is written logic one, the power to the analog comparator is switched off. this bit can be set at any time to turn o ff the analog comparator . this will reduce power consumption in active and idle mode. when changing the acd bit, the analog compar- acbg bandgap reference adc multiplexer output acme aden (1) bit 7 6543210 ? acme ? ? ? adts2 adts1 adts0 adcsrb read/write r r/w r r r r/w r/w r/w initial value0 0000000 bit 76543210 acd acbg aco aci acie acic acis1 acis0 acsr read/write r/w r/w r r/w r/w r/w r/w r/w initial value 0 0 n/a 0 0 0 0 0
229 atmega48/88/168 2545d?avr?07/04 ator interrupt must be disabled by clearing t he acie bit in acsr. otherwise an interrupt can occur when the bit is changed. bit 6 ? acbg: analog comparator bandgap select when this bit is set, a fixed bandgap referenc e voltage replaces the positive input to the analog comparator. when this bit is cleared, ain0 is applied to the positive input of the analog comparator. see ?internal voltage reference? on page 45. bit 5 ? aco: analog comparator output the output of the analog comparator is synchronized and then directly connected to aco. the synchronization introduces a delay of 1 - 2 clock cycles. bit 4 ? aci: analog comparator interrupt flag this bit is set by hardware when a comparator output event triggers the interrupt mode defined by acis1 and acis0. the analog comparator interrupt routine is executed if the acie bit is set and the i-bit in sreg is set. aci is cleared by hardware when execut- ing the corresponding interrupt handling vector. alternatively, aci is cleared by writing a logic one to the flag. bit 3 ? acie: analog comparator interrupt enable when the acie bit is written logic one and the i-bit in the status register is set, the ana- log comparator interrupt is activated. when written logic zero, the interrupt is disabled. bit 2 ? acic: analog comparator input capture enable when written logic one, this bit enables the input capture function in timer/counter1 to be triggered by the analog comparator. the comparator output is in this case directly connected to the input capture front-end logic, making the comparator utilize the noise canceler and edge select features of the timer/counter1 input capture interrupt. when written logic zero, no connection between the analog comparator and the input capture function exists. to make the comparator tr igger the timer/counter1 input capture inter- rupt, the icie1 bit in the timer interrupt mask register (timsk1) must be set. bits 1, 0 ? acis1, acis0: analog comparator interrupt mode select these bits determine which comparator events that trigger the analog comparator inter- rupt. the different settings are shown in table 96. when changing the acis1/acis0 bits, the anal og comparator interrupt must be dis- abled by clearing its interrupt enable bit in the acsr register. otherwise an interrupt can occur when the bits are changed. table 96. acis1/acis0 settings acis1 acis0 interrupt mode 0 0 comparator interrupt on output toggle. 01reserved 1 0 comparator interrupt on falling output edge. 1 1 comparator interrupt on rising output edge.
230 atmega48/88/168 2545d?avr?07/04 analog comparator multiplexed input it is possible to select any of the adc7..0 pins to replace the negative input to the ana- log comparator. the adc multiple xer is used to select this input, and consequently, the adc must be switched off to utilize this feature. if the analog comparator multiplexer enable bit (acme in adcsrb) is set and t he adc is switched off (aden in adcsra is zero), mux2..0 in admux select the input pin to replace the negative input to the ana- log comparator, as shown in table 97. if ac me is cleared or aden is set, ain1 is applied to the negative input to the analog comparator. digital input disable register 1 ? didr1 bit 7..2 ? res: reserved bits these bits are unused bits in the atm ega48/88/168, and will alwa ys read as zero. bit 1, 0 ? ain1d, ain0d: ain1, ain0 digital input disable when this bit is written logic one, the digital input buffer on the ain1/0 pin is disabled. the corresponding pin register bit will always read as zero when this bit is set. when an analog signal is applied to the ain1/0 pin and the digital input from this pin is not needed, this bit should be written logic one to reduce power consumption in the digital input buffer. table 97. analog comparator multiplexed input acme aden mux2..0 analog comparator negative input 0 x xxx ain1 1 1 xxx ain1 1 0 000 adc0 1 0 001 adc1 1 0 010 adc2 1 0 011 adc3 1 0 100 adc4 1 0 101 adc5 1 0 110 adc6 1 0 111 adc7 bit 76543210 ??????ain1dain0ddidr1 read/writerrrrrrr/wr/w initial value00000000
231 atmega48/88/168 2545d?avr?07/04 analog-to-digital converter features 10-bit resolution 0.5 lsb integral non-linearity 2 lsb absolute accuracy 13 - 260 s conversion time up to 15 ksps at maximum resolution 6 multiplexed single ended input channels 2 additional multiplexed single ended input channels (tqfp and mlf package only) optional left adjustment for adc result readout 0 - v cc adc input voltage range selectable 1.1v adc reference voltage free running or single conversion mode interrupt on adc co nversion complete sleep mode noise canceler the atmega48/88/168 features a 10-bit successive approximation adc. the adc is connected to an 8-channel analog multiplexer which allows eight single-ended voltage inputs constructed from the pins of port a. the single-ended voltage inputs refer to 0v (gnd). the adc contains a sample and hold circuit which ensures that the input voltage to the adc is held at a constant level during conv ersion. a block diagram of the adc is shown in figure 101. the adc has a separate analog supply voltage pin, av cc . av cc must not differ more than 0.3v from v cc . see the paragraph ?adc noise canceler? on page 237 on how to connect this pin. internal reference voltages of nominally 1.1v or av cc are provided on-chip. the voltage reference may be externally decoupled at the aref pin by a capacitor for better noise performance. the power reduction adc bit, pradc, in ?power reduction register - prr? on page 37 must be disabled by writing a logical zero to enable the adc.
232 atmega48/88/168 2545d?avr?07/04 figure 101. analog to digital converter block schematic operation the adc converts an analog input voltage to a 10-bit digital value through successive approximation. the minimum value represents gnd and the maximum value represents the voltage on the aref pin minus 1 lsb. optionally, av cc or an internal 1.1v refer- ence voltage may be connected to the aref pin by writing to the refsn bits in the admux register. the internal voltage refere nce may thus be decoupled by an external capacitor at the aref pin to improve noise immunity. the analog input channel is se lected by writing to the mux bits in admux. any of the adc input pins, as well as gnd and a fixed bandgap voltage reference, can be selected as single ended inputs to the adc. the adc is enabled by setting the adc enable bit, aden in adcsra. voltage refe rence and input channel select ions will not go into effect until aden is set. the adc does not consum e power when aden is cleared, so it is recommended to switch off the adc before entering power saving sleep modes. the adc generates a 10-bit result which is presented in the adc data registers, adch and adcl. by default, the result is presented right adjusted, but can optionally be presented left adjusted by setting the adlar bit in admux. adc conversion complete irq 8-bit data bus 15 0 adc multiplexer select (admux) adc ctrl. & status register (adcsra) adc data register (adch/adcl) mux2 adie adfr adsc aden adif adif mux1 mux0 adps0 adps1 adps2 mux3 conversion logic 10-bit dac + - sample & hold comparator internal 1.1v reference mux decoder avcc adc7 adc6 adc5 adc4 adc3 adc2 adc1 adc0 refs0 refs1 adlar channel selection adc[ 9 :0] adc multiplexer output aref bandgap reference prescaler gnd input mux
233 atmega48/88/168 2545d?avr?07/04 if the result is left adjusted and no more than 8-bit precision is requir ed, it is sufficient to read adch. otherwise, adcl must be read fi rst, then adch, to en sure that the content of the data registers belongs to the same conversion. once adcl is read, adc access to data registers is blocked. this means that if adcl has been read, and a conversion completes before adch is read, neither register is updated and the result from the con- version is lost. when adch is read, adc access to the adch and adcl registers is re-enabled. the adc has its own interrupt which can be triggered when a conversion completes. when adc access to the data registers is prohibited between reading of adch and adcl, the interrupt will trigger even if t he result is lost. starting a conversion a single conversion is started by dis abling the power reduction adc bit, pradc, in ?power reduction register - prr? on page 37 by writing a logical zero to it and writing a logical one to the adc start conversion bit, adsc. this bit stays high as long as the conversion is in progress and will be cleare d by hardware when th e conversion is com- pleted. if a different data channel is selected while a conversion is in progress, the adc will finish the current conversion befo re performing the channel change. alternatively, a conversion can be triggered automatically by various sources. auto trig- gering is enabled by setting the adc auto trigger enable bit, adate in adcsra. the trigger source is selected by setting t he adc trigger select bits, adts in adcsrb (see description of the adts bits for a list of the trigger sources). when a positive edge occurs on the selected trigger signal, the adc prescaler is reset and a conversion is started. this provides a method of starting c onversions at fixed intervals. if the trigger signal still is set when the conversion comple tes, a new conversion will not be started. if another positive edge occurs on the trigger signal during conversion, the edge will be ignored. note that an interrupt flag will be set even if the specific in terrupt is disabled or the global interrupt enable bit in sreg is cleared. a conversion can thus be triggered without causing an interrupt. however, the interrupt flag must be cleared in order to trig- ger a new conversion at the next interrupt event. figure 102. adc auto trigger logic using the adc interrupt flag as a trigger source makes the adc start a new conversion as soon as the ongoing conversion has finished. the adc then operates in free run- ning mode, constantly sampling and updating the adc data register. the first conversion must be started by writing a logical one to the adsc bit in adcsra. in this adsc adif source 1 source n adts[2:0] conversion logic prescaler start clk adc . . . . edge detector adate
234 atmega48/88/168 2545d?avr?07/04 mode the adc will perform successive c onversions independently of whether the adc interrupt flag, adif is cleared or not. if auto triggering is enabled, single conversions can be started by writing adsc in adcsra to one. adsc can also be used to determine if a conversion is in progress. the adsc bit will be read as one during a conversion, indep endently of how the conver- sion was started. prescaling and conversion timing figure 103. adc prescaler by default, the successive approximation circuitry requires an input clock frequency between 50 khz and 200 khz to get maximum resolution. if a lower resolution than 10 bits is needed, the input clock frequency to the adc can be higher than 200 khz to get a higher sample rate. the adc module contains a prescaler, which generates an acceptable adc clock fre- quency from any cpu frequency above 100 khz. the prescaling is set by the adps bits in adcsra. the prescaler starts counting from the moment the adc is switched on by setting the aden bit in adcsra. the prescaler keeps running for as long as the aden bit is set, and is continuously reset when aden is low. when initiating a single ended conversion by setting the adsc bit in adcsra, the con- version starts at the following rising edge of the adc clock cycle. a normal conversion takes 13 adc clock cycl es. the first conversion after the adc is switched on (aden in adcsra is set) takes 25 adc clock cycles in order to initialize the analog circuitry. the actual sample-and-hold takes place 1.5 adc clock cycles after the start of a normal conversion and 13.5 adc clock cycles after th e start of an first conversion. when a con- version is complete, the result is written to the adc data registers, and adif is set. in single conversion mode, adsc is cleared simultaneously. the software may then set adsc again, and a new conver sion will be initiated on the fi rst rising adc clock edge. when auto triggering is used, the prescaler is reset when the trigger event occurs. this assures a fixed delay from the trigger event to the start of conversion. in this mode, the sample-and-hold takes place two adc clock cycl es after the rising edge on the trigger source signal. three additional cpu clock cycles are used for synchronization logic. 7-bit adc prescaler adc clock source ck adps0 adps1 adps2 ck/128 ck/2 ck/4 ck/8 ck/16 ck/32 ck/64 reset aden start
235 atmega48/88/168 2545d?avr?07/04 in free running mode, a new co nversion will be started im mediately afte r the conver- sion completes, while adsc remains high. for a summary of conversion times, see table 98. figure 104. adc timing diagram, first conversion (single conversion mode) figure 105. adc timing diagram, single conversion figure 106. adc timing diagram, auto triggered conversion sign and msb of result lsb of result adc clock adsc sample & hold adif adch adcl cycle number aden 1 212 13 14 15 16 17 18 1 9 20 21 22 23 24 25 1 2 first conversion next conversion 3 mux and refs update mux and refs update conversion complete 1 2 3 4 5 6 7 8 9 10 11 12 13 sign and msb of result lsb of result adc clock adsc adif adch adcl cycle number 12 one conversion next conversion 3 sample & hold mux and refs update conversion complete mux and refs update 1 2 3 4 5 6 7 8 9 10 11 12 13 sign and msb of result lsb of result adc clock trigger source adif adch adcl cycle number 12 one conversion next conversion conversion complete prescaler reset adate prescaler reset sample & hold mux and refs update
236 atmega48/88/168 2545d?avr?07/04 figure 107. adc timing diagram, free running conversion changing channel or reference selection the muxn and refs1:0 bits in the admux register are single buffered through a tem- porary register to which the cpu has random access. this ensures that the channels and reference selection only takes place at a safe point during the conversion. the channel and reference selection is continuously updated until a conversion is started. once the conversion starts, the channel and reference selection is locked to ensure a sufficient sampling time for the adc. c ontinuous updating resumes in the last adc clock cycle before the conversion completes (a dif in adcsra is set). note that the conversion starts on the following rising adc clock edge after adsc is written. the user is thus advised not to write new channel or reference selection values to admux until one adc clock cycle after adsc is written. if auto triggering is used, the exact time of the triggering event can be indeterministic. special care must be taken when updating the admux register, in order to control which conversion will be affe cted by the new settings. if both adate and aden is written to one, an interrupt event can occur at any time. if the admux register is changed in this period, the user cannot tell if the next conversion is based on the old or the new settings. admux can be safely updated in the following ways: 1. when adate or aden is cleared. 2. during conversion, minimum one adc clock cycle after the trigger event. 3. after a conversion, before the interrupt flag used as trigger source is cleared. when updating admux in one of these conditions, the new settings will affect the next adc conversion. table 98. adc conversion time condition sample & hold (cycles from start of conversion) conversion time (cycles) first conversion 13.5 25 normal conversions, single ended 1.5 13 auto triggered conversions 2 13.5 11 12 13 sign and msb of result lsb of result adc clock adsc adif adch adcl cycle number 12 one conversion next conversion 34 conversion complete sample & hold mux and refs update
237 atmega48/88/168 2545d?avr?07/04 adc input channels when changing channel selections, the user should observe the following guidelines to ensure that the correct channel is selected: in single conversion mode, always select the channel before starting the conversion. the channel selection may be changed one adc clock cycle af ter writing one to adsc. however, the simplest method is to wait for the conversion to complete before changing the channel selection. in free running mode, always select the channel before starting the first conversion. the channel selection may be changed one adc clock cycle af ter writing one to adsc. however, the simplest method is to wait for the first conversion to complete, and then change the channel selection. since the next conversion has already started automati- cally, the next result will reflect the previo us channel selection. subsequent conversions will reflect the new channel selection. adc voltage reference the reference voltage for the adc (v ref ) indicates the conversion range for the adc. single ended channels that exceed v ref will result in codes close to 0x3ff. v ref can be selected as either av cc , internal 1.1v reference, or external aref pin. av cc is connected to the adc through a passiv e switch. the internal 1.1v reference is generated from the internal bandgap reference (v bg ) through an internal amplifier. in either case, the external aref pin is dire ctly connected to the adc, and the reference voltage can be made more immune to noise by connecting a capacitor between the aref pin and ground. v ref can also be measured at the aref pin with a high impedant voltmeter. note that v ref is a high impedant source, and only a capacitive load should be connected in a system. if the user has a fixed voltage source connected to the aref pin, the user may not use the other reference voltage options in the application, as they will be shorted to the external voltage. if no external voltage is applied to the aref pin, the user may switch between av cc and 1.1v as reference selection. the first adc conversion result after switching reference voltage source may be i naccurate, and the user is advised to dis- card this result. adc noise canceler the adc features a noise canceler that enables conversion during sleep mode to reduce noise induced from the cpu core and other i/o peripherals. the noise canceler can be used with adc noise reduction and idle mode. to make use of this feature, the following procedure should be used: 1. make sure that the adc is enabled and is not busy converting. single con- version mode must be selected and the adc conversion complete interrupt must be enabled. 2. enter adc noise reduction mode (or idle mode). the adc will start a con- version once the cpu has been halted. 3. if no other interrupts occur before the adc conversion completes, the adc interrupt will wake up the cpu and ex ecute the adc conversion complete interrupt routine. if another interrupt wakes up the cpu before the adc con- version is complete, that interrupt will be executed, and an adc conversion complete interrupt request will be ge nerated when the adc conversion completes. the cpu will remain in ac tive mode until a new sleep command is executed. note that the adc will not be automatically tu rned off when entering other sleep modes than idle mode and adc nois e reduction mode. the user is advised to write zero to aden before entering such sleep modes to avoid excessive power consumption.
238 atmega48/88/168 2545d?avr?07/04 analog input circuitry the analog input circuitry for single ended channels is illust rated in figure 108. an ana- log source applied to adcn is subjected to the pin capacitance and input leakage of that pin, regardless of whether that channel is selected as input for the adc. when the chan- nel is selected, the source must drive the s/h capacitor through the series resistance (combined resistance in the input path). the adc is optimized for analog signals with an output impedance of approximately 10 k ? or less. if such a source is used, the sampling time will be negligible. if a source with higher impedance is used, the sampling time will depend on how long time the source needs to charge the s/h capacitor, wi th can vary widely. the user is recom- mended to only use low impedant sources wit h slowly varying signals, since this minimizes the required charge transfer to the s/h capacitor. signal components higher than the nyquist frequency (f adc /2) should not be present for either kind of channels, to avoid distortion from unpredictable signal convolution. the user is advised to remove high frequency components with a low-pass filter before applying the signals as inputs to the adc. figure 108. analog input circuitry analog noise canceling techniques digital circuitry inside and outside the device generates emi which might affect the accuracy of analog measurements. if conversion accuracy is critical, the noise level can be reduced by applying the following techniques: 1. keep analog signal paths as short as possible. make sure analog tracks run over the analog ground plane, and keep them well away from high-speed switching digital tracks. 2. the av cc pin on the device should be connected to the digital v cc supply voltage via an lc network as shown in figure 109. 3. use the adc noise canceler function to reduce induced noise from the cpu. 4. if any adc [3..0] port pins are used as digital outputs, it is essential that these do not switch while a conversion is in progress. however, using the 2- wire interface (adc4 and adc5) will on ly affect the conversion on adc4 and adc5 and not the other adc channels. adcn i ih 1..100 k ? c s/h = 14 pf v cc /2 i il
239 atmega48/88/168 2545d?avr?07/04 figure 109. adc power connections adc accuracy definitions an n-bit single-ended adc converts a voltage linearly between gnd and v ref in 2 n steps (lsbs). the lowest code is read as 0, and the highest code is read as 2 n -1. several parameters describe the deviation from the ideal behavior: offset: the deviation of the first transition (0x000 to 0x001) compared to the ideal transition (at 0.5 lsb). ideal value: 0 lsb. figure 110. offset error gnd vcc pc5 (adc5/scl) pc4 (adc4/sda) pc3 (adc3) pc2 (adc2) pc1 (adc1) pc0 (adc0) adc7 gnd aref avcc adc6 pb5 10 h 100nf analog ground plane output code v ref input voltage ideal adc actual adc offset error
240 atmega48/88/168 2545d?avr?07/04 gain error: after adjusting for offset, the gain error is found as the deviation of the last transition (0x3fe to 0x3ff) compared to the ideal transition (at 1.5 lsb below maximum). ideal value: 0 lsb figure 111. gain error integral non-linearity (inl): after adjusting for offset and gain error, the inl is the maximum deviation of an actual transition compared to an ideal transition for any code. ideal value: 0 lsb. figure 112. integral non-linearity (inl) output code v ref input voltage ideal adc actual adc gain error output code v ref input voltage ideal adc actual adc inl
241 atmega48/88/168 2545d?avr?07/04 differential non-linearity ( dnl): the maximum deviation of the actual code width (the interval between two adjacent transitions) from the ideal code width (1 lsb). ideal value: 0 lsb. figure 113. differential non-linearity (dnl) quantization error: due to the quantization of the input voltage into a finite number of codes, a range of input voltages (1 l sb wide) will code to the same value. always 0.5 lsb. absolute accuracy: the maximum deviation of an actual (unadjusted) transition compared to an ideal transition for any code. this is the compound effect of offset, gain error, differential error, non-linearity, and quantization error. ideal value: 0.5 lsb. adc conversion result after the conversion is complete (adif is high), the conversion result can be found in the adc result registers (adcl, adch). for single ended conversion, the result is where v in is the voltage on the selected input pin and v ref the selected voltage refer- ence (see table 99 on page 242 and table 100 on page 242). 0x000 represents analog ground, and 0x3ff represents the selected reference voltage minus one lsb. adc multiplexer selection register ? admux bit 7:6 ? refs1:0: reference selection bits these bits select the voltage reference for the adc, as shown in table 99. if these bits are changed during a co nversion, the change will not go in effect until this conversion is complete (adif in adcsra is set). the internal voltage reference options may not be used if an external reference voltage is being applied to the aref pin. output code 0x3ff 0x000 0 v ref input voltage dnl 1 lsb adc v in 1024 ? v ref -------------------------- = bit 76543210 refs1 refs0 adlar ? mux3 mux2 mux1 mux0 admux read/write r/w r/w r/w r r/w r/w r/w r/w initial value00000000
242 atmega48/88/168 2545d?avr?07/04 bit 5 ? adlar: adc left adjust result the adlar bit affects the presentation of the adc conversion result in the adc data register. write one to adlar to left adjust the result. otherwise, the result is right adjusted. changing the adlar bit will affe ct the adc data register immediately, regardless of any ongoing conversions. for a complete description of this bit, see ?the adc data register ? adcl and adch? on page 244. bit 4 ? res: reserved bit this bit is an unused bit in the atmega4 8/88/168, and will always read as zero. bits 3:0 ? mux3:0: analog channel selection bits the value of these bits selects which analog inputs are connected to the adc. see table 100 for details. if these bits are changed during a conversion, the change will not go in effect until this conversion is complete (adif in adcsra is set). adc control and status register a ? adcsra table 99. voltage reference selections for adc refs1 refs0 voltage reference selection 0 0 aref, internal v ref turned off 01 av cc with external capacitor at aref pin 10reserved 1 1 internal 1.1v voltage reference with external capacitor at aref pin table 100. input channel selections mux3..0 single ended input 0000 adc0 0001 adc1 0010 adc2 0011 adc3 0100 adc4 0101 adc5 0110 adc6 0111 adc7 1000 (reserved) 1001 (reserved) 1010 (reserved) 1011 (reserved) 1100 (reserved) 1101 (reserved) 1110 1.1v (v bg ) 1111 0v (gnd) bit 76543210 aden adsc adate adif adie adps2 adps1 adps0 adcsra
243 atmega48/88/168 2545d?avr?07/04 bit 7 ? aden: adc enable writing this bit to one enables the adc. by writing it to ze ro, the adc is turned off. turn- ing the adc off while a conversion is in progress, will terminate this conversion. bit 6 ? adsc: adc start conversion in single conversion mode, write this bit to one to start each conversion. in free run- ning mode, write this bit to one to start th e first conversion. the first conversion after adsc has been written after the adc has been enabled, or if adsc is written at the same time as the adc is enabled, will take 25 adc clock cycles instead of the normal 13. this first conversion performs initialization of the adc. adsc will read as one as long as a conversion is in progress. when the conversion is complete, it returns to zero. writing zero to this bit has no effect. bit 5 ? adate: adc auto trigger enable when this bit is written to on e, auto triggering of the ad c is enabled. the adc will start a conversion on a positive edge of the selected trigger signal. the trigger source is selected by setting the adc trigge r select bits, adts in adcsrb. bit 4 ? adif: adc interrupt flag this bit is set when an adc conversion completes and the data registers are updated. the adc conversion complete interrupt is executed if the adie bit and the i-bit in sreg are set. adif is cleared by hardware when executing the corresponding interrupt handling vector. alternatively, adif is cleared by writing a logical one to the flag. beware that if doing a read-modify-write on adcsra, a pending interrupt can be dis- abled. this also applies if the sbi and cbi instructions are used. bit 3 ? adie: adc interrupt enable when this bit is written to one and the i-bit in sreg is set, the adc conversion com- plete interrupt is activated. bits 2:0 ? adps2:0: adc prescaler select bits these bits determine the division factor between the system clock frequency and the input clock to the adc. read/write r/w r/w r/w r/w r/w r/w r/w r/w initial value 0 0 0 0 0 0 0 0 table 101. adc prescaler selections adps2 adps1 adps0 division factor 000 2 001 2 010 4 011 8 100 16 101 32 110 64 1 1 1 128
244 atmega48/88/168 2545d?avr?07/04 the adc data register ? adcl and adch adlar = 0 adlar = 1 when an adc conversion is complete, the result is found in these two registers. when adcl is read, the adc data register is not updated until adch is read. conse- quently, if the result is left adjusted and no mo re than 8-bit precision is required, it is sufficient to read adch. otherwise, adcl must be read first, then adch. the adlar bit in admux, and the muxn bits in admux affect the way the result is read from the registers. if adlar is set, the result is left adjusted. if adlar is cleared (default), the result is right adjusted. adc9:0: adc conversion result these bits represent the result from the conversion, as detailed in ?adc conversion result? on page 241. adc control and status register b ? adcsrb bit 7, 5:3 ? res: reserved bits these bits are reserved for fu ture use. to ensure compatibility with future devices, these bist must be written to ze ro when adcsrb is written. bit 2:0 ? adts2:0: adc auto trigger source if adate in adcsra is written to one, the value of these bits se lects which source will trigger an adc conversion. if adate is cleared, the adts2:0 settings will have no effect. a conversion will be triggered by the rising edge of the selected interrupt flag. note that switching from a trigger source that is cleared to a trigger source that is set, will generate a positive e dge on the trigger signal. if aden in adcsra is set, this will bit 151413121110 9 8 ? ? ? ? ? ? adc9 adc8 adch adc7 adc6 adc5 adc4 adc3 adc2 adc1 adc0 adcl 76543210 read/writerrrrrrrr rrrrrrrr initial value00000000 00000000 bit 151413121110 9 8 adc9 adc8 adc7 adc6 adc5 adc4 adc3 adc2 adch adc1 adc0 ? ????? adcl 76543210 read/writerrrrrrrr rrrrrrrr initial value00000000 00000000 bit 76543210 ? acme ? ? ? adts2 adts1 adts0 adcsrb read/write r r/w r r r r/w r/w r/w initial value 0 0 0 0 0 0 0 0
245 atmega48/88/168 2545d?avr?07/04 start a conversion. switching to free runn ing mode (adts[2:0]=0 ) will not cause a trig- ger event, even if the adc interrupt flag is set . digital input disable register 0 ? didr0 bits 7:6 ? res: reserved bits these bits are reserved for fu ture use. to ensure compatibility with future devices, these bits must be written to ze ro when didr0 is written. bit 5..0 ? adc5d..adc0d: a dc5..0 digital input disable when this bit is written logic one, the digita l input buffer on the corresponding adc pin is disabled. the corresponding pin register bit will always read as zero when this bit is set. when an analog signal is applied to the adc5..0 pin and the digital input from this pin is not needed, this bit should be writt en logic one to reduce power consumption in the digital input buffer. note that adc pins adc7 and adc6 do not have digital input buffers, and therefore do not require digital input disable bits. table 102. adc auto trigger source selections adts2 adts1 adts0 trigger source 0 0 0 free running mode 0 0 1 analog comparator 0 1 0 external interrupt request 0 0 1 1 timer/counter0 compare match a 1 0 0 timer/counter0 overflow 1 0 1 timer/counter1 compare match b 1 1 0 timer/counter1 overflow 1 1 1 timer/counter1 capture event bit 76543210 ? ? adc5d adc4d adc3d adc2d adc1d adc0d didr0 read/write r r r/w r/w r/w r/w r/w r/w initial value00000000
246 atmega48/88/168 2545d?avr?07/04 debugwire on-chip debug system features complete program flow control emulates all on-chip func tions, both digital and analog, except reset pin real-time operation symbolic debugging support (both at c and assembler source level, or for other hlls) unlimited number of program break po ints (using software break points) non-intrusive operation electrical characteristics identical to real device automatic configuration system high-speed operation programming of non-volatile memories overview the debugwire on-chip debug system uses a on e-wire, bi-directional interface to con- trol the program flow, execute avr instructions in the cpu and to program the different non-volatile memories. physical interface when the debugwire enable (dwen) fuse is programmed and lock bits are unpro- grammed, the debugwire system within the target device is activated. the reset port pin is configured as a wire-and (open-drain) bi-directional i/o pin with pull-up enabled and becomes the communication gateway between target and emulator. figure 114. the debugwire setup figure 114 shows the schematic of a target mcu, with debugwire enabled, and the emulator connector. the system clock is not affected by debugwire and will always be the clock source selected by the cksel fuses. when designing a system where debugwire will be used, the following observations must be made for correct operation: pull-up resistors on the dw/(reset) line must not be smaller than 10k ? . the pull- up resistor is not required for debugwire functionality. connecting the reset pin directly to v cc will not work. dw gnd dw(reset) vcc 1.8 - 5.5v
247 atmega48/88/168 2545d?avr?07/04 capacitors connected to the reset pin must be disconnected when using debugwire. all external reset sources must be disconnected. software break points debugwire supports program memory break po ints by the avr break instruction. set- ting a break point in avr studio ? will insert a break inst ruction in the program memory. the instruction replaced by the br eak instruction will be stored. when pro- gram execution is continued, the stored in struction will be executed before continuing from the program memory. a break can be inserted manually by putting the break instruction in the program. the flash must be re-programmed each time a break point is changed. this is auto- matically handled by avr studio through the debugwire interface. the use of break points will therefore reduce the flash data retention. devices used for debugging pur- poses should not be shipped to end customers. limitations of debugwire the debugwire communication pin (dw) is physically located on the same pin as external reset (reset). an external reset source is therefore not supported when the debugwire is enabled. the debugwire system accurately emulates all i/o functions when running at full speed, i.e., when the program in the cpu is running. when the cpu is stopped, care must be taken while accessing some of the i/o registers via the debugger (avr studio). a programmed dwen fuse enables some parts of the clock system to be running in all sleep modes. this will increase the power consumption while in sleep. thus, the dwen fuse should be disabled when debugwire is not used. debugwire related register in i/o memory the following section describes the registers used with the debugwire. debugwire data register ? dwdr the dwdr register provides a communica tion channel from the running program in the mcu to the debugger. this register is only accessible by the debugwire and can therefore not be used as a general purpose register in the normal operations. bit 76543210 dwdr[7:0] dwdr read/write r/w r/w r/w r/w r/w r/w r/w r/w initial value00000000
248 atmega48/88/168 2545d?avr?07/04 self-programming the flash, atmega48 in atmega48, there is no read-while-write support, and no separate boot loader sec- tion. the spm instruction can be executed from the entire flash. the device provides a self-programming mechanism for downloading and uploading program code by the mcu itself. the self -programming can use any available data interface and associated protocol to read code and write (program) that code into the program memory. the program memory is updated in a page by page fashion. before programming a page with the data stored in the temporary page buffer, the page must be erased. the temporary page buffer is filled one word at a time using spm and the buffer can be filled either before the page erase command or between a page erase and a page write operation: alternative 1, fill the bu ffer before a page erase fill temporary page buffer perform a page erase perform a page write alternative 2, fill the bu ffer after page erase perform a page erase fill temporary page buffer perform a page write if only a part of the page needs to be changed, the rest of the page must be stored (for example in the temporary page buffer) before the erase, and then be re-written. when using alternative 1, the boot loader provi des an effective read-modify-write feature which allows the user software to first read the page, do the necessary changes, and then write back the modified data. if alternativ e 2 is used, it is not possible to read the old data while loading since the page is already erased. the temporary page buffer can be accessed in a random sequence. it is essential that the page address used in both the page erase and page write operation is addressing the same page. performing page erase by spm to execute page erase, set up the address in the z-pointer, write ?00000011? to spmcsr and execute spm within four clock cycles after writing spmcsr. the data in r1 and r0 is ignored. the page address must be written to pcpage in the z-register. other bits in the z-pointer will be ignored during this operation. the cpu is halted during the page erase operation. filling the temporary buffer (page loading) to write an instruction word, set up the addre ss in the z-pointer and data in r1:r0, write ?00000001? to spmcsr and execute spm within four clock cycles after writing spmcsr. the content of pcword in the z-register is used to address the data in the temporary buffer. the temporary buffer will auto-erase after a page write operation or by writing the rwwsre bit in spmcsr. it is also erased after a syst em reset. note that it is not possible to write more than one time to each address without erasing the tempo- rary buffer. if the eeprom is written in th e middle of an spm page load operation, a ll data loaded will be lost. performing a page write to execute page write, set up the address in the z-pointer, write ?00000101? to spmcsr and execute spm within four clock cycles after writing spmcsr. the data in r1 and r0 is ignored. the page address must be written to pcpage. other bits in the z-pointer must be written to zero during this operation.
249 atmega48/88/168 2545d?avr?07/04 the cpu is halted during the page write operation. addressing the flash during self- programming the z-pointer is used to address the spm commands. since the flash is organized in pages (see table 122 on page 274), the program counter can be treated as having two different sections. one section, consisting of the least significant bits, is addressing the wo rds within a page, while the most significant bits are addressing the pages. this is shown in figure 118. note that the page erase and page write operations are addressed independently. therefore it is of major impor- tance that the software addresses the same page in both the page erase and page write operation. the lpm instruction uses the z-pointer to store the address. since this instruction addresses the flash byte-by-byte, also th e lsb (bit z0) of the z-pointer is used. figure 115. addressing the flash during spm (1) note: 1. the different variables used in figur e 118 are listed in table 122 on page 274. bit 151413121110 9 8 zh (r31) z15 z14 z13 z12 z11 z10 z9 z8 zl (r30) z7z6z5z4z3z2z1z0 76543210 program memory 0 1 15 z - register bit 0 zpagemsb word address within a page page address within the flash zpcmsb instruction word pag e pcword[pagemsb:0]: 00 01 02 pageend pag e pcword pcpage pcmsb pagemsb program counter
250 atmega48/88/168 2545d?avr?07/04 store program memory control and status register ? spmcsr the store program memory control and status register contains the control bits needed to control the program memory operations. bit 7 ? spmie: spm interrupt enable when the spmie bit is written to one, and the i-bit in the status register is set (one), the spm ready interrupt will be enabled. the sp m ready interrupt will be executed as long as the selfprgen bit in the spmcsr register is cleared. bit 6 ? rwwsb: read-while-write section busy this bit is for compatibility with devices s upporting read-while-write. it will always read as zero in atmega48. bit 5 ? res: reserved bit this bit is a reserved bit in the atmega4 8/88/168 and will alwa ys read as zero. bit 4 ? rwwsre: read-while-write section read enable the functionality of this bit in atm ega48 is a subset of the functionality in atmega88/168. if the rwwsre bit is written while f illing the temporary page buffer, the temporary page buffer will be cleared and the data will be lost. bit 3 ? blbset: boot lock bit set the functionality of this bit in atm ega48 is a subset of the functionality in atmega88/168. an lpm inst ruction within three cycles after blbset and selfprgen are set in the spmcsr register, will read either the lock bits or th e fuse bits (depend- ing on z0 in the z-pointer) into the destination register. see ?reading the fuse and lock bits from software? on page 251 for details. bit 2 ? pgwrt: page write if this bit is written to one at the same time as selfprgen, the next spm instruction within four clock cycles executes page writ e, with the data stored in the temporary buffer. the page address is taken from the high part of the z-pointer. the data in r1 and r0 are ignored. the pgwrt bi t will auto-clear upon co mpletion of a page write, or if no spm instruction is executed within four clock cycles. the cpu is halted during the entire page write operation. bit 1 ? pgers: page erase if this bit is written to one at the same time as selfprgen, the next spm instruction within four clock cycles executes page eras e. the page address is taken from the high part of the z-pointer. the da ta in r1 and r0 are ignore d. the pgers bit will auto-clear upon completion of a page erase, or if no spm instruction is executed within four clock cycles. the cpu is halted during the entire page write operation. bit 0 ? selfprgen: self programming enable this bit enables the spm instruction for the next four clock cycles. if written to one together with either rwwsre, blbset, pgwrt, or pgers, the following spm instruction will have a special meaning, se e description above. if only selfprgen is written, the following spm inst ruction will store the value in r1:r0 in the temporary page buffer addressed by the z-pointer. the lsb of the z-pointer is ignored. the self- prgen bit will auto-clear upon completion of an spm in struction, or if no spm instruction is executed within four clock cycl es. during page erase and page write, the selfprgen bit remains high until the operation is completed. bit 7 6 5 4 3 2 1 0 spmie rwwsb ? rwwsre blbset pgwrt pgers selfprgen spmcsr read/write r/w r r r/w r/w r/w r/w r/w initial value 0 0 0 0 0 0 0 0
251 atmega48/88/168 2545d?avr?07/04 writing any other combination than ?10001?, ?01001?, ?00101?, ?00011? or ?00001? in the lower five bits will have no effect. eeprom write prevents writing to spmcsr note that an eeprom write operation will block all software programming to flash. reading the fuses and lock bits from software will al so be prevented during the eeprom write operation. it is recommended that th e user checks the status bit (eepe) in the eecr register and verifies that the bit is cleared before writing to the spmcsr register. reading the fuse and lock bits from software it is possible to read both the fuse and lock bits from software. to read the lock bits, load the z-pointer with 0x0001 and set t he blbset and selfprgen bits in spmcsr. when an lpm instruction is executed withi n three cpu cycles after the blbset and selfprgen bits are set in spmcsr, the value of the lock bits will be loaded in the destination regi ster. the blbset and selfprgen bi ts will auto-clear upon completion of reading the lock bits or if no lpm inst ruction is executed within three cpu cycles or no spm instruction is executed within four cpu cycles. when blbset and self- prgen are cleared, lpm will work as de scribed in the inst ruction set manual. the algorithm for reading the fuse low byte is similar to the one described above for reading the lock bits. to read the fuse low byte, load the z-pointer with 0x0000 and set the blbset and selfprgen bits in sp mcsr. when an lpm instruction is exe- cuted within three cycles after the bl bset and selfprgen bits are set in the spmcsr, the value of the fuse low byte (flb ) will be loaded in th e destination register as shown below.see table 119 on page 272 for a detailed description and mapping of the fuse low byte. similarly, when reading the fuse high by te (fhb), load 0x0003 in the z-pointer. when an lpm instruction is executed within th ree cycles after the blbset and selfprgen bits are set in the spmcsr, the value of th e fuse high byte will be loaded in the desti- nation register as shown below. see table 118 on page 272 for detailed description and mapping of the extended fuse byte. similarly, when reading the extended fuse byte (efb), load 0x0002 in the z-pointer. when an lpm instruction is executed within three cycles after the blbset and self- prgen bits are set in the spmcsr, the valu e of the extended fuse byte will be loaded in the destination register as shown below. see table 119 on page 272 for detailed description and mapping of the extended fuse byte. fuse and lock bits that are programmed, will be read as zero. fuse and lock bits that are unprogrammed, will be read as one. preventing flash corruption during periods of low v cc , the flash program can be corrupted because the supply volt- age is too low for the cpu and the flash to operate properly. these issues are the same as for board level systems using the flash, and the same design solutions should be applied. bit 76543210 rd ??????lb2lb1 bit 76543210 rd flb7 flb6 flb5 flb4 flb3 flb2 flb1 flb0 bit 76543210 rd fhb7 fhb6 fhb5 fhb4 fhb3 fhb2 fhb1 fhb0 bit 76543210 rd fhb7 fhb6 fhb5 fhb4 fhb3 fhb2 fhb1 fhb0
252 atmega48/88/168 2545d?avr?07/04 a flash program corruption can be caused by two situations when the voltage is too low. first, a regular write sequence to the flash requires a minimum voltage to operate cor- rectly. secondly, the cpu itself can execute in structions incorrectly, if the supply voltage for executing instructions is too low. flash corruption can easily be avoided by following these design recommendations (one is sufficient): 1. keep the avr reset active (low) duri ng periods of insufficient power supply voltage. this can be done by enabling the internal brown-out detector (bod) if the operating voltage matches the detection level. if not, an external low v cc reset protection circuit can be used. if a reset occurs while a write operation is in progress, the write operation will be comp leted provided that the power supply voltage is sufficient. 2. keep the avr core in power-down sleep mode during periods of low v cc . this will prevent the cpu from attempting to decode and execute instructions, effec- tively protecting the spmcsr register and thus the flash from unintentional writes. programming time for flash when using spm the calibrated rc oscillator is used to time flash accesses. table 108 shows the typi- cal programming time for flash accesses from the cpu. table 103. spm programming time symbol min programming time max programming time flash write (page erase, page write, and write lock bits by spm) 3.7 ms 4.5 ms
253 atmega48/88/168 2545d?avr?07/04 simple assembly code example for a boot loader note that the rwwsb bit will always be read as zero in atmega48. nevertheless, it is recommended to check this bit as shown in the code example, to ensure compatibility with devices supportin g read-while-write. ;-the routine writes one page of data from ram to flash ; the first data location in ram is pointed to by the y pointer ; the first data location in flash is pointed to by the z-pointer ;-error handling is not included ;-the routine must be placed inside the boot space ; (at least the do_spm sub routine). only code inside nrww section can ; be read during self-programming (page erase and page write). ;-registers used: r0, r1, temp1 (r16), temp2 (r17), looplo (r24), ; loophi (r25), spmcrval (r20) ; storing and restoring of registers is not included in the routine ; register usage can be optimized at the expense of code size ;-it is assumed that either the interrupt table is moved to the boot ; loader section or that the interrupts are disabled. .equ pagesizeb = pagesize*2 ;pagesizeb is page size in bytes, not words .org smallbootstart write_page: ; page erase ldi spmcrval, (1< 254 atmega48/88/168 2545d?avr?07/04 sbiw loophi:looplo, 1 ;use subi for pagesi- zeb<=256 brne rdloop ; return to rww section ; verify that rww section is safe to read return: in temp1, spmcsr sbrs temp1, rwwsb ; if rwwsb is set, the rww section is not ready yet ret ; re-enable the rww section ldi spmcrval, (1< 255 atmega48/88/168 2545d?avr?07/04 boot loader support ? read-while-write self-programming, atmega88 and atmega168 in atmega88 and atmega168, the boot loader support provides a real read-while- write self-programming mechanism for downloading and uploading program code by the mcu itself. this feature allows flexible application software updates controlled by the mcu using a flash-resident boot loader program. the boot loader program can use any available data interface and associated protocol to read code and write (pro- gram) that code into the flash memory, or read the code from the program memory. the program code within the boot loader section has the capability to write into the entire flash, including the boot loader memory. the boot loader can thus even modify itself, and it can also erase itself from the code if the feature is not needed anymore. the size of the boot loader memory is configurable with fuses and the boot loader has two sep- arate sets of boot lock bits which can be set independently. this gives the user a unique flexibility to select di fferent levels of protection. boot loader features read-while-write self-programming flexible boot memory size high security (separate boot lock bits for a flexible protection) separate fuse to select reset vector optimized page (1) size code efficien t algorithm efficient read-mod ify-write support note: 1. a page is a section in the flash consisting of several bytes (see table 122 on page 274) used during programming. the page organization does not affect normal operation. application and boot loader flash sections the flash memory is organized in two main sections, the application section and the boot loader section (see figure 117). the size of the different sections is configured by the bootsz fuses as shown in table 109 on page 268 and figure 117. these two sections can have different level of protection since they have different sets of lock bits. application section the application section is the section of the flash that is used for storing the application code. the protection level for the application section can be selected by the application boot lock bits (boot lock bits 0), see table 105 on page 259. the application section can never store any boot loader code since the spm instruction is disabled when exe- cuted from the application section. bls ? boot loader section while the application section is used for storing the application code, the the boot loader software must be located in the bls since the spm instruction can initiate a pro- gramming when executing from the bls only. the spm instruction can access the entire flash, including the bls itself. the protection level for the boot loader section can be selected by the boot loader lock bits (boot lock bits 1), see table 106 on page 259. read-while-write and no read-while-write flash sections whether the cpu supports read-while-write or if the cpu is halted during a boot loader software update is dependent on which address that is being programmed. in addition to the two sections that are conf igurable by the bootsz fuses as described above, the flash is also divided into two fixed sections, the read-while-write (rww) section and the no read-while-write (nrw w) section. the limit between the rww- and nrww sections is given in t able 110 on page 268 and figure 117 on page 258. the main difference between the two sections is: when erasing or writing a page located inside the rww section, the nrww section can be read during the operation. when erasing or writing a page located in side the nrww section, the cpu is halted during the entire operation.
256 atmega48/88/168 2545d?avr?07/04 note that the user software can never read any code that is located inside the rww section during a boot loader software operation. the syntax ?read-while-write sec- tion? refers to which section that is being programmed (erased or written), not which section that actually is being read during a boot loader software update. rww ? read-while-write section if a boot loader software update is programming a page inside the rww section, it is possible to read code from the flash, but only code that is located in the nrww sec- tion. during an on-going programming, the software must ensure that the rww section never is being read. if the user software is trying to read code that is located inside the rww section (i.e., by a call/jmp/lpm or an interrupt) during programming, the software might end up in an unknown state. to avoid this, the interrupts should either be disabled or moved to the boot loader section. the boot loader section is always located in the nrww section. the rww section busy bit (rwwsb) in the store program memory control and status r egister (spmcsr) will be read as lo gical one as long as the rww section is blocked for reading. after a programming is completed, the rwwsb must be cleared by software before reading code located in the rww section. see ?store pro- gram memory control and status register ? spmcsr? on page 260. for details on how to clear rwwsb. nrww ? no read-while-write section the code located in the nrww section can be read when the boot loader software is updating a page in the rww section. when the boot loader code updates the nrww section, the cpu is halted during the entire page erase or page write operation. table 104. read-while-write features which section does the z- pointer address during the programming? which section can be read during programming? is the cpu halted? read-while-write supported? rww section nrww section no yes nrww section none yes no
257 atmega48/88/168 2545d?avr?07/04 figure 116. read-while-write vs. no read-while-write read-while-write (rww) section no read-while-write (nrww) section z-pointer addresses rww section z-pointer addresses nrww section cpu is halted during the operation code located in nrww section can be read during the operation
258 atmega48/88/168 2545d?avr?07/04 figure 117. memory sections note: 1. the parameters in the figure above are given in table 109 on page 268. boot loader lock bits if no boot loader ca pability is needed, the entire flash is availabl e for application code. the boot loader has two separate sets of boot lock bits which can be set indepen- dently. this gives the user a unique flexibility to select different le vels of protection. the user can select: to protect the entire flash from a software update by the mcu. to protect only the boot loader flash section from a software update by the mcu. to protect only the application flash sect ion from a software update by the mcu. allow software update in the entire flash. see table 105 and table 106 for further details . the boot lock bits can be set in soft- ware and in serial or parallel programming mode, but they can be cleared by a chip erase command only. the general write lock (lock bit mode 2) does not control the programming of the flash memory by spm instruction. similarly, the general read/write lock (lock bit mode 1) does not control reading nor writing by lpm/spm, if it is attempted. 0x0000 flashend program memory bootsz = '11' application flash section boot loader flash section flashend program memory bootsz = '10' 0x0000 program memory bootsz = '01' program memory bootsz = '00' application flash section boot loader flash section 0x0000 flashend application flash section flashend end rww start nrww application flash section boot loader flash section boot loader flash section end rww start nrww end rww start nrww 0x0000 end rww, end application start nrww, start boot loader application flash section application flash section application flash section read-while-write section no read-while-write section read-while-write section no read-while-write section read-while-write section no read-while-write section read-while-write section no read-while-write section end application start boot loader end application start boot loader end application start boot loader
259 atmega48/88/168 2545d?avr?07/04 note: 1. ?1? means unprogrammed, ?0? means programmed note: 1. ?1? means unprogrammed, ?0? means programmed entering the boot loader program entering the boot loader takes place by a jump or call from the application program. this may be initiated by a trigger such as a command received via usart, or spi inter- face. alternatively, the boot reset fuse can be programmed so that the reset vector is pointing to the boot flash start address after a reset. in this case, the boot loader is started after a reset. after the application code is loaded, the program can start execut- ing the application code. note that the fuses cannot be changed by the mcu itself. this means that once the boot reset fuse is programmed, the reset vector will always point to the boot loader reset and the fuse can only be changed through the serial or parallel programming interface. note: 1. ?1? means unprogrammed, ?0? means programmed table 105. boot lock bit0 protection modes (application section) (1) blb0 mode blb02 blb01 protection 1 1 1 no restrictions for spm or lpm accessing the application section. 2 1 0 spm is not allowed to write to the application section. 3 0 0 spm is not allowed to write to the application section, and lpm executing from the boot loader section is not allowed to read from the application section. if interrupt vectors are placed in the boot loader section, interrupts are disabled while executing from the application section. 4 0 1 lpm executing from the b oot loader section is not allowed to read from the application section. if interrupt vectors are placed in the boot loader section, interrupts are disabled while executing from the application section. table 106. boot lock bit1 protection modes (boot loader section) (1) blb1 mode blb12 blb11 protection 1 1 1 no restrictions for spm or lpm accessing the boot loader section. 2 1 0 spm is not allowed to write to the boot loader section. 3 0 0 spm is not allowed to write to the boot loader section, and lpm executing from the application section is not allowed to read from the boot loader section. if interrupt vectors are placed in the application section, interrupts are disabled while executing from the boot loader section. 4 0 1 lpm executing from the application section is not allowed to read from the boot loader se ction. if interrupt vectors are placed in the application section, interrupts are disabled while executing from the boot loader section. table 107. boot reset fuse (1) bootrst reset address 1 reset vector = application reset (address 0x0000) 0 reset vector = boot loader reset (see table 109 on page 268)
260 atmega48/88/168 2545d?avr?07/04 store program memory control and status register ? spmcsr the store program memory control and status register contains the control bits needed to control the boot loader operations. bit 7 ? spmie: spm interrupt enable when the spmie bit is written to one, and the i-bit in the status register is set (one), the spm ready interrupt will be enabled. the sp m ready interrupt will be executed as long as the selfprgen bit in the spmcsr register is cleared. bit 6 ? rwwsb: read-while-write section busy when a self-programming (page erase or page write) operation to the rww section is initiated, the rwwsb will be set (one) by hardware. when the rwwsb bit is set, the rww section cannot be accessed. the rwws b bit will be cleared if the rwwsre bit is written to one after a self-programming operation is completed. alternatively the rwwsb bit will automatically be cleared if a page load operat ion is initiated. bit 5 ? res: reserved bit this bit is a reserved bit in the atmega48/88/168 and always read as zero. bit 4 ? rwwsre: read-while-write section read enable when programming (page erase or page write) to the rww section, the rww section is blocked for reading (the rwwsb will be set by hardware). to re-enable the rww section, the user software must wait until the programming is completed (selfprgen will be cleared). then, if the rwwsre bit is written to one at the same time as self- prgen, the next spm instruction within fo ur clock cycles re-enab les the rww section. the rww section cannot be re-enabled while the flash is busy with a page erase or a page write (selfprgen is se t). if the rwwsre bit is writ ten while the flash is being loaded, the flash load ope ration will abort and the da ta loaded will be lost. bit 3 ? blbset: boot lock bit set if this bit is written to one at the same time as selfprgen, the next spm instruction within four clock cycles sets boot lock bits and memory lock bits, according to the data in r0. the data in r1 and th e address in the z-pointer ar e ignored. the blbset bit will automatically be cleared upon completion of the lock bit set, or if no spm instruction is executed within four clock cycles. an lpm instruction within three cycles after blbset and selfprgen are set in the spmcsr register, will read either the lock bits or the fuse bits (depending on z0 in the z-pointer) into the destination register. see ?reading the fuse and lock bits from software? on page 264 for details. bit 2 ? pgwrt: page write if this bit is written to one at the same time as selfprgen, the next spm instruction within four clock cycles executes page writ e, with the data stored in the temporary buffer. the page address is taken from the high part of the z-pointer. the data in r1 and r0 are ignored. the pgwrt bi t will auto-clear upon co mpletion of a page write, or if no spm instruction is executed within four clock cycles. the cpu is halted during the entire page write operation if the nrww section is addressed. bit 1 ? pgers: page erase if this bit is written to one at the same time as selfprgen, the next spm instruction within four clock cycles executes page eras e. the page address is taken from the high bit 7 6 5 4 3 2 1 0 spmie rwwsb ? rwwsre blbset pgwrt pgers selfprgen spmcsr read/write r/w r r r/w r/w r/w r/w r/w initial value 0 0 0 0 0 0 0 0
261 atmega48/88/168 2545d?avr?07/04 part of the z-pointer. the da ta in r1 and r0 are ignore d. the pgers bit will auto-clear upon completion of a page erase, or if no spm instruction is executed within four clock cycles. the cpu is halted during the entire page write operation if the nrww section is addressed. bit 0 ? selfprgen: self programming enable this bit enables the spm instruction for the next four clock cycles. if written to one together with either r wwsre, blbset, pgwrt or pg ers, the following spm instruction will have a special meaning, se e description above. if only selfprgen is written, the following spm inst ruction will store the value in r1:r0 in the temporary page buffer addressed by the z-pointer. the lsb of the z-pointer is ignored. the self- prgen bit will auto-clear upon completion of an spm in struction, or if no spm instruction is executed within four clock cycl es. during page erase and page write, the selfprgen bit remains high until the operation is completed. writing any other combination than ?10001?, ?01001?, ?00101?, ?00011? or ?00001? in the lower five bits will have no effect. addressing the flash during self- programming the z-pointer is used to address the spm commands. since the flash is organized in pages (see table 122 on page 274), the program counter can be treated as having two different sections. one section, consisting of the least significant bits, is addressing the wo rds within a page, while the most significant bits are addressing the pages. this is1 shown in figure 118. note that the page erase and page write operations are addressed independently. therefore it is of major impor- tance that the boot loader software addresses the same page in both the page erase and page write operation. once a programming operation is initiated, the address is latched and the z-pointer can be used for other operations. the only spm operation that does not use the z-pointer is setting the boot loader lock bits. the content of the z-pointer is ignored and will have no effect on the operation. the lpm instruction does also use the z-pointer to store the address. since this instruction addresses the flash byte-by-byte, also th e lsb (bit z0) of the z-pointer is used. bit 151413121110 9 8 zh (r31) z15 z14 z13 z12 z11 z10 z9 z8 zl (r30) z7z6z5z4z3z2z1z0 76543210
262 atmega48/88/168 2545d?avr?07/04 figure 118. addressing the flash during spm (1) note: 1. the different variables used in figur e 118 are listed in table 111 on page 268. self-programming the flash the program memory is updated in a page by page fashion. before programming a page with the data stored in the temporary page buffer, the page must be erased. the temporary page buffer is filled one word at a time using spm and the buffer can be filled either before the page erase command or between a page erase and a page write operation: alternative 1, fill the bu ffer before a page erase fill temporary page buffer perform a page erase perform a page write alternative 2, fill the bu ffer after page erase perform a page erase fill temporary page buffer perform a page write if only a part of the page needs to be changed, the rest of the page must be stored (for example in the temporary page buffer) before the erase, and then be rewritten. when using alternative 1, the boot loader provi des an effective read-modify-write feature which allows the user software to first read the page, do the necessary changes, and then write back the modified data. if alternativ e 2 is used, it is not possible to read the old data while loading since the page is already erased. the temporary page buffer can be accessed in a random sequence. it is essential that the page address used in both the page erase and page write operation is addressing the same page. see ?simple assembly code example for a boot loader? on page 266 for an assembly code example. program memory 0 1 15 z - register bit 0 zpagemsb word address within a page page address within the flash zpcmsb instruction word pag e pcword[pagemsb:0]: 00 01 02 pageend pag e pcword pcpage pcmsb pagemsb program counter
263 atmega48/88/168 2545d?avr?07/04 performing page erase by spm to execute page erase, set up the address in the z-pointer, write ?x0000011? to spmcsr and execute spm within four clock cycles after writing spmcsr. the data in r1 and r0 is ignored. the page address must be written to pcpage in the z-register. other bits in the z-pointer will be ignored during this operation. page erase to the rww section: the nrww section can be read during the page erase. page erase to the nrww section: the cpu is halted during the operation. filling the temporary buffer (page loading) to write an instruction word, set up the addre ss in the z-pointer and data in r1:r0, write ?00000001? to spmcsr and execute spm within four clock cycles after writing spmcsr. the content of pcword in the z-register is used to address the data in the temporary buffer. the temporary buffer will auto-erase after a page write operation or by writing the rwwsre bit in spmcsr. it is also erased after a syst em reset. note that it is not possible to write more than one time to each address without erasing the tempo- rary buffer. if the eeprom is written in th e middle of an spm page load operation, a ll data loaded will be lost. performing a page write to execute page write, set up the address in the z-pointer, write ?x0000101? to spmcsr and execute spm within four clock cycles after writing spmcsr. the data in r1 and r0 is ignored. the page address must be written to pcpage. other bits in the z-pointer must be written to zero during this operation. page write to the rww section: the nrww section can be read during the page write. page write to the nrww section: the cpu is halted during the operation. using the spm interrupt if the spm interrupt is enabled, the spm interrupt will generate a constant interrupt when the selfprgen bit in spmcsr is cleared. this means that the interrupt can be used instead of polling the spmcsr register in software. when using the spm inter- rupt, the interrupt vectors should be moved to the bls section to avoid that an interrupt is accessing the rww section when it is blocked for reading. how to move the inter- rupts is described in ?watchdog timer? on page 46. consideration while updating bls special care must be taken if the user allows the boot loader section to be updated by leaving boot lock bit11 unprogrammed. an accidental write to the boot loader itself can corrupt the entire boot loader, and further software updates might be impossible. if it is not necessary to change the boot loader software itself, it is recommended to program the boot lock bit11 to protect the boot loader software from any internal software changes. prevent reading the rww section during self- programming during self-programming (either page erase or page write), the rww section is always blocked for reading. the user software itself must prevent that this section is addressed during the self pr ogramming oper ation. the rwwsb in the spmcsr will be set as long as the rww section is busy. during self-programming the interrupt vector table should be moved to the bls as described in ?watchdog timer? on page 46, or the interrupts must be disabled. before addressing the rww section after the programming is completed, the user software must clear the rwwsb by writing the rwwsre. see ?simple assembly code example for a bo ot loader? on page 266 for an example.
264 atmega48/88/168 2545d?avr?07/04 setting the boot loader lock bits by spm to set the boot loader lock bits, write the desired data to r0, write ?x0001001? to spmcsr and execute spm within four cloc k cycles after writing spmcsr. the only accessible lock bits are t he boot lock bits that may prevent the application and boot loader section from any software update by the mcu. see table 105 and table 106 for how the different settings of the boot loader bits affect the flash access. if bits 5..2 in r0 are clea red (zero), the corresponding bo ot lock bit will be programmed if an spm instruction is executed within four cycles after blbset and selfprgen are set in spmcsr. the z-pointer is don?t care during this operation, but for future compat- ibility it is recommended to load the z-pointer with 0x0001 (same as used for reading the lo ck bits). for future compatibility it is also recommended to set bits 7, 6, 1, and 0 in r0 to ?1? when writing the lock bits. when programming the lock bits the entire flash can be read during the operation. eeprom write prevents writing to spmcsr note that an eeprom write operation will block all software programming to flash. reading the fuses and lock bits from software will al so be prevented during the eeprom write operation. it is recommended that th e user checks the status bit (eepe) in the eecr register and verifies that the bit is cleared before writing to the spmcsr register. reading the fuse and lock bits from software it is possible to read both the fuse and lock bits from software. to read the lock bits, load the z-pointer with 0x0001 and set t he blbset and selfprgen bits in spmcsr. when an lpm instruction is executed withi n three cpu cycles after the blbset and selfprgen bits are set in spmcsr, the value of the lock bits will be loaded in the destination regi ster. the blbset and selfprgen bi ts will auto-clear upon completion of reading the lock bits or if no lpm inst ruction is executed within three cpu cycles or no spm instruction is executed within four cpu cycles. when blbset and self- prgen are cleared, lpm will work as de scribed in the inst ruction set manual. the algorithm for reading the fuse low byte is similar to the one described above for reading the lock bits. to read the fuse low byte, load the z-pointer with 0x0000 and set the blbset and selfprgen bits in sp mcsr. when an lpm instruction is exe- cuted within three cycles after the bl bset and selfprgen bits are set in the spmcsr, the value of the fuse low byte (flb ) will be loaded in th e destination register as shown below. refer to table 119 on page 272 for a detailed description and mapping of the fuse low byte. similarly, when reading the fuse high byte, load 0x0003 in the z-pointer. when an lpm instruction is executed within three cycles after the blbset and selfprgen bits are set in the spmcsr, the value of the fuse hi gh byte (fhb) will be loaded in the destina- tion register as shown below. refer to table 120 on page 273 for detailed description and mapping of the fuse high byte. bit 76543210 r0 1 1 blb12 blb11 blb02 blb01 1 1 bit 76543210 rd ? ? blb12 blb11 blb02 blb01 lb2 lb1 bit 76543210 rd flb7 flb6 flb5 flb4 flb3 flb2 flb1 flb0 bit 76543210 rd fhb7 fhb6 fhb5 fhb4 fhb3 fhb2 fhb1 fhb0
265 atmega48/88/168 2545d?avr?07/04 when reading the extended fuse byte, load 0x0002 in the z-pointer. when an lpm instruction is executed within three cycles after the blbset and selfprgen bits are set in the spmcsr, the value of the extended fuse byte (efb) will be loaded in the destination register as shown below. refer to table 118 on page 272 for detailed description and mapping of the extended fuse byte. fuse and lock bits that are programmed, will be read as zero. fuse and lock bits that are unprogrammed, will be read as one. preventing flash corruption during periods of low v cc , the flash program can be corrupted because the supply volt- age is too low for the cpu and the flash to operate properly. these issues are the same as for board level systems using the flash, and the same design solutions should be applied. a flash program corruption can be caused by two situations when the voltage is too low. first, a regular write sequence to the flash requires a minimum voltage to operate cor- rectly. secondly, the cpu itself can execute in structions incorrectly, if the supply voltage for executing instructions is too low. flash corruption can easily be avoided by following these design recommendations (one is sufficient): 1. if there is no need for a boot loader update in the system, program the boot loader lock bits to prevent any boot loader software updates. 2. keep the avr reset active (low) duri ng periods of insufficient power supply voltage. this can be done by enabling the internal brown-out detector (bod) if the operating voltage matches the detection level. if not, an external low v cc reset protection circuit can be used. if a reset occurs while a write operation is in progress, the write operation will be comp leted provided that the power supply voltage is sufficient. 3. keep the avr core in power-down sleep mode during periods of low v cc . this will prevent the cpu from attempting to decode and execute instructions, effec- tively protecting the spmcsr register and thus the flash from unintentional writes. programming time for flash when using spm the calibrated rc oscillator is used to time flash accesses. table 108 shows the typi- cal programming time for flash accesses from the cpu. bit 76543210 rd ? ? ? ? efb3 efb2 efb1 efb0 table 108. spm programming time symbol min programming time max programming time flash write (page erase, page write, and write lock bits by spm) 3.7 ms 4.5 ms
266 atmega48/88/168 2545d?avr?07/04 simple assembly code example for a boot loader ;-the routine writes one page of data from ram to flash ; the first data location in ram is pointed to by the y pointer ; the first data location in flash is pointed to by the z-pointer ;-error handling is not included ;-the routine must be placed inside the boot space ; (at least the do_spm sub routine). only code inside nrww section can ; be read during self-programming (page erase and page write). ;-registers used: r0, r1, temp1 (r16), temp2 (r17), looplo (r24), ; loophi (r25), spmcrval (r20) ; storing and restoring of registers is not included in the routine ; register usage can be optimized at the expense of code size ;-it is assumed that either the interrupt table is moved to the boot ; loader section or that the interrupts are disabled. .equ pagesizeb = pagesize*2 ;pagesizeb is page size in bytes, not words .org smallbootstart write_page: ; page erase ldi spmcrval, (1< 267 atmega48/88/168 2545d?avr?07/04 sbiw loophi:looplo, 1 ;use subi for pagesizeb<=256 brne rdloop ; return to rww section ; verify that rww section is safe to read return: in temp1, spmcsr sbrs temp1, rwwsb ; if rwwsb is set, the rww section is not ready yet ret ; re-enable the rww section ldi spmcrval, (1< 268 atmega48/88/168 2545d?avr?07/04 atmega88 boot loader parameters in table 109 through table 111, the parameters used in the description of the self pro- gramming are given. note: the different bootsz fuse configurations are shown in figure 117. for details about these two section, see ?nrww ? no read-while-write section? on page 256 and ?rww ? read-while-write section? on page 256 note: 1. z15:z13: always ignored z0: should be zero for all spm commands, byte select for the lpm instruction. see ?addressing the flash during self-programming? on page 261 for details about the use of z-pointer during self-programming. table 109. boot size configuration, atmega88 bootsz1 bootsz0 boot size pages application flash section boot loader flash section end application section boot reset address (start boot loader section) 11 128 words 4 0x000 - 0xf7f 0xf80 - 0xfff 0xf7f 0xf80 10 256 words 8 0x000 - 0xeff 0xf00 - 0xfff 0xeff 0xf00 01 512 words 16 0x000 - 0xdff 0xe00 - 0xfff 0xdff 0xe00 00 1024 words 32 0x000 - 0xbff 0xc00 - 0xfff 0xbff 0xc00 table 110. read-while-write limit, atmega88 section pages address read-while-write secti on (rww) 96 0x000 - 0xbff no read-while-write section (nrww) 32 0xc00 - 0xfff table 111. explanation of different variables used in figure 118 and the mapping to the z-pointer, atmega88 variable corresponding z-value (1) description pcmsb 11 most significant bit in the program counter. (the program counter is 12 bits pc[11:0]) pagemsb 4 most significant bit which is used to address the words within one page (32 words in a page requires 5 bits pc [4:0]). zpcmsb z12 bit in z-register that is mapped to pcmsb. because z0 is not used, the zpcmsb equals pcmsb + 1. zpagemsb z5 bit in z-register that is mapped to pagemsb. because z0 is not used, the zpagemsb equals pagemsb + 1. pcpage pc[11:5] z12:z6 program counter page address: page select, for page erase and page write pcword pc[4:0] z5:z1 program counter word address: word select, for filling temporary buffer (must be zero during page write operation)
269 atmega48/88/168 2545d?avr?07/04 atmega168 boot loader parameters in table 112 through table 114, the parameters used in the description of the self pro- gramming are given. note: the different bootsz fuse configurations are shown in figure 117. for details about these two section, see ?nrww ? no read-while-write section? on page 256 and ?rww ? read-while-write section? on page 256 note: 1. z15:z14: always ignored z0: should be zero for all spm commands, byte select for the lpm instruction. see ?addressing the flash during self-programming? on page 261 for details about the use of z-pointer during self-programming. table 112. boot size configuration, atmega168 bootsz1 bootsz0 boot size pages application flash section boot loader flash section end application section boot reset address (start boot loader section) 11 128 words 2 0x0000 - 0x1f7f 0x1f80 - 0x1fff 0x1f7f 0x1f80 10 256 words 4 0x0000 - 0x1eff 0x1f00 - 0x1fff 0x1eff 0x1f00 01 512 words 8 0x0000 - 0x1dff 0x1e00 - 0x1fff 0x1dff 0x1e00 00 1024 words 16 0x0000 - 0x1bff 0x1c00 - 0x1fff 0x1bff 0x1c00 table 113. read-while-write limit, atmega168 section pages address read-while-write section (rww) 112 0x0000 - 0x1bff no read-while-write section (nrww) 16 0x1c00 - 0x1fff table 114. explanation of different variables used in figure 118 and the mapping to the z-pointer, atmega168 variable corresponding z-value (1) description pcmsb 12 most significant bit in the program counter. (the program counter is 12 bits pc[11:0]) pag e m s b 5 most significant bit which is used to address the words within one page (64 words in a page requires 6 bits pc [5:0]) zpcmsb z13 bit in z-register that is mapped to pcmsb. because z0 is not used, the zpcmsb equals pcmsb + 1. zpagemsb z6 bit in z-register that is mapped to pagemsb. because z0 is not used, the zpagemsb equals pagemsb + 1. pcpage pc[12:6] z13:z7 program counter page address: page select, for page erase and page write pcword pc[5:0] z6:z1 program counter word address: word select, for filling temporary buffer (must be zero during page write operation)
270 atmega48/88/168 2545d?avr?07/04 memory programming program and data memory lock bits the atmega88/168 provides six lock bits which can be left unprogrammed (?1?) or can be programmed (?0?) to obtain the additional features listed in table 116. the lock bits can only be erased to ?1? with the chip erase command.the atmega48 has no sepa- rate boot loader section. the spm instruction is enabled for the whole flash if the selfprgen fuse is programmed (?0?), otherwise it is disabled. notes: 1. ?1? means unprogrammed, ?0? means programmed 2. only on atmega88/168. notes: 1. program the fuse bits and boot lock bits before programming the lb1 and lb2. 2. ?1? means unprogrammed, ?0? means programmed table 115. lock bit byte (1) lock bit byte bit no de scription default value 7 ? 1 (unprogrammed) 6 ? 1 (unprogrammed) blb12 (2) 5 boot lock bit 1 (unprogrammed) blb11 (2) 4 boot lock bit 1 (unprogrammed) blb02 (2) 3 boot lock bit 1 (unprogrammed) blb01 (2) 2 boot lock bit 1 (unprogrammed) lb2 1 lock bit 1 (unprogrammed) lb1 0 lock bit 1 (unprogrammed) table 116. lock bit protection modes (1)(2) memory lock bits protection type lb mode lb2 lb1 1 1 1 no memory lock features enabled. 210 further programming of the flash and eeprom is disabled in parallel and serial programming mode. the fuse bits are locked in both serial and parallel programming mode. (1) 300 further programming and verification of the flash and eeprom is disabled in parall el and serial programming mode. the boot lock bits and fuse bits are locked in both serial and parallel programming mode. (1)
271 atmega48/88/168 2545d?avr?07/04 notes: 1. program the fuse bits and boot lock bits before programming the lb1 and lb2. 2. ?1? means unprogrammed, ?0? means programmed table 117. lock bit protection modes (1)(2) . only atmega88/168. blb0 mode blb02 blb01 111 no restrictions for spm or lpm accessing the application section. 2 1 0 spm is not allowed to write to the application section. 300 spm is not allowed to write to the application section, and lpm executing from the boot loader section is not allowed to read from the application section. if interrupt vectors are placed in the boot loader section, interrupts are disabled while executing from the application section. 401 lpm executing from the boot loader section is not allowed to read from the application section. if interrupt vectors are placed in the boot loader section, interrupts are disabled while executing from the application section. blb1 mode blb12 blb11 111 no restrictions for spm or lpm accessing the boot loader section. 2 1 0 spm is not allowed to write to the boot loader section. 300 spm is not allowed to write to the boot loader section, and lpm executing from the application section is not allowed to read from the boot loader section. if interrupt vectors are placed in the application section, interrupts are disabled while executing from the boot loader section. 401 lpm executing from the application section is not allowed to read from the boot loader se ction. if interrupt vectors are placed in the application section, interrupts are disabled while executing from the boot loader section.
272 atmega48/88/168 2545d?avr?07/04 fuse bits the atmega48/88/168 has three fuse bytes. table 118 - table 121 describe briefly the functionality of all the fuses and how they are mapped into the fuse bytes. note that the fuses are read as logical zero, ?0?, if they are programmed. note: 1. the default value of bootsz1..0 results in maximum boot size. see table 124 on page 275 for details. table 118. extended fuse byte for mega48 extended fuse byte bit no description default value ?7? 1 ?6? 1 ?5? 1 ?4? 1 ?3? 1 ?2? 1 ?1? 1 selfprgen 0 self programming enable 1 (unprogrammed) table 119. extended fuse byte for mega88/168 extended fuse byte bit no description default value ?7? 1 ?6? 1 ?5? 1 ?4? 1 ?3? 1 bootsz1 2 select boot size (see table 113 for details) 0 (programmed) (1) bootsz0 1 select boot size (see table 113 for details) 0 (programmed) (1) bootrst 0 select reset vector 1 (unprogrammed)
273 atmega48/88/168 2545d?avr?07/04 notes: 1. see ?alternate functions of port c? on page 73 for description of rstdisbl fuse. 2. the spien fuse is not accessible in serial programming mode. 3. see ?watchdog timer control register - wdtcsr? on page 49 for details. 4. see table 21 on page 43 for bodlevel fuse decoding. note: 1. the default value of sut1..0 results in maximum start-up time for the default clock source. see table 12 on page 30 for details. 2. the default setting of cksel3..0 result s in internal rc oscillator @ 8 mhz. see table 11 on page 30 for details. 3. the ckout fuse allows the system clock to be output on po rtb0. see ?clock out- put buffer? on page 32 for details. 4. see ?system clock prescaler? on page 33 for details. the status of the fuse bits is not affected by chip erase. note that the fuse bits are locked if lock bit1 (lb1) is programmed. program the fuse bits before programming the lock bits. table 120. fuse high byte high fuse byte bit no de scription default value rstdisbl (1) 7 external reset disable 1 (unprogrammed) dwen 6 debugwire enable 1 (unprogrammed) spien (2) 5 enable serial program and data downloading 0 (programmed, spi programming enabled) wdton (3) 4 watchdog timer always on 1 (unprogrammed) eesave 3 eeprom memory is preserved through the chip erase 1 (unprogrammed), eeprom not reserved bodlevel2 (4) 2 brown-out detector trigger level 1 (unprogrammed) bodlevel1 (4) 1 brown-out detector trigger level 1 (unprogrammed) bodlevel0 (4) 0 brown-out detector trigger level 1 (unprogrammed) table 121. fuse low byte low fuse byte bit no description default value ckdiv8 (4) 7 divide clock by 8 0 (programmed) ckout (3) 6 clock output 1 (unprogrammed) sut1 5 select start-up time 1 (unprogrammed) (1) sut0 4 select start-up time 0 (programmed) (1) cksel3 3 select clock source 0 (programmed) (2) cksel2 2 select clock source 0 (programmed) (2) cksel1 1 select clock source 1 (unprogrammed) (2) cksel0 0 select clock source 0 (programmed) (2)
274 atmega48/88/168 2545d?avr?07/04 latching of fuses the fuse values are latched when the device enters programming mode and changes of the fuse values will have no effect until the part leaves programming mode. this does not apply to the eesave fuse which will take effect once it is programmed. the fuses are also latched on power-up in normal mode. signature bytes all atmel microcontrollers have a three-byte signature code which identifies the device. this code can be read in both serial and parallel mode, also when the device is locked. the three bytes reside in a separate address space. atmega48 signature bytes 1. 0x000: 0x1e (indicates manufactured by atmel). 2. 0x001: 0x92 (indicates 4kb flash memory). 3. 0x002: 0x05 (indicates atmega48 device when 0x001 is 0x92). atmega88 signature bytes 1. 0x000: 0x1e (indicates manufactured by atmel). 2. 0x001: 0x93 (indicates 8kb flash memory). 3. 0x002: 0x0a (indicates atmega 88 device when 0x001 is 0x93). atmega168 signature bytes 1. 0x000: 0x1e (indicates manufactured by atmel). 2. 0x001: 0x94 (indicates 16kb flash memory). 3. 0x002: 0x06 (indicates atmega168 device when 0x001 is 0x94). calibration byte the atmega48/88/168 has a by te calibration value for the internal rc oscillator. this byte resides in the high byte of address 0x000 in the signature address space. during reset, this byte is automatically written into the osccal register to ensure correct fre- quency of the calibrated rc oscillator. page size table 122. no. of words in a page and no. of pages in the flash device flash size page size pcword no. of pages pcpage pcmsb atmega48 2k words (4k bytes) 32 words pc[4:0] 64 pc[10:5] 10 atmega88 4k words (8k bytes) 32 words pc[4:0] 128 pc[11:5] 11 atmega168 8k words (16k bytes) 64 words pc[5:0] 128 pc[12:6] 12 table 123. no. of words in a page a nd no. of pages in the eeprom device eeprom size page size pcword no. of pages pcpage eeamsb atmega48 256 bytes 4 bytes eea[1:0] 64 eea[7:2] 7 atmega88 512 bytes 4 bytes eea[1:0] 128 eea[8:2] 8 atmega168 512 bytes 4 bytes eea[1:0] 128 eea[8:2] 8
275 atmega48/88/168 2545d?avr?07/04 parallel programming parameters, pin mapping, and commands this section describes how to parallel program and verify flash program memory, eeprom data memory, memory lock bits, an d fuse bits in th e atmega48/88/168. pulses are assumed to be at least 250 ns unless otherwise noted. signal names in this section, some pins of the atmega48/88/168 are referenced by signal names describing their functionality during parallel programming, see figure 119 and table 124. pins not described in the following table are referenced by pin names. the xa1/xa0 pins determine the action executed when the xtal1 pin is given a posi- tive pulse. the bit coding is shown in table 126. when pulsing wr or oe , the command loaded determines the action executed. the dif- ferent commands are shown in table 127. figure 119. parallel programming table 124. pin name mapping signal name in programming mode pin name i/o function rdy/bsy pd1 o 0: device is busy programming, 1: device is ready for new command oe pd2 i output enable (active low) wr pd3 i write pulse (active low) bs1 pd4 i byte select 1 (?0? selects low byte, ?1? selects high byte) xa0 pd5 i xtal action bit 0 xa1 pd6 i xtal action bit 1 vcc +5v gnd xtal1 pd1 pd2 pd3 pd4 pd5 pd6 pc[1:0]:pb[5:0] data reset pd7 +12 v bs1 xa0 xa1 oe rdy/bsy pagel pc2 wr bs2 avcc +5v
276 atmega48/88/168 2545d?avr?07/04 pag e l p d 7 i program memory and eeprom data page load bs2 pc2 i byte select 2 (?0? selects low byte, ?1? selects 2?nd high byte) data {pc[1:0]: pb[5:0]} i/o bi-directional data bus (output when oe is low) table 125. pin values used to enter programming mode pin symbol value pagel prog_enable[3] 0 xa1 prog_enable[2] 0 xa0 prog_enable[1] 0 bs1 prog_enable[0] 0 table 126. xa1 and xa0 coding xa1 xa0 action when xtal1 is pulsed 0 0 load flash or eeprom address (high or low address byte determined by bs1). 0 1 load data (high or low data byte for flash determined by bs1). 1 0 load command 1 1 no action, idle table 127. command byte bit coding command byte command executed 1000 0000 chip erase 0100 0000 write fuse bits 0010 0000 write lock bits 0001 0000 write flash 0001 0001 write eeprom 0000 1000 read signature bytes and calibration byte 0000 0100 read fuse and lock bits 0000 0010 read flash 0000 0011 read eeprom table 124. pin name mapping (continued) signal name in programming mode pin name i/o function
277 atmega48/88/168 2545d?avr?07/04 serial programming pin mapping table 128. pin mapping serial programming symbol pins i/o description mosi pb3 i serial data in miso pb4 o serial data out sck pb5 i serial clock
278 atmega48/88/168 2545d?avr?07/04 parallel programming enter programming mode the following algorithm puts the device in parallel programming mode: 1. apply 4.5 - 5.5v between v cc and gnd. 2. set reset to ?0? and toggle xtal1 at least six times. 3. set the prog_enable pins listed in table 125 on page 276 to ?0000? and wait at least 100 ns. 4. apply 11.5 - 12.5v to reset . any activity on prog_enable pins within 100 ns after +12v has been applied to reset , will cause the device to fail entering pro- gramming mode. 5. wait at least 50 s before sending a new command. considerations for efficient programming the loaded command and address are retained in the device during programming. for efficient programming, the following should be considered. the command needs only be loaded once when writing or reading multiple memory locations. skip writing the data value 0xff, that is the contents of the entire eeprom (unless the eesave fuse is programmed) and flash after a chip erase. address high byte needs only be loaded before programming or reading a new 256 word window in flash or 256 byte eeprom . this consideration also applies to signature bytes reading. chip erase the chip erase will er ase the flash and eeprom (1) memories plus lock bits. the lock bits are not reset until the program memory has been completely erased. the fuse bits are not changed. a chip erase must be performed before the flash and/or eeprom are reprogrammed. note: 1. the eeprpom memory is preserved du ring chip erase if the eesave fuse is programmed. load command ?chip erase? 1. set xa1, xa0 to ?10?. this enables command loading. 2. set bs1 to ?0?. 3. set data to ?1000 0000?. this is the command for chip erase. 4. give xtal1 a positive pulse. this loads the command. 5. give wr a negative pulse. this starts the chip erase. rdy/bsy goes low. 6. wait until rdy/bsy goes high before loading a new command. programming the flash the flash is organized in pages, see table 122 on page 274. when programming the flash, the program data is latched into a page buffer. this allows one page of program data to be programmed simultaneously. the following procedure describes how to pro- gram the entire flash memory: a. load command ?write flash? 1. set xa1, xa0 to ?10?. this enables command loading. 2. set bs1 to ?0?. 3. set data to ?0001 0000?. this is the command for write flash. 4. give xtal1 a positive pulse. this loads the command. b. load address low byte
279 atmega48/88/168 2545d?avr?07/04 1. set xa1, xa0 to ?00?. this enables address loading. 2. set bs1 to ?0?. this selects low address. 3. set data = address low byte (0x00 - 0xff). 4. give xtal1 a positive pulse. this loads the address low byte. c. load data low byte 1. set xa1, xa0 to ?01?. this enables data loading. 2. set data = data low byte (0x00 - 0xff). 3. give xtal1 a positive pulse. this loads the data byte. d. load data high byte 1. set bs1 to ?1?. this selects high data byte. 2. set xa1, xa0 to ?01?. this enables data loading. 3. set data = data high byte (0x00 - 0xff). 4. give xtal1 a positive pulse. this loads the data byte. e. latch data 1. set bs1 to ?1?. this selects high data byte. 2. give pagel a positive pulse. this latches the data bytes. (see figure 121 for signal waveforms) f. repeat b through e until the entire buffer is filled or until all data within the page is loaded. while the lower bits in the address are mapped to words within the page, the higher bits address the pages with in the flash. this is illustrated in figure 120 on page 280. note that if less than eight bits are required to address words in the page (pagesize < 256), the most significant bit(s) in the address low byte are used to address the page when performing a page write. g. load address high byte 1. set xa1, xa0 to ?00?. this enables address loading. 2. set bs1 to ?1?. this selects high address. 3. set data = address high byte (0x00 - 0xff). 4. give xtal1 a positive pulse. this loads the address high byte. h. program page 1. give wr a negative pulse. this starts programming of the entire page of data. rdy/bsy goes low. 2. wait until rdy/bsy goes high (see figure 121 for signal waveforms). i. repeat b through h until the entire flash is programmed or until all data has been programmed. j. end page programming 1. 1. set xa1, xa0 to ?10?. this enables command loading. 2. set data to ?0000 0000?. this is the command for no operation. 3. give xtal1 a positive pulse. this loads the command, and the internal write sig- nals are reset.
280 atmega48/88/168 2545d?avr?07/04 figure 120. addressing the flash which is organized in pages (1) note: 1. pcpage and pcword are listed in table 122 on page 274. figure 121. programming the flash waveforms (1) note: 1. ?xx? is don?t care. the letters re fer to the programming description above. programming the eeprom the eeprom is organized in pages, see table 123 on page 274. when programming the eeprom, the program data is latched into a page buffer . this allows one page of data to be programmed simultaneously. the programming algorithm for the eeprom data memory is as follows (refer to ?programming the flash? on page 278 for details on command, address and data loading): 1. a: load command ?0001 0001?. 2. g: load address high byte (0x00 - 0xff). 3. b: load address low byte (0x00 - 0xff). 4. c: load data (0x00 - 0xff). 5. e: latch data (give pagel a positive pulse). program memory word address within a page page address within the flash instruction word pag e pcword[pagemsb:0]: 00 01 02 pageend pag e pcword pcpage pcmsb pagemsb program counter rdy/bsy wr oe reset +12v pagel bs2 0x10 addr. low addr. high data data low data high addr. low data low data high xa1 xa0 bs1 xtal1 xx xx xx abcdeb cdegh f
281 atmega48/88/168 2545d?avr?07/04 k: repeat 3 through 5 until the entire buffer is filled. l: program eeprom page 1. set bs1 to ?0?. 2. give wr a negative pulse. this starts programming of the eeprom page. rdy/bsy goes low. 3. wait until to rdy/bsy goes high before programming the next page (see figure 122 for signal waveforms). figure 122. programming the eeprom waveforms reading the flash the algorithm for reading the flash memory is as follows (refer to ?programming the flash? on page 278 for details on command and address loading): 1. a: load command ?0000 0010?. 2. g: load address high byte (0x00 - 0xff). 3. b: load address low byte (0x00 - 0xff). 4. set oe to ?0?, and bs1 to ?0?. the flash word low byte can now be read at data. 5. set bs1 to ?1?. the flash word high byte can now be read at data. 6. set oe to ?1?. reading the eeprom the algorithm for readi ng the eeprom memory is as follows (refer to ?programming the flash? on page 278 for details on command and address loading): 1. a: load command ?0000 0011?. 2. g: load address high byte (0x00 - 0xff). 3. b: load address low byte (0x00 - 0xff). 4. set oe to ?0?, and bs1 to ?0?. the eeprom data byte can now be read at data. 5. set oe to ?1?. rdy/bsy wr oe reset +12v pagel bs2 0x11 addr. high data addr. low data addr. low data xx xa1 xa0 bs1 xtal1 xx agbceb c el k
282 atmega48/88/168 2545d?avr?07/04 programming the fuse low bits the algorithm for programming the fuse low bits is as follows (refer to ?programming the flash? on page 278 for details on command and data loading): 1. a: load command ?0100 0000?. 2. c: load data low byte. bit n = ?0? programs and bit n = ?1? erases the fuse bit. 3. give wr a negative pulse and wait for rdy/bsy to go high. programming the fuse high bits the algorithm for programming the fuse high bits is as follows (refer to ?programming the flash? on page 278 for details on command and data loading): 1. a: load command ?0100 0000?. 2. c: load data low byte. bit n = ?0? programs and bit n = ?1? erases the fuse bit. 3. set bs1 to ?1? and bs2 to ?0?. this selects high data byte. 4. give wr a negative pulse and wait for rdy/bsy to go high. 5. set bs1 to ?0?. this selects low data byte. programming the extended fuse bits the algorithm for programming the extended fuse bits is as follows (refer to ?program- ming the flash? on page 278 for details on command and data loading): 1. 1. a: load command ?0100 0000?. 2. 2. c: load data low byte. bit n = ?0? programs and bit n = ?1? erases the fuse bit. 3. 3. set bs1 to ?0? and bs2 to ?1?. this selects extended data byte. 4. 4. give wr a negative pulse and wait for rdy/bsy to go high. 5. 5. set bs2 to ?0?. this selects low data byte. figure 123. programming the fuses waveforms programming the lock bits the algorithm for programming the lock bits is as follows (refer to ?programming the flash? on page 278 for details on command and data loading): 1. a: load command ?0010 0000?. 2. c: load data low byte. bit n = ?0? programs the lock bit. if lb mode 3 is pro- grammed (lb1 and lb2 is programmed), it is not possible to program the boot lock bits by any external programming mode. 3. give wr a negative pulse and wait for rdy/bsy to go high. the lock bits can only be cleared by executing chip erase. rdy/bsy wr oe reset +12v pagel 0x40 data data xx xa1 xa0 bs1 xtal1 ac 0x40 data xx ac write fuse low byte write fuse high byte 0x40 data xx ac write extended fuse byte bs2
283 atmega48/88/168 2545d?avr?07/04 reading the fuse and lock bits the algorithm for reading the fuse and lock bits is as follows (refer to ?programming the flash? on page 278 for details on command loading): 1. a: load command ?0000 0100?. 2. set oe to ?0?, bs2 to ?0? and bs1 to ?0?. the status of the fuse low bits can now be read at data (?0? means programmed). 3. set oe to ?0?, bs2 to ?1? and bs1 to ?1?. the status of the fuse high bits can now be read at data (?0? means programmed). 4. set oe to ?0?, bs2 to ?1?, and bs1 to ?0?. the status of the extended fuse bits can now be read at data (?0? means programmed). 5. set oe to ?0?, bs2 to ?0? and bs1 to ?1?. the status of the lock bits can now be read at data (?0? means programmed). 6. set oe to ?1?. figure 124. mapping between bs1, bs2 and the fuse and lock bits during read reading the signature bytes the algorithm for reading the signature bytes is as follows (refer to ?programming the flash? on page 278 for details on command and address loading): 1. a: load command ?0000 1000?. 2. b: load address low byte (0x00 - 0x02). 3. set oe to ?0?, and bs1 to ?0?. the selected signature byte can now be read at data. 4. set oe to ?1?. reading the calibration byte the algorithm for reading the calibration byte is as follows (refer to ?programming the flash? on page 278 for details on command and address loading): 1. a: load command ?0000 1000?. 2. b: load address low byte, 0x00. 3. set oe to ?0?, and bs1 to ?1?. the calibration byte can now be read at data. 4. set oe to ?1?. lock bits 0 1 bs2 fuse high byte 0 1 bs1 data fuse low byte 0 1 bs2 extended fuse byte
284 atmega48/88/168 2545d?avr?07/04 parallel programming characteristics figure 125. parallel programming timing, including some general timing requirements figure 126. parallel programming timing, loading sequence with timing requirements (1) note: 1. the timing requirements shown in figure 125 (i.e., t dvxh , t xhxl , and t xldx ) also apply to loading operation. data & contol (data, xa0/1, bs1, bs2) xtal1 t xhxl t wlwh t dvxh t xldx t plwl t wlrh wr rdy/bsy pagel t phpl t plbx t bvph t xlwl t wlbx t bvwl wlrl xtal1 pagel t plxh xlxh t t xlph addr0 (low byte) data (low byte) data (high byte) addr1 (low byte) data bs1 xa0 xa1 load address (low byte) load data (low byte) load data (high byte) load data load address (low byte)
285 atmega48/88/168 2545d?avr?07/04 figure 127. parallel programming timing, reading sequence (within the same page) with timing requirements (1) note: 1. the timing requirements shown in figure 125 (i.e., t dvxh , t xhxl , and t xldx ) also apply to reading operation. table 129. parallel programming characteristics, v cc = 5v 10% symbol parameter min typ max units v pp programming enable voltage 11.5 12.5 v i pp programming enable current 250 a t dvxh data and control valid before xtal1 high 67 ns t xlxh xtal1 low to xtal1 high 200 ns t xhxl xtal1 pulse width high 150 ns t xldx data and control hold after xtal1 low 67 ns t xlwl xtal1 low to wr low 0 ns t xlph xtal1 low to pagel high 0 ns t plxh pagel low to xtal1 high 150 ns t bvph bs1 valid before pagel high 67 ns t phpl pagel pulse width high 150 ns t plbx bs1 hold after pagel low 67 ns t wlbx bs2/1 hold after wr low 67 ns t plwl pagel low to wr low 67 ns t bvwl bs1 valid to wr low 67 ns t wlwh wr pulse width low 150 ns t wlrl wr low to rdy/bsy low 0 1 s t wlrh wr low to rdy/bsy high (1) 3.7 4.5 ms t wlrh_ce wr low to rdy/bsy high for chip erase (2) 7.5 9 ms t xlol xtal1 low to oe low 0 ns xtal1 oe addr0 (low byte) data (low byte) data (high byte) addr1 (low byte) data bs1 xa0 xa1 load address (low byte) read data (low byte) read data (high byte) load address (low byte) t bvdv t oldv t xlol t ohdz
286 atmega48/88/168 2545d?avr?07/04 notes: 1. t wlrh is valid for the write flash, write eeprom, write fuse bits and write lock bits commands. 2. t wlrh_ce is valid for the chip erase command. serial downloading both the flash and eeprom memory arrays can be programmed using the serial spi bus while reset is pulled to gnd. the serial interface consists of pins sck, mosi (input) and miso (o utput). after reset is set low, the programming enable instruction needs to be executed first before program/ erase operations can be executed. note, in table 128 on page 277, the pin mapping for spi programming is listed. not all parts use the spi pins dedicated for the internal spi interface. figure 128. serial programming and verify (1) notes: 1. if the device is clocked by the inter nal oscillator, it is no need to connect a clock source to the xtal1 pin. 2. v cc - 0.3v < av cc < v cc + 0.3v, however, av cc should always be within 1.8 - 5.5v when programming the eeprom, an auto-erase cy cle is built into the self-timed pro- gramming operation (in the serial mode only ) and there is no need to first execute the chip erase instruction. the chip erase operation turns the content of every memory location in both the program and eeprom arrays into 0xff. depending on cksel fuses, a valid clock must be present. the minimum low and high periods for the serial clock (sck) input are defined as follows: low:> 2 cpu clock cycles for f ck < 12 mhz, 3 cpu clock cycles for f ck >= 12 mhz high:> 2 cpu clock cycles for f ck < 12 mhz, 3 cpu clock cycles for f ck >= 12 mhz t bvdv bs1 valid to data valid 0 250 ns t oldv oe low to data valid 250 ns t ohdz oe high to data tri-stated 250 ns table 129. parallel programming characteristics, v cc = 5v 10% (continued) symbol parameter min typ max units vcc gnd xtal1 sck miso mosi reset +1.8 - 5.5v avcc +1.8 - 5.5v (2)
287 atmega48/88/168 2545d?avr?07/04 serial programming algorithm when writing serial data to the atmega48/88/168, data is clocked on the rising edge of sck. when reading data from the atmega48/88/168, data is clocked on the falling edge of sck. see figure 129 for timing details. to program and verify the atmega48/88/168 in the serial programming mode, the fol- lowing sequence is recommended (see four byte instruction formats in table 131 ): 1. power-up sequence: apply power between v cc and gnd while reset and sck are set to ?0?. in some systems, the programmer can not guarantee that sck is held low during power-up. in this case, reset must be given a positive pulse of at least two cpu clock cycles duration after sck has been set to ?0?. 2. wait for at least 20 ms and enable serial programming by sending the program- ming enable serial instruction to pin mosi. 3. the serial programming instructions will not work if the communication is out of synchronization. when in sync. the second byte (0x53), will echo back when issuing the third byte of the programming enable instruction. whether the echo is correct or not, all four bytes of the instruction must be transmitted. if the 0x53 did not echo back, give reset a positive pulse and issue a new programming enable command. 4. the flash is programmed one page at a time. the memory page is loaded one byte at a time by supplying the 6 lsb of the address and data together with the load program memory page instruction. to ensure correct loading of the page, the data low byte must be loaded before data high byte is applied for a given address. the program memory page is stored by loading the write program memory page instruction with the 7 msb of the address. if polling ( rdy/bsy ) is not used, the user must wait at least t wd_flash before issuing the next page. (see table 130.) accessing the serial programming interface before the flash write operation completes can result in incorrect programming. 5. a: the eeprom array is programmed one byte at a time by supplying the address and data to gether with the appr opriate write instruction. an eeprom memory location is first automatically er ased before new data is written. if polling ( rdy/bsy ) is not used, the user must wait at least t wd_eeprom before issuing the next byte. (see table 130.) in a chip erased device, no 0xffs in the data file(s) need to be programmed. b: the eeprom array is programmed one pa ge at a time. the memory page is loaded one byte at a time by supplying the 6 lsb of the address and data together with the load eeprom memory page in struction. the eeprom mem- ory page is stored by loading the wr ite eeprom memory page instruction with the 7 msb of the address. when usin g eeprom page access only byte loca- tions loaded with the load eeprom memory page instruction is altered. the remaining locations rema in unchanged. if polling ( rdy/bsy ) is not used, the used must wait at least t wd_eeprom before issuing the next page (see table 123). in a chip erased device, no 0xff in the data file(s) need to be programmed. 6. any memory location can be verified by using the read instruction which returns the content at the selected address at serial output miso. 7. at the end of the programming session, reset can be set high to commence normal operation. 8. power-off sequence (if needed): set reset to ?1?. tu r n v cc power off.
288 atmega48/88/168 2545d?avr?07/04 . figure 129. serial programming waveforms table 130. minimum wait delay befo re writing the next fl ash or eeprom location symbol minimum wait delay t wd_flash 4.5 ms t wd_eeprom 3.6 ms t wd_erase 9.0 ms msb msb lsb lsb serial clock input (sck) serial data input (mosi) (miso) sample serial data output table 131. serial programmin g instruction set instruction instruction format operation byte 1 byte 2 byte 3 byte4 programming enable 1010 1100 0101 0011 xxxx xxxx xxxx xxxx enable serial programming after reset goes low. chip erase 1010 1100 100x xxxx xxxx xxxx xxxx xxxx chip erase eeprom and flash. read program memory 0010 h 000 000 a aaaa bbbb bbbb oooo oooo read h (high or low) data o from program memory at word address a : b . load program memory page 0100 h 000 000x xxxx xxxx bbbb iiii iiii write h (high or low) data i to program memory page at word address b . data low byte must be loaded before data high byte is applied within the same address. write program memory page 0100 1100 000 a aaaa bbbb xxxx xxxx xxxx write program memory page at address a : b . read eeprom memory 1010 0000 000x xx aa bbbb bbbb oooo oooo read data o from eeprom memory at address a : b . write eeprom memory 1100 0000 000x xx aa bbbb bbbb iiii iiii write data i to eeprom memory at address a : b . load eeprom memory page (page access) 1100 0001 0000 0000 0000 00 bb iiii iiii load data i to eeprom memory page buffer. after data is loaded, program eeprom page. write eeprom memory page (page access) 1100 0010 00xx xx aa bbbb bb 00 xxxx xxxx write eeprom page at address a : b . read lock bits 0101 1000 0000 0000 xxxx xxxx xx oo oooo read lock bits. ?0? = programmed, ?1? = unprogrammed. see table 115 on page 270 for details.
289 atmega48/88/168 2545d?avr?07/04 note: a = address high bits, b = address low bits, h = 0 - low byte, 1 - high byte, o = data out, i = data in, x = don?t care spi serial programming characteristics for characteristics of the spi module see ?spi timing characteristics? on page 295. write lock bits 1010 1100 111x xxxx xxxx xxxx 11 ii iiii write lock bits. set bits = ?0? to program lock bits. see table 115 on page 270 for details. read signature byte 0011 0000 000x xxxx xxxx xx bb oooo oooo read signature byte o at address b . write fuse bits 1010 1100 1010 0000 xxxx xxxx iiii iiii set bits = ?0? to program, ?1? to unprogram. see table 121 on page 273 for details. write fuse high bits 1010 1100 1010 1000 xxxx xxxx iiii iiii set bits = ?0? to program, ?1? to unprogram.see table 120 on page 273 for details. write extended fuse bits 1010 1100 1010 0100 xxxx xxxx xxxx xxii set bits = ?0? to program, ?1? to unprogram. see table 118 on page 272 for details. read fuse bits 0101 0000 0000 0000 xxxx xxxx oooo oooo read fuse bits. ?0? = programmed, ?1? = unprogrammed.see table 121 on page 273 for details. read fuse high bits 0101 1000 0000 1000 xxxx xxxx oooo oooo read fuse high bits. ?0? = pro- grammed, ?1? = unprogrammed. see table 120 on page 273 for details. read extended fuse bits 0101 0000 0000 1000 xxxx xxxx oooo oooo read extended fuse bits. ?0? = pro- grammed, ?1? = unprogrammed. see table 118 on page 272 for details. read calibration byte 0011 1000 000x xxxx 0000 000 b oooo oooo read calibration byte at address b . poll rdy/bsy 1111 0000 0000 0000 xxxx xxxx xxxx xxx o if o = ?1?, a programming operation is still busy. wait until this bit returns to ?0? before applying another command. table 131. serial programming instruction set (continued) instruction instruction format operation byte 1 byte 2 byte 3 byte4
290 atmega48/88/168 2545d?avr?07/04 electrical characteristics absolute maximum ratings* dc characteristics operating temperature.................................. -55 c to +125 c *notice: stresses beyond those listed under ?absolute maximum ratings? may cause permanent dam- age to the device. this is a stress rating only and functional operation of the device at these or other conditions beyond those indicated in the operational sections of th is specification is not implied. exposure to absolute maximum rating conditions for extended periods may affect device reliability. storage temperature ..................................... -65c to +150c voltage on any pin except reset with respect to ground ................................-0.5v to v cc +0.5v voltage on reset with respect to ground......-0.5v to +13.0v maximum operating voltage ............................................ 6.0v dc current per i/o pin ............................................... 40.0 ma dc current v cc and gnd pins................................ 200.0 ma t a = -40 c to 85 c, v cc = 1.8v to 5.5v (unless otherwise noted) symbol parameter condition min. (5) typ. max. (5) units v il input low voltage,except xtal1 and reset pin v cc = 1.8v - 2.4v v cc = 2.4v - 5.5v -0.5 -0.5 0.2v cc (1) 0.3v cc (1) v v il1 input low voltage, xtal1 pin v cc = 1.8v - 5.5v -0.5 0.1v cc (1) v v il2 input low voltage, reset pin v cc = 1.8v - 5.5v -0.5 0.1v cc (1) v v ih input high voltage, except xtal1 and reset pins v cc = 1.8v - 2.4v v cc = 2.4v - 5.5v 0.7v cc (2) 0.6v cc (2) v cc + 0.5 v cc + 0.5 v v ih1 input high voltage, xtal1 pin v cc = 1.8v - 2.4v v cc = 2.4v - 5.5v 0.8v cc (2) 0.7v cc (2) v cc + 0.5 v cc + 0.5 v v ih2 input high voltage, reset pin v cc = 1.8v - 5.5v 0.9v cc (2) v cc + 0.5 v v ol output low voltage (3) , except pc6 i ol = 10ma, v cc = 5v i ol = 5ma, v cc = 3v 0.7 0.5 v v ol1 output low voltage (3) , pc6 tbd tbd v v oh output high voltage (4) , except pc6 i oh = -20ma, v cc = 5v i oh = -10ma, v cc = 3v 4.2 2.3 v v oh1 output high voltage (4) , pc6 tbd tbd v i il input leakage current i/o pin v cc = 5.5v, pin low (absolute value) 1a i ih input leakage current i/o pin v cc = 5.5v, pin high (absolute value) 1a r rst reset pull-up resistor 30 60 k ? r pu i/o pin pull-up resistor 20 50 k ?
291 atmega48/88/168 2545d?avr?07/04 notes: 1. ?max? means the highest value where the pin is guaranteed to be read as low 2. ?min? means the lowest value where t he pin is guaranteed to be read as high 3. although each i/o port can sink more than the test conditions (20ma at v cc = 5v, 10ma at v cc = 3v) under steady state conditions (non-transient), th e following must be observed: atmega48: 1] the sum of all iol, for ports c0 - c5, should not exceed 70 ma. 2] the sum of all iol, for ports c6 , d0 - d4, should not exceed 70 ma. 3] the sum of all iol, for ports b0 - b7, d5 - d7, should not exceed 70 ma. atmega88/168: 1] the sum of all iol, for ports c0 - c5, should not exceed 100 ma. 2] the sum of all iol, for ports c6 , d0 - d4, should not exceed 100 ma. 3] the sum of all iol, for ports b0 - b7, d5 - d7, should not exceed 100 ma. if iol exceeds the test condition, vol may exceed the related specification. pins are not guar anteed to sink current greater than the listed test condition. 4. although each i/o port can source more than the test conditions (20ma at v cc = 5v, 10ma at v cc = 3v) under steady state conditions (non-transient), th e following must be observed: atmega48: 1] the sum of all ioh, for ports c0 - c5, should not exceed 70 ma. 2] the sum of all ioh, for ports c6 , d0 - d4, should not exceed 70 ma. 3] the sum of all ioh, for ports b0 - b7, d5 - d7, should not exceed 70 ma. atmega88/168: 1] the sum of all ioh, for ports c0 - c5, should not exceed 100 ma. 2] the sum of all ioh, for ports c6 , d0 - d4, should not exceed 100 ma. 3] the sum of all ioh, for ports b0 - b7, d5 - d7, should not exceed 100 ma. if ioh exceeds the test condition, voh may exceed the rela ted specification. pins are not guaranteed to source current greater than the listed test condition. i cc power supply current (6) active 1mhz, v cc = 2v (atmega48/88/168v) 0.55 ma active 4mhz, v cc = 3v (atmega48/88/168l) 3.5 ma active 8mhz, v cc = 5v (atmega48/88/168) 12 ma idle 1mhz, v cc = 2v (atmega48/88/168v) 0.25 0.5 ma idle 4mhz, v cc = 3v (atmega48/88/168l) 1.5 ma idle 8mhz, v cc = 5v (atmega48/88/168) 5.5 ma power-down mode wdt enabled, v cc = 3v <8 15 a wdt disabled, v cc = 3v <1 2 a v acio analog comparator input offset voltage v cc = 5v v in = v cc /2 <10 40 mv i aclk analog comparator input leakage current v cc = 5v v in = v cc /2 -50 50 na t acid analog comparator propagation delay v cc = 2.7v v cc = 4.0v 750 500 ns t a = -40 c to 85 c, v cc = 1.8v to 5.5v (unless otherwise noted) (continued) symbol parameter condition min. (5) typ. max. (5) units
292 atmega48/88/168 2545d?avr?07/04 5. all dc characteristics contained in this datasheet are based on simulation and characterization of other avr microcontrol- lers manufactured in the same process technology. these valu es are preliminary values repr esenting design targets, and will be updated after characterization of actual silicon 6. values with ?power reduction register - prr? disabled (0x00). external clock drive waveforms figure 130. external clock drive waveforms external clock drive note: all dc characteristics contained in this datasheet are based on simulation and charac- terization of other avr microcontrollers ma nufactured in the same process technology. these values are preliminary values repres enting design targets, and will be updated after characterization of actual silicon. v il1 v ih1 table 132. external clock drive symbol parameter v cc =1.8-5.5v v cc =2.7-5.5v v cc =4.5-5.5v units min. max. min. max. min. max. 1/t clcl oscillator frequency 0208016mhz t clcl clock period 500 125 62.5 ns t chcx high time 200 50 25 ns t clcx low time 200 50 25 ns t clch rise time 2.0 1.6 0.5 s t chcl fall time 2.0 1.6 0.5 s ? t clcl change in period from one clock cycle to the next 22 2%
293 atmega48/88/168 2545d?avr?07/04 maximum speed vs. v cc maximum frequency is dependent on v cc. as shown in figure 131 and figure 132, the maximum frequency vs. v cc curve is linear between 1.8v < v cc < 2.7v and between 2.7v < v cc < 4.5v. figure 131. maximum frequency vs. v cc , atmega48v/88v/168v figure 132. maximum frequency vs. v cc , atmega48/88/168 10 mhz 4 mhz 1.8v 2.7v 5.5v safe operating area 20 mhz 10 mhz 2.7v 4.5v 5.5v safe operating area
294 atmega48/88/168 2545d?avr?07/04 2-wire serial inte rface characteristics table 133 describes the requirements for devices connected to the 2-wire serial bus. the atmega48/88/168 2-wire serial interface meets or exceeds these requirements under the noted conditions. timing symbols refer to figure 133. notes: 1. in atmega48/88/168, this parameter is characterized and not 100% tested. 2. required only for f scl > 100 khz. 3. c b = capacitance of one bus line in pf. 4. f ck = cpu clock frequency table 133. 2-wire serial bus requirements symbol parameter condition min max units v il input low-voltage -0.5 0.3 v cc v v ih input high-voltage 0.7 v cc v cc + 0.5 v v hys (1) hysteresis of schmitt trigger inputs 0.05 v cc (2) ?v v ol (1) output low-voltage 3 ma sink current 0 0.4 v t r (1) rise time for both sda and scl 20 + 0.1c b (3)(2) 300 ns t of (1) output fall time from v ihmin to v ilmax 10 pf < c b < 400 pf (3) 20 + 0.1c b (3)(2) 250 ns t sp (1) spikes suppressed by input filter 0 50 (2) ns i i input current each i/o pin 0.1v cc < v i < 0.9v cc -10 10 a c i (1) capacitance for each i/o pin ? 10 pf f scl scl clock frequency f ck (4) > max(16f scl , 250khz) (5) 0 400 khz rp value of pull-up resistor f scl 100 khz f scl > 100 khz t hd;sta hold time (repeated) start condition f scl 100 khz 4.0 ? s f scl > 100 khz 0.6 ? s t low low period of the scl clock f scl 100 khz (6) 4.7 ? s f scl > 100 khz (7) 1.3 ? s t high high period of the scl clock f scl 100 khz 4.0 ? s f scl > 100 khz 0.6 ? s t su;sta set-up time for a repeated start condition f scl 100 khz 4.7 ? s f scl > 100 khz 0.6 ? s t hd;dat data hold time f scl 100 khz 0 3.45 s f scl > 100 khz 0 0.9 s t su;dat data setup time f scl 100 khz 250 ? ns f scl > 100 khz 100 ? ns t su;sto setup time for stop condition f scl 100 khz 4.0 ? s f scl > 100 khz 0.6 ? s t buf bus free time between a stop and start condition f scl 100 khz 4.7 ? s f scl > 100 khz 1.3 ? s v cc 0,4v ? 3ma --------------------------- - 1000ns c b ---------------- - ? v cc 0,4v ? 3ma --------------------------- - 300ns c b ------------- - ?
295 atmega48/88/168 2545d?avr?07/04 5. this requirement applies to all atmega 48/88/168 2-wire serial interface operatio n. other devices connected to the 2-wire serial bus need only obey the general f scl requirement. 6. the actual low period generated by the atmega48/88/168 2-wire serial interface is (1/f scl - 2/f ck ), thus f ck must be greater than 6 mhz for the low time requir ement to be strictly met at f scl = 100 khz. 7. the actual low period generated by the atm ega48/88/168 2-wire serial interface is (1/f scl - 2/f ck ), thus the low time require- ment will not be strictly met for f scl > 308 khz when f ck = 8 mhz. still, atmega48/88/168 devices connected to the bus may communicate at full speed (400 khz) with other atmega48/88/ 168 devices, as well as any other device with a proper t low acceptance margin. figure 133. 2-wire serial bus timing spi timing characteristics see figure 134 and figure 135 for details. note: 1. in spi programming mode the minimum sck high/low period is: - 2 t clcl for f ck < 12 mhz - 3 t clcl for f ck > 12 mhz 2. all dc characteristics contained in this datasheet are based on simulation and char- acterization of other avr microcontrolle rs manufactured in the same process technology. these values are preliminary va lues representing design targets, and will be updated after characterization of actual silicon. t su;sta t low t high t low t of t hd;sta t hd;dat t su;dat t su;sto t buf scl sda t r table 134. spi timing parameters description mode min typ max 1 sck period master see table 71 ns 2 sck high/low master 50% duty cycle 3 rise/fall time master 3.6 4 setup master 10 5hold master 10 6 out to sck master 0.5 t sck 7 sck to out master 10 8 sck to out high master 10 9ss low to out slave 15 10 sck period slave 4 t ck 11 sck high/low (1) slave 2 t ck 12 rise/fall time slave 1600 13 setup slave 10 14 hold slave t ck 15 sck to out slave 15 16 sck to ss high slave 20 17 ss high to tri-state slave 10 18 ss low to sck slave 20
296 atmega48/88/168 2545d?avr?07/04 figure 134. spi interface timing requirements (master mode) figure 135. spi interface timing requirements (slave mode) mo si (data output) sck (cpol = 1) mi so (data input) sck (cpol = 0) ss msb lsb lsb msb ... ... 61 22 3 45 8 7 mi so (data output) sck (cpol = 1) mo si (data input) sck (cpol = 0) ss msb lsb lsb msb ... ... 10 11 11 12 13 14 17 15 9 x 16
297 atmega48/88/168 2545d?avr?07/04 adc characteristics ? preliminary data note: all dc characteristics contained in this datasheet are based on simulation and characterization of other avr microcontroll ers manufactured in the same process technology. these values are preliminary values representing design targets, and will be updated after characterization of actual silicon. table 135. adc characteristics symbol parameter condition min typ max units resolution 10 bits absolute accuracy (including inl, dnl, quantization error, gain and offset error) v ref = 4v, v cc = 4v, adc clock = 200 khz 22.5lsb v ref = 4v, v cc = 4v, adc clock = 1 mhz 4.5 lsb v ref = 4v, v cc = 4v, adc clock = 200 khz noise reduction mode 2lsb v ref = 4v, v cc = 4v, adc clock = 1 mhz noise reduction mode 4.5 lsb integral non-linearity (inl) v ref = 4v, v cc = 4v, adc clock = 200 khz 0.5 lsb differential non-linearity (dnl) v ref = 4v, v cc = 4v, adc clock = 200 khz 0.25 lsb gain error v ref = 4v, v cc = 4v, adc clock = 200 khz 2lsb offset error v ref = 4v, v cc = 4v, adc clock = 200 khz 2lsb conversion time free running conversion 13 260 s clock frequency 50 1000 khz av cc analog supply voltage v cc - 0.3 v cc + 0.3 v v ref reference voltage 1.0 av cc v v in input voltage gnd v ref v input bandwidth 38.5 khz v int internal voltage reference 1.0 1.1 1.2 v r ref reference input resistance 32 k ? r ain analog input resistance 100 m ?
298 atmega48/88/168 2545d?avr?07/04 atmega48/88/168 typical characteristics ? preliminary data the following charts show typical behavior. these figures are not tested during manu- facturing. all current consumption measurements are performed with all i/o pins configured as inputs and with internal pull-ups enabled. a sine wave generator with rail- to-rail output is used as clock source. all active- and idle current consumption measurements are done with all bits in the prr register set and thus, the corresponding i/o modules are turned off. also the analog comparator is disabled during these measurements. table 136 on page 304 and table 137 on page 304 show the additional current consumption compared to i cc active and i cc idle for every i/o module controlled by the power reduction register. see ?power reduction register? on page 37 for details. the power consumption in power-down mode is independent of clock selection. the current consumption is a function of several factors such as: operating voltage, operating frequency, loading of i/o pins, switching rate of i/o pins, code executed and ambient temperature. the dominating factors are operating voltage and frequency. the current drawn from capacitive loaded pins may be estimated (for one pin) as c l * v cc *f where c l = load capacitance, v cc = operating voltage and f = average switch- ing frequency of i/o pin. the parts are characterized at frequencies higher than test limits. parts are not guaran- teed to function properly at frequencies higher than the ordering code indicates. the difference between current consumption in power-down mode with watchdog timer enabled and power-down mode with watchdog timer disabled represents the dif- ferential current drawn by the watchdog timer. active supply current figure 136. active supply current vs. frequency (0.1 - 1.0 mhz) active supply current vs. frequency 0.1 - 1.0 mhz 5.5 v 5.0 v 4.5 v 4.0 v 3.3 v 2.7 v 1.8 v 0 0.2 0.4 0.6 0.8 1 1.2 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0. 9 1 frequency (mhz) i cc (ma)
299 atmega48/88/168 2545d?avr?07/04 figure 137. active supply current vs. frequency (1 - 24 mhz) figure 138. active supply current vs. v cc (internal rc oscillator, 128 khz) active supply current vs. frequency 1 - 24 mhz 0 2 4 6 8 10 12 14 16 18 04812162024 frequency (mhz) i cc (ma) 2.7v 1.8v 3.3v 4.0v 4.5v 5.0v 5.5v active supply current vs. v cc internal rc oscillator, 128 khz 85 c 25 c -40 c 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 1.5 2 2.5 3 3.5 4 4.5 5 5.5 v cc (v) i cc (ma)
300 atmega48/88/168 2545d?avr?07/04 figure 139. active supply current vs. v cc (internal rc oscillator, 1 mhz) figure 140. active supply current vs. v cc (internal rc oscillator, 8 mhz) active supply current vs. v cc internal rc oscillator, 1 mhz 85 c 25 c -40 c 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.5 2 2.5 3 3.5 4 4.5 5 5.5 v cc (v) i cc (ma) active supply current vs. v cc internal rc oscillator, 8 mhz 85 c 25 c -40 c 0 1 2 3 4 5 6 7 1.5 2 2.5 3 3.5 4 4.5 5 5.5 v cc (v) i cc (ma)
301 atmega48/88/168 2545d?avr?07/04 figure 141. active supply current vs. v cc (32 khz external oscillator) idle supply current figure 142. idle supply current vs. frequency (0.1 - 1.0 mhz) active supply current vs. v cc 32 khz external oscillator 25 c 0 10 20 30 40 50 60 1.5 2 2.5 3 3.5 4 4.5 5 5.5 v cc (v) i cc (ua) idle supply current vs. frequency 0.1 - 1.0 mhz 5.5 v 5.0 v 4.5 v 4.0 v 3.3 v 2.7 v 1.8 v 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0. 9 1 frequency (mhz) i cc (ma)
302 atmega48/88/168 2545d?avr?07/04 figure 143. idle supply current vs. frequency (1 - 24 mhz) figure 144. idle supply current vs. v cc (internal rc o scillator, 128 khz) idle supply current vs. frequency 1 - 24 mhz 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 0 4 8 12162024 frequency (mhz) i cc (ma) 2.7v 1.8v 3.3v 4.0v 4.5v 5.0v 5.5v idle supply current vs. v cc internal rc oscillator, 128 khz 85 c 25 c -40 c 0 0.005 0.01 0.015 0.02 0.025 0.03 1.5 2 2.5 3 3.5 4 4.5 5 5.5 v cc (v) i cc (ma)
303 atmega48/88/168 2545d?avr?07/04 figure 145. idle supply current vs. v cc (internal rc oscillator, 1 mhz) figure 146. idle supply current vs. v cc (internal rc oscillator, 8 mhz) idle supply current vs. v cc internal rc oscillator, 1 mhz 85 c 25 c -40 c 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 1.5 2 2.5 3 3.5 4 4.5 5 5.5 v cc (v) i cc (ma) idle supply current vs. v cc internal rc oscillator, 8 mhz 85 c 25 c -40 c 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.5 2 2.5 3 3.5 4 4.5 5 5.5 v cc (v) i cc (ma)
304 atmega48/88/168 2545d?avr?07/04 figure 147. idle supply current vs. v cc (32 khz external oscillator) supply current of io modules the tables and formulas below can be used to calculate the additional current consump- tion for the different i/o modules in active and idle mode. the enabling or disabling of the i/o modules are controlled by the power reduction register. see ?power reduction register? on page 37 for details. idle supply current vs. v cc 32 khz external oscillator 25 c 0 5 10 15 20 25 30 1.5 2 2.5 3 3.5 4 4.5 5 5.5 v cc (v) i cc (ua) table 136. additional current consumption for the different i/o modules (absolute values) prr bit typical numbers v cc = 2v, f = 1mhz v cc = 3v, f = 4mhz v cc = 5v, f = 8mhz prusart0 8.0 ua 51 ua 220 ua prtwi 12 ua 75 ua 315 ua prtim2 11 ua 72 ua 300 ua prtim1 5.0 ua 32 ua 130 ua prtim0 4.0 ua 24 ua 100 ua prspi 15 ua 95 ua 400 ua pradc 12 ua 75 ua 315 ua table 137. additional current consumption (percentage) in active and idle mode prr bit additional current consumption compared to active with external clock (see figure 136 and figure 137) additional current consumption compared to idle with external clock (see figure 142 and figure 143) prusart0 3.3% 18% prtwi 4.8% 26% prtim2 4.7% 25%
305 atmega48/88/168 2545d?avr?07/04 it is possible to calculate the typical current consumption based on the numbers from table 2 for other v cc and frequency settings than listed in table 1. example 1 calculate the expected current consumption in idle mode with usart0, timer1, and twi enabled at v cc = 3.0v and f = 1mhz. from table 2, third column, we see that we need to add 18% for the usart0, 26% for t he twi, and 11% for the timer1 module. reading from figure 3, we find that the id le current consumption is ~0,075ma at v cc = 3.0v and f = 1mhz. the total current consum ption in idle mode with usart0, timer1, and twi enabled, gives: example 2 same conditions as in example 1, but in active mode instead. from table 2, second col- umn we see that we need to add 3.3% for the usart0, 4.8% for the twi, and 2.0% for the timer1 module. reading from figure 1, we find that the active current consumption is ~0,42ma at v cc = 3.0v and f = 1mhz. the total current consumption in idle mode with usart0, timer1, and twi enabled, gives: example 3 all i/o modules should be enabled. calculat e the expected current consumption in active mode at v cc = 3.6v and f = 10mhz. we find the active current consumption with- out the i/o modules to be ~ 4.0ma (from figure 2). then, by using the numbers from table 2 - second column, we find the total current consumption: prtim1 2.0% 11% prtim0 1.6% 8.5% prspi 6.1% 33% pradc 4.9% 26% table 137. additional current consumption (percentage) in active and idle mode (continued) prr bit additional current consumption compared to active with external clock (see figure 136 and figure 137) additional current consumption compared to idle with external clock (see figure 142 and figure 143) i cc total 0.075 ma 10.180.260.11 +++ () ? 0.116 ma ? i cc total 0.42 ma 1 0.033 0.048 0.02 +++ () ? 0.46 ma ? i cc total 4.0 ma 1 0.033 0.048 0.047 0.02 0.016 0.061 0.049 +++++++ () ? 5.1 ma ?
306 atmega48/88/168 2545d?avr?07/04 power-down supply current figure 148. power-down supply current vs. v cc (watchdog timer disabled) figure 149. power-down supply current vs. v cc (watchdog timer enabled) power-down supply current vs. v cc watchdog timer disabled 85 c 25 c -40 c 0 0.5 1 1.5 2 2.5 1.5 2 2.5 3 3.5 4 4.5 5 5.5 v cc (v) i cc (ua) power-down supply current vs. v cc watchdog timer enabled 85 c 25 c -40 c 0 2 4 6 8 10 12 1.5 2 2.5 3 3.5 4 4.5 5 5.5 v cc (v) i cc (ua)
307 atmega48/88/168 2545d?avr?07/04 power-save supply current figure 150. power-save supply current vs. v cc (watchdog timer disabled) standby supply current figure 151. standby supply current vs. v cc (low power cr ystal oscillator) power-save supply current vs. v cc watchdog timer disabled 25 c 0 2 4 6 8 10 12 1.5 2 2.5 3 3.5 4 4.5 5 5.5 v cc (v) i cc (ua) standby supply current vs. v cc low power crystal oscillator 6 mhz xtal 6 mhz res. 4 mhz xtal 4 mhz res. 455khz res. 32 khz xtal 2 mhz xtal 2 mhz res. 1 mhz res. 0 20 40 60 80 100 120 140 160 180 1.5 2 2.5 3 3.5 4 4.5 5 5.5 v cc (v) i cc (ua)
308 atmega48/88/168 2545d?avr?07/04 figure 152. standby supply current vs. v cc (full swing crystal oscillator) pin pull-up figure 153. i/o pin pull-up resistor current vs. input voltage (v cc = 5v) standby supply current vs. v cc full swing crystal oscillator 6 mhz xtal (ckopt) 4 mhz xtal (ckopt) 2 mhz xtal (ckopt) 16 mhz xtal 12 mhz xtal 0 50 100 150 200 250 300 350 400 450 500 1.5 2 2.5 3 3.5 4 4.5 5 5.5 v cc (v) i cc (ua) i/o pin pull-up resistor current vs. input voltage v cc = 5v 85 c 25 c -40 c 0 20 40 60 80 100 120 140 160 0123456 v op (v) i op (ua)
309 atmega48/88/168 2545d?avr?07/04 figure 154. i/o pin pull-up resistor current vs. input voltage (v cc = 2.7v) figure 155. reset pull-up resist or current vs. reset pin voltage (v cc = 5v) i/o pin pull-up resistor current vs. input voltage v cc = 2.7v 85 c 25 c -40 c 0 10 20 30 40 50 60 70 80 9 0 0 0.5 1 1.5 2 2.5 3 v op (v) i op (ua) reset pull-up resistor current vs. reset pin voltage v cc = 5v 0 20 40 60 80 100 120 0123456 v reset (v) i reset (ua) -40oc 25oc 85oc
310 atmega48/88/168 2545d?avr?07/04 figure 156. reset pull-up resist or current vs. reset pin voltage (v cc = 2.7v) pin driver strength figure 157. i/o pin source current vs. output voltage (v cc = 5v) reset pull-up resistor current vs. reset pin voltage v cc = 2.7v -40 c 0 10 20 30 40 50 60 70 0 0.5 1 1.5 2 2.5 3 v reset (v) i reset (ua) 25 c 85 c i/o pin source current vs. output voltage v cc = 5v 85 c 25 c -40 c 0 10 20 30 40 50 60 70 80 9 0 0123456 v oh (v) i oh (ma)
311 atmega48/88/168 2545d?avr?07/04 figure 158. i/o pin source current vs. output voltage (v cc = 2.7v) figure 159. i/o pin source current vs. output voltage (v cc = 1.8v) i/o pin source current vs. output voltage v cc = 2.7v 85 c 25 c -40 c 0 5 10 15 20 25 30 35 0 0.5 1 1.5 2 2.5 3 v oh (v) i oh (ma) i/o pin source current vs. output voltage v cc = 1.8v 85 c 25 c -40 c 0 1 2 3 4 5 6 7 8 9 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 v oh (v) i oh (ma)
312 atmega48/88/168 2545d?avr?07/04 figure 160. i/o pin sink current vs. output voltage (v cc = 5v) figure 161. i/o pin sink current vs. output voltage (v cc = 2.7v) i/o pin sink current vs. output voltage v cc = 5v 85 c 25 c 0 10 20 30 40 50 60 70 80 0 0.5 1 1.5 2 2.5 v ol (v) i ol (ma) i/o pin sink current vs. output voltage v cc = 2.7v 85 c 25 c -40 c 0 5 10 15 20 25 30 35 40 0 0.5 1 1.5 2 2.5 v ol (v) i ol (ma)
313 atmega48/88/168 2545d?avr?07/04 figure 162. i/o pin sink current vs. output voltage (v cc = 1.8v) pin thresholds and hysteresis figure 163. i/o pin input threshold voltage vs. v cc (vih, i/o pin read as '1') i/o pin sink current vs. output voltage v cc = 1.8v 85 c 25 c -40 c 0 2 4 6 8 10 12 14 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 v ol (v) i ol (ma) i/o pin input threshold voltage vs. v cc vih, io pin read as '1' 85 c 25 c -40 c 0 0.5 1 1.5 2 2.5 3 1.5 2 2.5 3 3.5 4 4.5 5 5.5 v cc (v) threshold (v)
314 atmega48/88/168 2545d?avr?07/04 figure 164. i/o pin input threshold voltage vs. v cc (vil, i/o pin read as '0') figure 165. reset input thresh old voltage vs. v cc (vih, reset pin read as '1') i/o pin input threshold voltage vs. v cc vil, io pin read as '0' 85 c 25 c -40 c 0 0.5 1 1.5 2 2.5 3 1.5 2 2.5 3 3.5 4 4.5 5 5.5 v cc (v) threshold (v) reset input threshold voltage vs. v cc vih, io pin read as '1' 85 c 25 c -40 c 0 0.5 1 1.5 2 2.5 3 1.5 2 2.5 3 3.5 4 4.5 5 5.5 v cc (v) threshold (v)
315 atmega48/88/168 2545d?avr?07/04 figure 166. reset input thresh old voltage vs. v cc (vil, reset pin read as '0') figure 167. reset input pin hysteresis vs. v cc reset input threshold voltage vs. v cc vil, io pin read as '0' 85 c 25 c -40 c 0 0.5 1 1.5 2 2.5 3 1.5 2 2.5 3 3.5 4 4.5 5 5.5 v cc (v) threshold (v) reset pin input hysteresis vs. v cc vil 0 100 200 300 400 500 600 2 2.5 3 3.5 4 4.5 5 5.5 v cc (v) input hysteresis (mv)
316 atmega48/88/168 2545d?avr?07/04 bod thresholds and analog comparator offset figure 168. bod thresholds vs. temper ature (bodlevel is 4.0v) figure 169. bod thresholds vs. temper ature (bodlevel is 2.7v) bod thresholds vs. temperature bodlevel is 4.0v 4.2 4.25 4.3 4.35 4.4 4.45 4.5 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 9 0 100 temperature (c) threshold (v) rising vcc falling vcc bod thresholds vs. temperature bodlevel is 2.7v 2.6 2.65 2.7 2.75 2.8 2.85 2. 9 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 9 0 100 temperature (c) threshold (v) rising vcc falling vcc
317 atmega48/88/168 2545d?avr?07/04 figure 170. bod thresholds vs. temper ature (bodlevel is 1.8v) figure 171. bandgap voltage vs. v cc bod thresholds vs. temperature bodlevel is 1.8v 1.76 1.78 1.8 1.82 1.84 1.86 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 9 0 100 temperature (c) threshold (v) rising vcc falling vcc bandgap voltage vs. v cc -40 c 85 c 1.08 1.085 1.0 9 1.0 9 5 1.1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 v cc (v) bandgap voltage (v )
318 atmega48/88/168 2545d?avr?07/04 figure 172. analog comparator offset voltage vs. common mode voltage (v cc =5v) figure 173. analog comparator offset voltage vs. common mode voltage (v cc =2.7v) analog comparator offset voltage vs. common mode voltage v cc =5v -40 c 85 c 0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.00 9 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 common mode voltage (v) analog comparator offset voltage (v) analog comparator offset voltage vs. common mode voltage v cc =5v -40 c 85 c 0 0.5 1 1.5 2 2.5 3 3.5 4 0 0.5 1 1.5 2 2.5 common mode voltage (v) analog comparator offset voltage (mv)
319 atmega48/88/168 2545d?avr?07/04 internal osci llator speed figure 174. watchdog oscillator frequency vs. v cc figure 175. calibrated 8 mhz rc oscillator frequen cy vs. temperature watchdog oscillator frequency vs. v cc 85 c 25 c -40 c 9 5 100 105 110 115 120 1.5 2 2.5 3 3.5 4 4.5 5 5.5 v cc (v) f rc (khz) calibrated 8 mhz rc oscillator frequency vs. temperature 5.0 v 2.7 v 1.8 v 7.4 7.5 7.6 7.7 7.8 7. 9 8 8.1 8.2 8.3 8.4 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 9 0 100 temperature (c) f rc (mhz)
320 atmega48/88/168 2545d?avr?07/04 figure 176. calibrated 8 mhz rc osc illator frequency vs. v cc figure 177. calibrated 8 mhz rc oscillator frequen cy vs. osccal value calibrated 8mhz rc oscillator frequency vs. v 85 ?c 25 ?c -40 ?c 7.4 7.6 7.8 8 8.2 8.4 8.6 1.5 2 2.5 3 3.5 4 4.5 5 5.5 v cc (v) f rc (mhz) cc calibrated 8mhz rc oscillator frequency vs. osccal value 85 c 25 c -40 c 3.5 5.5 7.5 9 .5 11.5 13.5 0 1632486480 9 6 112 128 144 160 176 1 9 2 208 224 240 osccal value f rc (mhz)
321 atmega48/88/168 2545d?avr?07/04 current consumption of peripheral units figure 178. brownout detector current vs. v cc figure 179. adc current vs. v cc (adc at 50 khz) brownout detector current vs. v cc 85 ?c 25 ?c -40 ?c 18 20 22 24 26 28 30 32 1.5 2 2.5 3 3.5 4 4.5 5 5.5 v cc (v) i cc (ua) aref vs. v cc adc at 50 khz 85 c 25 c -40 c 150 200 250 300 350 400 450 500 1.5 2 2.5 3 3.5 4 4.5 5 5.5 v cc (v) i cc (ua)
322 atmega48/88/168 2545d?avr?07/04 figure 180. aref current vs. v cc (adc at 1 mhz) figure 181. analog comparator current vs. v cc aref vs. v cc adc at 1 mhz 85 ?c 25 ?c -40 ?c 0 20 40 60 80 100 120 140 160 180 1.5 2 2.5 3 3.5 4 4.5 5 5.5 v cc (v) i cc (ua) analog comparator current vs. v cc 85 ?c 25 ?c -40 ?c 0 20 40 60 80 100 120 140 1.5 2 2.5 3 3.5 4 4.5 5 5.5 v cc (v) i cc (ua)
323 atmega48/88/168 2545d?avr?07/04 figure 182. programming current vs. v cc current consumption in reset and reset pulse width figure 183. reset supply current vs. v cc (0.1 - 1.0 mhz, excluding current through the reset pull-up) programming current vs. v 85 ?c 25 ?c -40 ?c 0 2 4 6 8 10 12 14 1.5 2 2.5 3 3.5 4 4.5 5 5.5 v cc (v) i cc (ma) cc reset supply current vs. v cc 0.1 - 1.0 mhz, excluding current through the reset pull-up 5.5 v 5.0 v 4.5 v 4.0 v 3.3 v 2.7 v 1.8 v 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0. 9 1 frequency (mhz) i cc (ma)
324 atmega48/88/168 2545d?avr?07/04 figure 184. reset supply current vs. v cc (1 - 24 mhz, excluding current through the reset pull-up) figure 185. reset pulse width vs. v cc reset supply current vs. v cc 1 - 24 mhz, excluding current through the reset pull-up 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 0 4 8 12162024 frequency (mhz) i cc (ma) 2.7v 1.8v 3.3v 4.0v 4.5v 5.0v 5.5v reset pulse width vs. v cc 85 ?c 25 ?c -40 ?c 0 500 1000 1500 2000 2500 1.5 2 2.5 3 3.5 4 4.5 5 5.5 v cc (v) pulsewidth (ns)
325 atmega48/88/168 2545d?avr?07/04 register summary address name bit 7 bit 6 bit 5 bit 4 bit 3 bit 2 bit 1 bit 0 page (0xff) reserved ? ? ? ? ? ? ? ? (0xfe) reserved ? ? ? ? ? ? ? ? (0xfd) reserved ? ? ? ? ? ? ? ? (0xfc) reserved ? ? ? ? ? ? ? ? (0xfb) reserved ? ? ? ? ? ? ? ? (0xfa) reserved ? ? ? ? ? ? ? ? (0xf9) reserved ? ? ? ? ? ? ? ? (0xf8) reserved ? ? ? ? ? ? ? ? (0xf7) reserved ? ? ? ? ? ? ? ? (0xf6) reserved ? ? ? ? ? ? ? ? (0xf5) reserved ? ? ? ? ? ? ? ? (0xf4) reserved ? ? ? ? ? ? ? ? (0xf3) reserved ? ? ? ? ? ? ? ? (0xf2) reserved ? ? ? ? ? ? ? ? (0xf1) reserved ? ? ? ? ? ? ? ? (0xf0) reserved ? ? ? ? ? ? ? ? (0xef) reserved ? ? ? ? ? ? ? ? (0xee) reserved ? ? ? ? ? ? ? ? (0xed) reserved ? ? ? ? ? ? ? ? (0xec) reserved ? ? ? ? ? ? ? ? (0xeb) reserved ? ? ? ? ? ? ? ? (0xea) reserved ? ? ? ? ? ? ? ? (0xe9) reserved ? ? ? ? ? ? ? ? (0xe8) reserved ? ? ? ? ? ? ? ? (0xe7) reserved ? ? ? ? ? ? ? ? (0xe6) reserved ? ? ? ? ? ? ? ? (0xe5) reserved ? ? ? ? ? ? ? ? (0xe4) reserved ? ? ? ? ? ? ? ? (0xe3) reserved ? ? ? ? ? ? ? ? (0xe2) reserved ? ? ? ? ? ? ? ? (0xe1) reserved ? ? ? ? ? ? ? ? (0xe0) reserved ? ? ? ? ? ? ? ? (0xdf) reserved ? ? ? ? ? ? ? ? (0xde) reserved ? ? ? ? ? ? ? ? (0xdd) reserved ? ? ? ? ? ? ? ? (0xdc) reserved ? ? ? ? ? ? ? ? (0xdb) reserved ? ? ? ? ? ? ? ? (0xda) reserved ? ? ? ? ? ? ? ? (0xd9) reserved ? ? ? ? ? ? ? ? (0xd8) reserved ? ? ? ? ? ? ? ? (0xd7) reserved ? ? ? ? ? ? ? ? (0xd6) reserved ? ? ? ? ? ? ? ? (0xd5) reserved ? ? ? ? ? ? ? ? (0xd4) reserved ? ? ? ? ? ? ? ? (0xd3) reserved ? ? ? ? ? ? ? ? (0xd2) reserved ? ? ? ? ? ? ? ? (0xd1) reserved ? ? ? ? ? ? ? ? (0xd0) reserved ? ? ? ? ? ? ? ? (0xcf) reserved ? ? ? ? ? ? ? ? (0xce) reserved ? ? ? ? ? ? ? ? (0xcd) reserved ? ? ? ? ? ? ? ? (0xcc) reserved ? ? ? ? ? ? ? ? (0xcb) reserved ? ? ? ? ? ? ? ? (0xca) reserved ? ? ? ? ? ? ? ? (0xc9) reserved ? ? ? ? ? ? ? ? (0xc8) reserved ? ? ? ? ? ? ? ? (0xc7) reserved ? ? ? ? ? ? ? ? (0xc6) udr0 usart i/o data register 180 (0xc5) ubrr0h usart baud rate register high 184 (0xc4) ubrr0l usart baud rate register low 184 (0xc3) reserved ? ? ? ? ? ? ? ? (0xc2) ucsr0c umsel01 umsel00 upm01 upm00 usbs0 ucsz01 /udord0 ucsz00 / ucpha0 ucpol0 183/196 (0xc1) ucsr0b rxcie0 txcie0 udrie0 rxen0 txen0 ucsz02 rxb80 txb80 182 (0xc0) ucsr0a rxc0 txc0 udre0 fe0 dor0 upe0 u2x0 mpcm0 180
326 atmega48/88/168 2545d?avr?07/04 (0xbf) reserved ? ? ? ? ? ? ? ? (0xbe) reserved ? ? ? ? ? ? ? ? (0xbd) twamr twam6 twam5 twam4 twam3 twam2 twam1 twam0 ?209 (0xbc) twcr twint twea twsta twsto twwc twen ?twie 206 (0xbb) twdr 2-wire serial interface data register 208 (0xba) twar twa6 twa5 twa4 tw a3 twa2 twa1 twa0 twgce 208 (0xb9) twsr tws7 tws6 tws5 tws4 tws3 ?twps1twps0 207 (0xb8) twbr 2-wire serial interface bit rate register 206 (0xb7) reserved ? ? ? ? ? ? ? (0xb6) assr ? exclk as2 tcn2ub ocr2aub ocr2bub tcr2aub tcr2bub 150 (0xb5) reserved ? ? ? ? ? ? ? ? (0xb4) ocr2b timer/counter2 output compare register b 147 (0xb3) ocr2a timer/counter2 output compare register a 147 (0xb2) tcnt2 timer/counter2 (8-bit) 147 (0xb1) tccr2b foc2a foc2b ? ? wgm22 cs22 cs21 cs20 146 (0xb0) tccr2a com2a1 com2a0 com2b1 com2b0 ? ?wgm21wgm20 143 (0xaf) reserved ? ? ? ? ? ? ? ? (0xae) reserved ? ? ? ? ? ? ? ? (0xad) reserved ? ? ? ? ? ? ? ? (0xac) reserved ? ? ? ? ? ? ? ? (0xab) reserved ? ? ? ? ? ? ? ? (0xaa) reserved ? ? ? ? ? ? ? ? (0xa9) reserved ? ? ? ? ? ? ? ? (0xa8) reserved ? ? ? ? ? ? ? ? (0xa7) reserved ? ? ? ? ? ? ? ? (0xa6) reserved ? ? ? ? ? ? ? ? (0xa5) reserved ? ? ? ? ? ? ? ? (0xa4) reserved ? ? ? ? ? ? ? ? (0xa3) reserved ? ? ? ? ? ? ? ? (0xa2) reserved ? ? ? ? ? ? ? ? (0xa1) reserved ? ? ? ? ? ? ? ? (0xa0) reserved ? ? ? ? ? ? ? ? (0x9f) reserved ? ? ? ? ? ? ? ? (0x9e) reserved ? ? ? ? ? ? ? ? (0x9d) reserved ? ? ? ? ? ? ? ? (0x9c) reserved ? ? ? ? ? ? ? ? (0x9b) reserved ? ? ? ? ? ? ? ? (0x9a) reserved ? ? ? ? ? ? ? ? (0x99) reserved ? ? ? ? ? ? ? ? (0x98) reserved ? ? ? ? ? ? ? ? (0x97) reserved ? ? ? ? ? ? ? ? (0x96) reserved ? ? ? ? ? ? ? ? (0x95) reserved ? ? ? ? ? ? ? ? (0x94) reserved ? ? ? ? ? ? ? ? (0x93) reserved ? ? ? ? ? ? ? ? (0x92) reserved ? ? ? ? ? ? ? ? (0x91) reserved ? ? ? ? ? ? ? ? (0x90) reserved ? ? ? ? ? ? ? ? (0x8f) reserved ? ? ? ? ? ? ? ? (0x8e) reserved ? ? ? ? ? ? ? ? (0x8d) reserved ? ? ? ? ? ? ? ? (0x8c) reserved ? ? ? ? ? ? ? ? (0x8b) ocr1bh timer/counter1 - output compare register b high byte 129 (0x8a) ocr1bl timer/counter1 - outp ut compare register b low byte 129 (0x89) ocr1ah timer/counter1 - output compare register a high byte 129 (0x88) ocr1al timer/counter1 - output compare register a low byte 129 (0x87) icr1h timer/counter1 - input capture register high byte 129 (0x86) icr1l timer/counter1 - input capture register low byte 129 (0x85) tcnt1h timer/counter1 - counter register high byte 129 (0x84) tcnt1l timer/counter1 - counter register low byte 129 (0x83) reserved ? ? ? ? ? ? ? ? (0x82) tccr1c foc1a foc1b ? ? ? ? ? ?128 (0x81) tccr1b icnc1 ices1 ? wgm13 wgm12 cs12 cs11 cs10 127 (0x80) tccr1a com1a1 com1a0 com1b1 com1b0 ? ?wgm11wgm10 125 (0x7f) didr1 ? ? ? ? ? ?ain1dain0d 230 (0x7e) didr0 ? ? adc5d adc4d adc3d adc2d adc1d adc0d 245 address name bit 7 bit 6 bit 5 bit 4 bit 3 bit 2 bit 1 bit 0 page
327 atmega48/88/168 2545d?avr?07/04 (0x7d) reserved ? ? ? ? ? ? ? ? (0x7c) admux refs1 refs0 adlar ? mux3 mux2 mux1 mux0 241 (0x7b) adcsrb ?acme ? ? ? adts2 adts1 adts0 244 (0x7a) adcsra aden adsc adate adif adie adps2 adps1 adps0 242 (0x79) adch adc data register high byte 244 (0x78) adcl adc data register low byte 244 (0x77) reserved ? ? ? ? ? ? ? ? (0x76) reserved ? ? ? ? ? ? ? ? (0x75) reserved ? ? ? ? ? ? ? ? (0x74) reserved ? ? ? ? ? ? ? ? (0x73) reserved ? ? ? ? ? ? ? ? (0x72) reserved ? ? ? ? ? ? ? ? (0x71) reserved ? ? ? ? ? ? ? ? (0x70) timsk2 ? ? ? ? ? ocie2b ocie2a toie2 148 (0x6f) timsk1 ? ?icie1 ? ? ocie1b ocie1a toie1 130 (0x6e) timsk0 ? ? ? ? ? ocie0b ocie0a toie0 100 (0x6d) pcmsk2 pcint23 pcint22 pcint21 pcint20 pcint19 pcint18 pcint17 pcint16 83 (0x6c) pcmsk1 ? pcint14 pcint13 pcint12 pcint11 pcint10 pcint9 pcint8 83 (0x6b) pcmsk0 pcint7 pcint6 pcint5 pc int4 pcint3 pcint2 pcint1 pcint0 84 (0x6a) reserved ? ? ? ? ? ? ? ? (0x69) eicra ? ? ? ?isc11isc10isc01isc00 80 (0x68) pcicr ? ? ? ? ? pcie2 pcie1 pcie0 (0x67) reserved ? ? ? ? ? ? ? ? (0x66) osccal oscillator calibration register 30 (0x65) reserved ? ? ? ? ? ? ? ? (0x64) prr prtwi prtim2 prtim0 ? prtim1 prspi prusart0 pradc 37 (0x63) reserved ? ? ? ? ? ? ? ? (0x62) reserved ? ? ? ? ? ? ? ? (0x61) clkpr clkpce ? ? ? clkps3 clkps2 clkps1 clkps0 33 (0x60) wdtcsr wdif wdie wdp3 wdce wde wdp2 wdp1 wdp0 49 0x3f (0x5f) sreg i t h s v n z c 9 0x3e (0x5e) sph ? ? ? ? ? (sp10) 5. sp9 sp8 11 0x3d (0x5d) spl sp7 sp6 sp5 sp4 sp3 sp2 sp1 sp0 11 0x3c (0x5c) reserved ? ? ? ? ? ? ? ? 0x3b (0x5b) reserved ? ? ? ? ? ? ? ? 0x3a (0x5a) reserved ? ? ? ? ? ? ? ? 0x39 (0x59) reserved ? ? ? ? ? ? ? ? 0x38 (0x58) reserved ? ? ? ? ? ? ? ? 0x37 (0x57) spmcsr spmie (rwwsb) 5. ? (rwwsre) 5. blbset pgwrt pgers selfprgen 260 0x36 (0x56) reserved ? ? ? ? ? ? ? ? 0x35 (0x55) mcucr ? ? ?pud ? ? ivsel ivce 0x34 (0x54) mcusr ? ? ? ? wdrf borf extrf porf 0x33 (0x53) smcr ? ? ? ?sm2sm1sm0se 35 0x32 (0x52) reserved ? ? ? ? ? ? ? ? 0x31 (0x51) reserved ? ? ? ? ? ? ? ? 0x30 (0x50) acsr acd acbg aco aci acie acic acis1 acis0 228 0x2f (0x4f) reserved ? ? ? ? ? ? ? ? 0x2e (0x4e) spdr spi data register 160 0x2d (0x4d) spsr spif wcol ? ? ? ? ? spi2x 160 0x2c (0x4c) spcr spie spe dord mstr cpol cpha spr1 spr0 158 0x2b (0x4b) gpior2 general purpose i/o register 2 23 0x2a (0x4a) gpior1 general purpose i/o register 1 23 0x29 (0x49) reserved ? ? ? ? ? ? ? ? 0x28 (0x48) ocr0b timer/counter0 output compare register b 0x27 (0x47) ocr0a timer/counter0 output compare register a 0x26 (0x46) tcnt0 timer/counter0 (8-bit) 0x25 (0x45) tccr0b foc0a foc0b ? ? wgm02 cs02 cs01 cs00 0x24 (0x44) tccr0a com0a1 com0a0 com0b1 com0b0 ? ?wgm01wgm00 0x23 (0x43) gtccr tsm ? ? ? ? ? psrasy psrsync 103/152 0x22 (0x42) eearh (eeprom address register high byte) 5. 18 0x21 (0x41) eearl eeprom address register low byte 18 0x20 (0x40) eedr eeprom data register 18 0x1f (0x3f) eecr ? ? eepm1 eepm0 eerie eempe eepe eere 18 0x1e (0x3e) gpior0 general purpose i/o register 0 23 0x1d (0x3d) eimsk ? ? ? ? ? ?int1int0 81 0x1c (0x3c) eifr ? ? ? ? ? ? intf1 intf0 82 address name bit 7 bit 6 bit 5 bit 4 bit 3 bit 2 bit 1 bit 0 page
328 atmega48/88/168 2545d?avr?07/04 note: 1. for compatibility with future devices, reserved bits shoul d be written to zero if accessed. reserved i/o memory addresse s should never be written. 2. i/o registers within the address range 0x00 - 0x1f are direct ly bit-accessible using the sbi and cbi instructions. in these registers, the value of single bits can be ch ecked by using the sbis and sbic instructions. 3. some of the status flags are cleared by writing a logical one to them. note that, unlike most other avrs, the cbi and sbi instructions will only operate on the specified bit, and can ther efore be used on registers containing such status flags. the cbi and sbi instructions work wit h registers 0x00 to 0x1f only. 4. when using the i/o specific commands in and out, the i/o addresses 0x00 - 0x3f must be used. when addressing i/o registers as data space using ld and st instructions, 0x20 must be added to these addr esses. the atmega48/88/168 is a complex microcontroller with more peripheral units than can be supported within the 64 location reserved in opcode for the in and out instructions. for the extended i/o space from 0x60 - 0xff in sram, only the st/sts/std and ld/lds/ldd instructions can be used. 5. only valid for atmega88/168. 0x1b (0x3b) pcifr ? ? ? ? ? pcif2 pcif1 pcif0 0x1a (0x3a) reserved ? ? ? ? ? ? ? ? 0x19 (0x39) reserved ? ? ? ? ? ? ? ? 0x18 (0x38) reserved ? ? ? ? ? ? ? ? 0x17 (0x37) tifr2 ? ? ? ? ? ocf2b ocf2a tov2 148 0x16 (0x36) tifr1 ? ?icf1 ? ? ocf1b ocf1a tov1 130 0x15 (0x35) tifr0 ? ? ? ? ? ocf0b ocf0a tov0 0x14 (0x34) reserved ? ? ? ? ? ? ? ? 0x13 (0x33) reserved ? ? ? ? ? ? ? ? 0x12 (0x32) reserved ? ? ? ? ? ? ? ? 0x11 (0x31) reserved ? ? ? ? ? ? ? ? 0x10 (0x30) reserved ? ? ? ? ? ? ? ? 0x0f (0x2f) reserved ? ? ? ? ? ? ? ? 0x0e (0x2e) reserved ? ? ? ? ? ? ? ? 0x0d (0x2d) reserved ? ? ? ? ? ? ? ? 0x0c (0x2c) reserved ? ? ? ? ? ? ? ? 0x0b (0x2b) portd portd7 portd6 portd 5 portd4 portd3 portd2 portd1 portd0 79 0x0a (0x2a) ddrd ddd7 ddd6 ddd5 ddd4 ddd3 ddd2 ddd1 ddd0 79 0x09 (0x29) pind pind7 pind6 pi nd5 pind4 pind3 pind2 pind1 pind0 79 0x08 (0x28) portc ? portc6 portc5 portc4 portc3 portc2 portc1 portc0 79 0x07 (0x27) ddrc ? ddc6 ddc5 ddc4 ddc3 ddc2 ddc1 ddc0 79 0x06 (0x26) pinc ? pinc6 pinc5 pinc4 pinc3 pinc2 pinc1 pinc0 79 0x05 (0x25) portb portb7 portb6 portb 5 portb4 portb3 portb2 portb1 portb0 79 0x04 (0x24) ddrb ddb7 ddb6 ddb 5 ddb4 ddb3 ddb2 ddb1 ddb0 79 0x03 (0x23) pinb pinb7 pinb6 pinb5 pinb4 pinb3 pinb2 pinb1 pinb0 79 0x02 (0x22) reserved ? ? ? ? ? ? ? ? 0x01 (0x21) reserved ? ? ? ? ? ? ? ? 0x0 (0x20) reserved ? ? ? ? ? ? ? ? address name bit 7 bit 6 bit 5 bit 4 bit 3 bit 2 bit 1 bit 0 page
329 atmega48/88/168 2545d?avr?07/04 instruction set summary mnemonics operands description operation flags #clocks arithmetic and logic instructions add rd, rr add two registers rd rd + rr z,c,n,v,h 1 adc rd, rr add with carry two registers rd rd + rr + c z,c,n,v,h 1 adiw rdl,k add immediate to word rdh:rdl rdh:rdl + k z,c,n,v,s 2 sub rd, rr subtract two registers rd rd - rr z,c,n,v,h 1 subi rd, k subtract constant from register rd rd - k z,c,n,v,h 1 sbc rd, rr subtract with carry two registers rd rd - rr - c z,c,n,v,h 1 sbci rd, k subtract with carry constant from reg. rd rd - k - c z,c,n,v,h 1 sbiw rdl,k subtract immediate from word rdh:rdl rdh:rdl - k z,c,n,v,s 2 and rd, rr logical and registers rd rd ? rr z,n,v 1 andi rd, k logical and register and constant rd rd ? k z,n,v 1 or rd, rr logical or registers rd rd v rr z,n,v 1 ori rd, k logical or register and constant rd rd v k z,n,v 1 eor rd, rr exclusive or registers rd rd rr z,n,v 1 com rd one?s complement rd 0xff ? rd z,c,n,v 1 neg rd two?s complement rd 0x00 ? rd z,c,n,v,h 1 sbr rd,k set bit(s) in register rd rd v k z,n,v 1 cbr rd,k clear bit(s) in register rd rd ? (0xff - k) z,n,v 1 inc rd increment rd rd + 1 z,n,v 1 dec rd decrement rd rd ? 1 z,n,v 1 tst rd test for zero or minus rd rd ? rd z,n,v 1 clr rd clear register rd rd rd z,n,v 1 ser rd set register rd 0xff none 1 mul rd, rr multiply unsigned r1:r0 rd x rr z,c 2 muls rd, rr multiply signed r1:r0 rd x rr z,c 2 mulsu rd, rr multiply signed with unsigned r1:r0 rd x rr z,c 2 fmul rd, rr fractional multiply unsigned r1:r0 (rd x rr) << 1 z,c 2 fmuls rd, rr fractional multiply signed r1:r0 (rd x rr) << 1 z,c 2 fmulsu rd, rr fractional multiply signed with unsigned r1:r0 (rd x rr) << 1 z,c 2 branch instructions rjmp k relative jump pc pc + k + 1 none 2 ijmp indirect jump to (z) pc z none 2 jmp (1) k direct jump pc knone3 rcall k relative subroutine call pc pc + k + 1 none 3 icall indirect call to (z) pc znone3 call (1) k direct subroutine call pc knone4 ret subroutine return pc stack none 4 reti interrupt return pc stack i 4 cpse rd,rr compare, skip if equal if (rd = rr) pc pc + 2 or 3 none 1/2/3 cp rd,rr compare rd ? rr z, n,v,c,h 1 cpc rd,rr compare with carry rd ? rr ? c z, n,v,c,h 1 cpi rd,k compare register with immediate rd ? k z, n,v,c,h 1 sbrc rr, b skip if bit in register cleared if (rr(b)=0) pc pc + 2 or 3 none 1/2/3 sbrs rr, b skip if bit in register is set if (rr(b)=1) pc pc + 2 or 3 none 1/2/3 sbic p, b skip if bit in i/o register cleared if (p(b)=0) pc pc + 2 or 3 none 1/2/3 sbis p, b skip if bit in i/o register is set if (p(b)=1) pc pc + 2 or 3 none 1/2/3 brbs s, k branch if status flag set if (sreg(s) = 1) then pc pc+k + 1 none 1/2 brbc s, k branch if status flag cleared if (sreg(s) = 0) then pc pc+k + 1 none 1/2 breq k branch if equal if (z = 1) then pc pc + k + 1 none 1/2 brne k branch if not equal if (z = 0) then pc pc + k + 1 none 1/2 brcs k branch if carry set if (c = 1) then pc pc + k + 1 none 1/2 brcc k branch if carry cleared if (c = 0) then pc pc + k + 1 none 1/2 brsh k branch if same or higher if (c = 0) then pc pc + k + 1 none 1/2 brlo k branch if lower if (c = 1) then pc pc + k + 1 none 1/2 brmi k branch if minus if (n = 1) then pc pc + k + 1 none 1/2 brpl k branch if plus if (n = 0) then pc pc + k + 1 none 1/2 brge k branch if greater or equal, signed if (n v= 0) then pc pc + k + 1 none 1/2 brlt k branch if less than zero, signed if (n v= 1) then pc pc + k + 1 none 1/2 brhs k branch if half carry flag set if (h = 1) then pc pc + k + 1 none 1/2 brhc k branch if half carry flag cleared if (h = 0) then pc pc + k + 1 none 1/2 brts k branch if t flag set if (t = 1) then pc pc + k + 1 none 1/2 brtc k branch if t flag cleared if (t = 0) then pc pc + k + 1 none 1/2 brvs k branch if overflow flag is set if (v = 1) then pc pc + k + 1 none 1/2 brvc k branch if overflow flag is cleared if (v = 0) then pc pc + k + 1 none 1/2
330 atmega48/88/168 2545d?avr?07/04 brie k branch if interrupt enabled if ( i = 1) then pc pc + k + 1 none 1/2 brid k branch if interrupt disabled if ( i = 0) then pc pc + k + 1 none 1/2 bit and bit-test instructions sbi p,b set bit in i/o register i/o(p,b) 1none2 cbi p,b clear bit in i/o register i/o(p,b) 0none2 lsl rd logical shift left rd(n+1) rd(n), rd(0) 0 z,c,n,v 1 lsr rd logical shift right rd(n) rd(n+1), rd(7) 0 z,c,n,v 1 rol rd rotate left through carry rd(0) c,rd(n+1) rd(n),c rd(7) z,c,n,v 1 ror rd rotate right through carry rd(7) c,rd(n) rd(n+1),c rd(0) z,c,n,v 1 asr rd arithmetic shift right rd(n) rd(n+1), n=0..6 z,c,n,v 1 swap rd swap nibbles rd(3..0) rd(7..4),rd(7..4) rd(3..0) none 1 bset s flag set sreg(s) 1 sreg(s) 1 bclr s flag clear sreg(s) 0 sreg(s) 1 bst rr, b bit store from register to t t rr(b) t 1 bld rd, b bit load from t to register rd(b) tnone1 sec set carry c 1c1 clc clear carry c 0 c 1 sen set negative flag n 1n1 cln clear negative flag n 0 n 1 sez set zero flag z 1z1 clz clear ze ro flag z 0 z 1 sei global interrupt enable i 1i1 cli global interrupt disable i 0 i 1 ses set signed test flag s 1s1 cls clear signed test flag s 0 s 1 sev set twos complement overflow. v 1v1 clv clear twos complement overflow v 0 v 1 set set t in sreg t 1t1 clt clear t in sreg t 0 t 1 seh set half carry flag in sreg h 1h1 clh clear half carry flag in sreg h 0 h 1 data transfer instructions mov rd, rr move between registers rd rr none 1 movw rd, rr copy register word rd+1:rd rr+1:rr none 1 ldi rd, k load immediate rd knone1 ld rd, x load indirect rd (x) none 2 ld rd, x+ load indirect and post-inc. rd (x), x x + 1 none 2 ld rd, - x load indirect and pre-dec. x x - 1, rd (x) none 2 ld rd, y load indirect rd (y) none 2 ld rd, y+ load indirect and post-inc. rd (y), y y + 1 none 2 ld rd, - y load indirect and pre-dec. y y - 1, rd (y) none 2 ldd rd,y+q load indirect with displacement rd (y + q) none 2 ld rd, z load indirect rd (z) none 2 ld rd, z+ load indirect and post-inc. rd (z), z z+1 none 2 ld rd, -z load indirect and pre-dec. z z - 1, rd (z) none 2 ldd rd, z+q load indirect with displacement rd (z + q) none 2 lds rd, k load direct from sram rd (k) none 2 st x, rr store indirect (x) rr none 2 st x+, rr store indirect and post-inc. (x) rr, x x + 1 none 2 st - x, rr store indirect and pre-dec. x x - 1, (x) rr none 2 st y, rr store indirect (y) rr none 2 st y+, rr store indirect and post-inc. (y) rr, y y + 1 none 2 st - y, rr store indirect and pre-dec. y y - 1, (y) rr none 2 std y+q,rr store indirect with displacement (y + q) rr none 2 st z, rr store indirect (z) rr none 2 st z+, rr store indirect and post-inc. (z) rr, z z + 1 none 2 st -z, rr store indirect and pre-dec. z z - 1, (z) rr none 2 std z+q,rr store indirect with displacement (z + q) rr none 2 sts k, rr store direct to sram (k) rr none 2 lpm load program memory r0 (z) none 3 lpm rd, z load program memory rd (z) none 3 lpm rd, z+ load program memory and post-inc rd (z), z z+1 none 3 spm store program memory (z) r1:r0 none - in rd, p in port rd pnone1 out p, rr out port p rr none 1 push rr push register on stack stack rr none 2 mnemonics operands description operation flags #clocks
331 atmega48/88/168 2545d?avr?07/04 note: 1. these instructions are only available in atmega168. pop rd pop register from stack rd stack none 2 mcu control instructions nop no operation none 1 sleep sleep (see specific descr. for sleep function) none 1 wdr watchdog reset (see specific descr. for wdr/timer) none 1 break break for on-chip debug only none n/a mnemonics operands description operation flags #clocks
332 atmega48/88/168 2545d?avr?07/04 ordering information atmega48 note: 1. this device can also be supplied in wafer form. please contact your local atmel sales office for detailed ordering infor mation and minimum quantities. 2. pb-free packaging alternative 3. see figure 131 on page 293 and figure 132 on page 293. speed (mhz) power supply ordering code package operation range 10 (3) 1.8 - 5.5 atmega48v-10ai atmega48v-10pi atmega48v-10mi atmega48v-10aj (2) atmega48v-10pj (2) atmega48v-10mj (2) 32a 28p3 32m1-a 32a 28p3 32m1-a industrial (-40 c to 85 c) 20 (3) 2.7 - 5.5 atmega48-20ai atmega48-20pi atmega48-20mi atmega48-20aj (2) atmega48-20pj (2) atmega48-20mj (2) 32a 28p3 32m1-a 32a 28p3 32m1-a industrial (-40 c to 85 c) package type 32a 32-lead, thin (1.0 mm) plastic quad flat package (tqfp) 28p3 28-lead, 0.300? wide, plastic dual inline package (pdip) 32m1-a 32-pad, 5 x 5 x 1.0 body, lead pitch 0.50 mm micro lead frame package (mlf)
333 atmega48/88/168 2545d?avr?07/04 atmega88 note: 1. this device can also be supplied in wafer form. please contact your local atmel sales office for detailed ordering infor mation and minimum quantities. 2. pb-free packaging alternative 3. see figure 131 on page 293 and figure 132 on page 293. speed (mhz) power supply ordering code package operation range 10 (3) 1.8 - 5.5 atmega88v-10ai atmega88v-10pi atmega88v-10mi atmega88v-10aj (2) atmega88v-10pj (2) atmega88v-10mj (2) 32a 28p3 32m1-a 32a 28p3 32m1-a industrial (-40 c to 85 c) 20 (3) 2.7 - 5.5 atmega88-20ai atmega88-20pi atmega88-20mi atmega88-20aj (2) atmega88-20pj (2) atmega88-20mj (2) 32a 28p3 32m1-a 32a 28p3 32m1-a industrial (-40 c to 85 c) package type 32a 32-lead, thin (1.0 mm) plastic quad flat package (tqfp) 28p3 28-lead, 0.300? wide, plastic dual inline package (pdip) 32m1-a 32-pad, 5 x 5 x 1.0 body, lead pitch 0.50 mm micro lead frame package (mlf)
334 atmega48/88/168 2545d?avr?07/04 atmega168 note: 1. this device can also be supplied in wafer form. please contact your local atmel sales office for detailed ordering infor mation and minimum quantities. 2. pb-free packaging alternative 3. see figure 131 on page 293 and figure 132 on page 293. speed (mhz) power supply ordering code package operation range 10 (3) 1.8 - 5.5 atmega168v-10ai atmega168v-10pi ATMEGA168V-10MI atmega168v-10aj (2) atmega168v-10pj (2) atmega168v-10mj (2) 32a 28p3 32m1-a 32a 28p3 32m1-a industrial (-40 c to 85 c) 20 (3) 2.7 - 5.5 atmega168-20ai atmega168-20pi atmega168-20mi atmega168-20aj (2) atmega168-20pj (2) atmega168-20mj (2) 32a 28p3 32m1-a 32a 28p3 32m1-a industrial (-40 c to 85 c) package type 32a 32-lead, thin (1.0 mm) plastic quad flat package (tqfp) 28p3 28-lead, 0.300? wide, plastic dual inline package (pdip) 32m1-a 32-pad, 5 x 5 x 1.0 body, lead pitch 0.50 mm micro lead frame package (mlf)
335 atmega48/88/168 2545d?avr?07/04 packaging information 32a 2325 orchard parkway san jose, ca 95131 title drawing no. r rev. 32a, 32-lead, 7 x 7 mm body size, 1.0 mm body thickness, 0.8 mm lead pitch, thin profile plastic quad flat package (tqfp) b 32a 10/5/2001 pin 1 identifier 0?~7? pin 1 l c a1 a2 a d1 d e e1 e b notes: 1. this package conforms to jedec reference ms-026, variation aba. 2. dimensions d1 and e1 do not include mold protrusion. allowable protrusion is 0.25 mm per side. dimensions d1 and e1 are maximum plastic body size dimensions including mold mismatch. 3. lead coplanarity is 0.10 mm maximum. a 1.20 a1 0.05 0.15 a2 0.95 1.00 1.05 d 8.75 9.00 9.25 d1 6.90 7.00 7.10 note 2 e 8.75 9.00 9.25 e1 6.90 7.00 7.10 note 2 b 0.30 0.45 c 0.09 0.20 l 0.45 0.75 e 0.80 typ common dimensions (unit of measure = mm) symbol min nom max note
336 atmega48/88/168 2545d?avr?07/04 28p3 2325 orchard parkway san jose, ca 95131 title drawing no. r rev. 28p3 , 28-lead (0.300"/7.62 mm wide) plastic dual inline package (pdip) b 28p3 09/28/01 pin 1 e1 a1 b ref e b1 c l seating plane a 0?~ 15? d e eb b2 (4 places) common dimensions (unit of measure = mm) symbol min nom max note a 4.5724 a1 0.508 d 34.544 ? 34.798 note 1 e 7.620 8.255 e1 7.112 7.493 note 1 b 0.381 0.533 b1 1.143 1.397 b2 0.762 1.143 l 3.175 3.429 c 0.203 0.356 eb 10.160 e 2.540 typ note: 1. dimensions d and e1 do not include mold flash or protrusion. mold flash or protrusion shall not exceed 0.25 mm (0.010").
337 atmega48/88/168 2545d?avr?07/04 32m1-a 2325 orchard parkway san jose, ca 95131 title drawing no. r rev. 32m1-a , 32-pad, 5 x 5 x 1.0 mm body, lead pitch 0.50 mm micro lead frame package (mlf) c 32m1-a 01/15/03 common dimensions (unit of measure = mm) symbol min nom max note pin 1 id d1 d e1 e e b a3 a2 a1 a d2 e2 0.08 c l 1 2 3 p p 0 1 2 3 a 0.80 0.90 1.00 a1 ? 0.02 0.05 a2 ? 0.65 1.00 a3 0.20 ref b 0.18 0.23 0.30 d 5.00 bsc d1 4.75 bsc d2 2.95 3.10 3.25 e 5.00 bsc e1 4.75bsc e2 2.95 3.10 3.25 e 0.50 bsc l 0.30 0.40 0.50 p ? ? 0.60 ? ? 12 o notes: 1. jedec standard mo-220, fig. 2 (anvil singulation), vhhd-2. top view side view bottom view 0 pin 1 id
338 atmega48/88/168 2545d?avr?07/04 errata atmega48 the revision letter in this section refers to the revision of the atmega48 device. rev a wrong values read after erase only operation watchdog timer interrupt disabled start-up time with crystal oscill ator is higher than expected high power consumption in power-down with external clock asynchronous oscillator does not stop in power-down 1. wrong values read after erase only operation at supply voltages below 2.7 v, an eeprom location that is erased by the erase only operation may read as programmed (0x00). problem fix/workaround if it is necessary to read an eeprom location after eras e only, use an atomic write operation with 0xff as data in order to erase a location. in any case, the write only operation can be used as intended. thus no special considerations are needed as long as the erased location is not read before it is programmed. 2. watchdog timer interrupt disabled if the watchdog timer interrupt flag is not cleared before a new timeout occurs, the watchdog will be disabled, and the interrupt flag will automatically be cleared. this is only applicable in interrupt only mode. if the watchdog is configured to reset the device in the watchdog time-out following an interrupt, the device works correctly. problem fix / workaround make sure there is enough time to always service the first timeout event before a new watchdog timeout occurs. this is done by selecting a long enough time-out period. 3. start-up time with crystal oscillator is higher than expected the clock counting part of the start-up time is about 2 times higher than expected for all start-up periods when running on an external crystal. this applies only when waking up by reset. wake-up from power down is not affected. for most settings, the clock counting parts is a small fraction of the overall start-up time, and thus, the problem can be ignored. the exception is when using a very low frequency crystal like for instance a 32 khz clock crystal. problem fix / workaround no known workaround. 4. high power consumption in power-down with external clock the power consumption in power down with an active external clock is about 10 times higher than when using internal rc or external oscillators. problem fix / workaround stop the external clock when the device is in power down. 5. asynchronous oscillator does not stop in power-down the asynchronous oscillator does not stop when entering power down mode. this leads to higher power consumption than expected. problem fix / workaround manually disable the asynchronous timer before entering power down.
339 atmega48/88/168 2545d?avr?07/04 errata atmega88 the revision letter in this section refers to the revision of the atmega88 device. rev. a writing to eeprom does not wo rk at low operating voltages part may hang in reset 1. writing to eeprom does not wo rk at low operating voltages writing to the eeprom does not work at low voltages. problem fix/workaround do not write the eeprom at voltages below 4.5 volts. this will be corrected in rev. b. 2. part may hang in reset some parts may get stuck in a reset state when a reset signal is applied when the internal reset state-machine is in a specific state. the in ternal reset state-machine is in this state for approximately 10 ns immediately before the part wakes up after a reset, and in a 10 ns window when altering the system clock prescaler. the problem is most often seen during in-system programming of the device. there are theoreti- cal possibilities of this happening also in run-mode. the follo wing three cases can trigger the device to get stuck in a reset-state: - two succeeding resets are applied where the second reset occurs in the 10ns win- dow before the device is out of the reset-state caused by the first reset. - a reset is applied in a 10 ns window while the system clock prescaler value is updated by software. - leaving spi-programming mode generates an internal reset signal that can trigger this case. the two first cases can occur during normal operating mode, while the last case occurs only during programming of the device. problem fix/workaround the first case can be avoided during run -mode by ensuring that only one reset source is active. if an external reset push button is used, the reset start-up time should be selected such that the reset line is fully debounced during the start-up time. the second case can be avoided by not using the system clock prescaler. the third case occurs during in-system programming only. it is most frequently seen when using the internal rc at maximum frequency. if the device gets stuck in the reset-state, turn power off, then on again to get the device out of this state.
340 atmega48/88/168 2545d?avr?07/04 errata atmega168 the revision letter in this section refers to the revision of the atmega168 device. rev a wrong values read after erase only operation part may hang in reset 1. wrong values read after erase only operation at supply voltages below 2.7 v, an eeprom location that is erased by the erase only operation may read as programmed (0x00). problem fix/workaround if it is necessary to read an eeprom location after eras e only, use an atomic write operation with 0xff as data in order to erase a location. in any case, the write only operation can be used as intended. thus no special considerations are needed as long as the erased location is not read before it is programmed. 2. part may hang in reset some parts may get stuck in a reset state when a reset signal is applied when the internal reset state-machine is in a specific state. the in ternal reset state-machine is in this state for approximately 10 ns immediately before the part wakes up after a reset, and in a 10 ns window when altering the system clock prescaler. the problem is most often seen during in-system programming of the device. there are theoreti- cal possibilities of this happening also in run-mode. the follo wing three cases can trigger the device to get stuck in a reset-state: - two succeeding resets are applied where the second reset occurs in the 10ns win- dow before the device is out of the reset-state caused by the first reset. - a reset is applied in a 10 ns window while the system clock prescaler value is updated by software. - leaving spi-programming mode generates an internal reset signal that can trigger this case. the two first cases can occur during normal operating mode, while the last case occurs only during programming of the device. problem fix/workaround the first case can be avoided during run -mode by ensuring that only one reset source is active. if an external reset push button is used, the reset start-up time should be selected such that the reset line is fully debounced during the start-up time. the second case can be avoided by not using the system clock prescaler. the third case occurs during in-system programming only. it is most frequently seen when using the internal rc at maximum frequency. if the device gets stuck in the reset-state, turn power off, then on again to get the device out of this state.
341 atmega48/88/168 2545d?avr?07/04 datasheet change log please note that the referring page numbers in this section are referred to this docu- ment. the referring revision in this section are referring to the document revision. changes from rev. 2545c-04/04 to rev. 2545d-07/04 changes from rev. 2545b-01/04 to rev. 2545c-04/04 changes from rev. 2545a-09/03 to rev. 2545b-01/04 1. updated instructions used with wdtcsr in relevant code examples. 2. updated table 8 on page 28, table 21 on page 43, table 112 on page 269, table 114 on page 269, and table 131 on page 288. 3. updated ?system clock prescaler? on page 33. 4. moved ?timer/counter2 interrupt mask register ? timsk2? and ?timer/counter2 interrupt flag register ? tifr2? to ?8-bit timer/counter register description? on page 143. 5. updated cross-reference in ?electrical interconnection? on page 199. 6. updated equation in ?bit rate generator unit? on page 204. 7. added ?page size? on page 274. 8. updated ?serial programming algorithm? on page 287. 9. updated ?ordering information? for ?atmega168? on page 334 10. updated ?errata atmega88? on page 339 and ?errata atmega168? on page 340. 1. speed grades changed: - 12mhz to 10mhz - 24mhz to 20mhz 2. updated ?maximum speed vs. vcc? on page 293. 3. updated ?ordering information? on page 332. 4. updated ?errata atmega88? on page 339. 1. added pdip to ?i/o and packages?, updated ?speed grade? and power consumption estimates in ?features? on page 1. 2. updated ?stack pointer? on page 11 with ramend as recommended stack pointer value. 3. added section ?power reduction register? on page 37 and a note regarding the use of the prr bits to 2-wire, timer/counters, usart, analog comparator and adc sections. 4. updated ?watchdog timer? on page 46. 5. updated figure 55 on page 125 and table 56 on page 126. 6. extra compare match interrupt ocf2b added to features in section ?8- bit timer/counter2 with pwm and asynchronous operation? on page 132 7. updated table 19 on page 37, table 102 on page 245, table 118 to table 121 on page 272 to 273 and table 98 on page 236. added note 2 to table 115 on page 270. fixed typo in table 42 on page 81. 8. updated whole ?atmega48/88/168 typical characteristics ? preliminary data? on page 298. 9. added item 2 to 5 in ?errata atmega48? on page 338.
342 atmega48/88/168 2545d?avr?07/04 10. renamed the following bits: - spmen to selfprgen - psr2 to psrasy - psr10 to psrsync - watchdog reset to watchdog system reset 11. updated c code examples containing old iar syntax. 12. updated blbset description in ?store program memory control and status register ? spmcsr? on page 260.
i atmega48/88/168 2545d?avr?07/04 table of contents features............... .............. .............. ............... .............. .............. .......... 1 pin configurations............ .............. ............... .............. .............. .......... 2 disclaimer ............................................................................................................. 2 overview.............. .............. .............. ............... .............. .............. .......... 3 block diagram ...................................................................................................... 3 comparison between atmega48, atmega88, and atmega168......................... 4 pin descriptions.................................................................................................... 5 about code examples........... ................ ................. ................ ............. 6 avr cpu core ................ ................ ............... .............. .............. .......... 7 introduction ........................................................................................................... 7 architectural overview.......................................................................................... 7 alu ? arithmetic logic unit.................................................................................. 8 status register ..................................................................................................... 9 general purpose register file ........................................................................... 10 stack pointer ...................................................................................................... 11 instruction execution timing............................................................................... 12 reset and interrupt handling.............................................................................. 12 avr atmega48/88/168 memories ................ .............. .............. ........ 15 in-system reprogrammable flash program memory ........................................ 15 sram data memory........................................................................................... 16 eeprom data memory............ ................ ................ ................ ................ .......... 17 i/o memory ......................................................................................................... 23 system clock and clock options ............. .............. .............. ........... 24 clock systems and their distribution .................................................................. 24 clock sources..................................................................................................... 25 low power crystal oscillator.............................................................................. 26 full swing crystal oscillator ..................... .......................................................... 28 low frequency crystal oscillator ....................................................................... 29 calibrated internal rc oscillator .............. .......................................................... 30 128 khz internal oscillator.................................................................................. 31 external clock..................................................................................................... 31 clock output buffer ............................................................................................ 32 timer/counter oscillator........................... .......................................................... 32 system clock prescaler...................................................................................... 33 power management and sleep modes......... .............. .............. ........ 35 idle mode ............................................................................................................ 36 adc noise reduction mode............................................................................... 36 power-down mode.............................................................................................. 36 power-save mode............................................................................................... 36
ii atmega48/88/168 2545d?avr?07/04 standby mode..................................................................................................... 37 power reduction register .................................................................................. 37 minimizing power consumption ......................................................................... 38 system control and reset ..... ............... ................. ................ ........... 40 internal voltage reference ................................................................................. 45 watchdog timer ................................................................................................. 46 interrupts ................ ................ ................ ................. ................ ........... 51 interrupt vectors in atmega48........................................................................... 51 interrupt vectors in atmega88........................................................................... 53 interrupt vectors in atmega168......................................................................... 57 i/o-ports........ ................. ................ .............. .............. .............. ........... 62 introduction ......................................................................................................... 62 ports as general digital i/o ................................................................................ 63 alternate port functions ..................................................................................... 67 register description for i/o ports ....................................................................... 79 external interrupts.......... .............. .............. .............. .............. ........... 80 8-bit timer/counter0 with pw m............... ................ .............. ........... 85 overview............................................................................................................. 85 timer/counter clock sources............................................................................. 86 counter unit........................................................................................................ 86 output compare unit.......................................................................................... 87 compare match output unit ............................................................................... 89 modes of operation ............................................................................................ 90 timer/counter timing diagrams......................................................................... 94 8-bit timer/counter register description ........................................................... 95 timer/counter0 and timer/counter1 pr escalers ............. ............. 102 16-bit timer/counter1 with pwm............. ................ .............. ......... 104 overview........................................................................................................... 104 accessing 16-bit registers ............................................................................... 106 timer/counter clock sources........................................................................... 110 counter unit...................................................................................................... 110 input capture unit............................................................................................. 111 output compare units ...................................................................................... 113 compare match output unit ............................................................................. 114 modes of operation .......................................................................................... 115 timer/counter timing diagrams....................................................................... 123 16-bit timer/counter register description ....................................................... 125 8-bit timer/counter2 with pw m and asynchronous operation .. 132
iii atmega48/88/168 2545d?avr?07/04 overview........................................................................................................... 132 timer/counter clock sources........................................................................... 133 counter unit...................................................................................................... 133 output compare unit........................................................................................ 134 compare match output unit ............................................................................. 136 modes of operation .......................................................................................... 137 timer/counter timing diagrams....................................................................... 141 8-bit timer/counter register description ......................................................... 143 asynchronous operation of the timer/counter ................................................. 149 timer/counter prescaler................................................................................... 152 serial peripheral interface ? spi................ .............. .............. ......... 153 ss pin functionality.......................................................................................... 158 data modes ...................................................................................................... 160 usart0 ............... ................. ................ ................. ................ ........... 162 overview........................................................................................................... 162 clock generation .............................................................................................. 163 frame formats ................................................................................................. 166 usart initialization.......................................................................................... 167 data transmission ? the usart transmitter ................................................. 168 data reception ? the usart receiver .......................................................... 171 asynchronous data reception ......................................................................... 175 multi-processor communication mode ............................................................. 179 usart register description ............................................................................ 180 examples of baud rate setting........................................................................ 185 usart in spi mode ............. ................ ................. ................ ........... 189 overview........................................................................................................... 189 clock generation .............................................................................................. 189 spi data modes and timing............................................................................. 190 frame formats ................................................................................................. 190 data transfer.................................................................................................... 192 usart mspim register description ............................................................... 194 avr usart mspim vs. avr spi............................................................................................................ 197 2-wire serial interface....... .............. ............... .............. .............. ...... 198 features............................................................................................................ 198 2-wire serial interface bus definition................................................................ 198 data transfer and frame format ..................................................................... 199 multi-master bus systems, arbitration and synchronization ............................ 202 overview of the twi module ............................................................................ 204 twi register description.................................................................................. 206 using the twi ................................................................................................... 209 transmission modes......................................................................................... 213
iv atmega48/88/168 2545d?avr?07/04 multi-master systems and arbitration............................................................... 226 analog comparator ............. ................ ................. ................ ........... 228 analog comparator multiplexed input .............................................................. 230 analog-to-digital converter. ............... ................. ................ ........... 231 features............................................................................................................ 231 starting a conversion ....................................................................................... 233 prescaling and conversion timing ................................................................... 234 changing channel or reference selection ...................................................... 236 adc noise canceler......................................................................................... 237 adc conversion result.................................................................................... 241 debugwire on-chip debug s ystem .............. .............. ............ ...... 246 features............................................................................................................ 246 overview........................................................................................................... 246 physical interface ............................................................................................. 246 software break points ...................................................................................... 247 limitations of debugwire ................................................................................ 247 debugwire related register in i/o memory ................................................... 247 self-programming the flash, atmega48................ .............. ......... 248 addressing the flash during self-programming .............................................. 249 boot loader support ? read-while -write self-programming, atmega88 and atmega168 ......... .............. .............. .............. ......... 255 boot loader features ....................................................................................... 255 application and boot loader flash sections .................................................... 255 read-while-write and no read-while-write flash sections........................... 255 boot loader lock bits....................................................................................... 258 entering the boot loader program ................................................................... 259 addressing the flash during self-programming .............................................. 261 self-programming the flash ............................................................................. 262 memory programming........... ................ ................. ................ ......... 270 program and data memory lock bits .............................................................. 270 fuse bits........................................................................................................... 272 signature bytes ................................................................................................ 274 calibration byte ................................................................................................ 274 page size ......................................................................................................... 274 parallel programming parameters, pin mapping, and commands .................. 275 serial programming pin mapping ..................................................................... 277 parallel programming ....................................................................................... 278 serial downloading........................................................................................... 286 electrical characteristics...... ................ ................. ................ ......... 290
v atmega48/88/168 2545d?avr?07/04 absolute maximum ratings*............................................................................. 290 dc characteristics............................................................................................ 290 external clock drive waveforms ...................................................................... 292 external clock drive ......................................................................................... 292 maximum speed vs. v cc ........................................................................................................................ 293 2-wire serial interface characteristics .............................................................. 294 spi timing characteristics ............................................................................... 295 adc characteristics ? preliminary data........................................................... 297 atmega48/88/168 typica l characteristics ? preliminary data.... 298 active supply current ....................................................................................... 298 idle supply current ........................................................................................... 301 supply current of io modules .......................................................................... 304 power-down supply current ............................................................................ 306 power-save supply current ............................................................................. 307 standby supply current.................................................................................... 307 pin pull-up ........................................................................................................ 308 pin driver strength ........................................................................................... 310 pin thresholds and hysteresis ......................................................................... 313 bod thresholds and analog comparator offset ............................................. 316 internal oscillator speed ....................... ........................................................... 319 current consumption of peripheral units ......................................................... 321 current consumption in reset and reset pulse width..................................... 323 register summary ..... ................. ................ .............. .............. ......... 325 instruction set summary ...... ................ ................. ................ ......... 329 ordering information........... ................ ................. ................ ........... 332 atmega48 ........................................................................................................ 332 atmega88 ........................................................................................................ 333 atmega168 ...................................................................................................... 334 packaging information .......... ................ ................. ................ ......... 335 32a ................................................................................................................... 335 28p3 ................................................................................................................. 336 32m1-a ............................................................................................................. 337 errata atmega48 .......... ................ .............. .............. .............. ......... 338 rev a ................................................................................................................ 338 errata atmega88 .......... ................ .............. .............. .............. ......... 339 rev. a ............................................................................................................... 339 errata atmega168 ............... ................ ................. ................ ........... 340 rev a ................................................................................................................ 340
vi atmega48/88/168 2545d?avr?07/04 datasheet change log.......... ................ ................. ................ ......... 341 changes from rev. 2545c-04/04 to rev. 2545d-07/04................................... 341 changes from rev. 2545b-01/04 to rev. 2545c-04/04 ................................... 341 changes from rev. 2545a-09/03 to rev. 2545b-01/04 ................................... 341 table of contents ................ .............. .............. .............. .............. ......... i
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