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19-1942; Rev 3; 6/03
I2C-Compatible RTC in a TDFN
General Description
The MAX6900, I2CTM-bus-compatible real-time clock (RTC) in a 6-pin TDFN package contains a real-time clock/calendar and 31-byte 8-bit wide of static random access memory (SRAM). The real-time clock/calendar provides seconds, minutes, hours, day, date, month, and year information. The end of the month date is automatically adjusted for months with fewer than 31 days, including corrections for leap year up to the year 2100. The clock operates in either the 24hr or 12hr format with an AM/PM indicator.
Features
o Real-Time Clock Counts Seconds, Minutes, Hours, Date, Month, Day, and Year o Leap Year Compensation Valid up to Year 2100 o Fast (400kHz) I2C-Bus-Compatible Interface from 2.0V to 5.5V o 31 8 SRAM for Scratchpad Data Storage o Uses Standard 32.768kHz, 12.5pF Load, Watch Crystal o Ultra-Low 225nA (typ) Timekeeping Current o Single-Byte or Multiple-Byte (Burst Mode) Data Transfer for Read or Write of Clock Registers or SRAM o 6-Pin 3mm x 3mm x 0.8mm TDFN Surface-Mount Package o No External Crystal Bias Resistors or Capacitors Required
MAX6900
Applications
Portable Instruments Point-of-Sale Equipment Intelligent Instruments Battery-Powered Products
I2C is a trademark of Philips Corp. Purchase of I2C components of Maxim Integrated Products, Inc., or one of its sublicensed Associated Companies, conveys a license under the Philips I2C Patent Rights to use these components in an I2C system, provided that the system conforms to the I2C Standard Specification as defined by Philips.
Ordering Information
PART MAX6900ETT-T TEMP RANGE -40C to +85C PINPACKAGE 6 TDFN TOP MARK AEU
Related Real-Time Clock Products
PART MAX6900 MAX6901 MAX6902 SERIAL BUS I2C compatible 3-wire SPITM compatible SRAM 31 8 31
ALARM FUNCTION -- Polled Polled
OUTPUT FREQUENCY -- 32kHz --
PIN-PACKAGE 6 TDFN 8 TDFN 8 TDFN
8
31 8
SPI is a trademark of Motorola, Inc.
Pin Configuration
TOP VIEW
Typical Operating Circuit
VCC
VCC 1
6
SDA
VCC
0.01F RPU RPU
VCC
X1 2
MAX6900
5
SCL C
5
SCL
1 VCC
X1
2 CRYSTAL
X2 3
4
GND RPU = tr/Cbus
6
MAX6900 3 SDA X2 GND 4
TDFN
________________________________________________________________ Maxim Integrated Products
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For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at 1-888-629-4642, or visit Maxim's website at www.maxim-ic.com.
I2C-Compatible RTC in a TDFN MAX6900
ABSOLUTE MAXIMUM RATINGS
VCC to GND ..............................................................-0.3V to +6V All Other Pins to GND ................................-0.3V to (VCC + 0.3V) Input Current All Pins ............................................................................20mA Output Current All Outputs .......................................................................20mA Rate of Rise, VCC ............................................................100V/s Continuous Power Dissipation (TA = +70C) 6-Pin TDFN (derate 24.4mW/C above +70C) .......1951.0mW Operating Temperature Range ...............................TMIN to TMAX MAX6900 ETT-T .......................TMIN = -40C, TMAX = +85C Junction Temperature .....................................................+150C Storage Temperature Range ............................ -65C to +150C ESD Protection (all pins, Human Body model) ..................2000V Lead Temperature (soldering, 10s) ............................+300C
Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
DC ELECTRICAL CHARACTERISTICS
(VCC = +2.0V to +5.5V, TA = TMIN to TMAX, unless otherwise noted. Typical values are at VCC = +3.3V, TA = +25C.) (Note 1)
PARAMETER Operating Voltage Range Active Supply Current (Note 2) Timekeeping Supply Current (Note 3) 2-WIRE DIGITAL INPUTS SCL, SDA Input High Voltage Input Low Voltage Input Hysteresis (Note 5) Input Leakage Current (Note 4) Input Capacitance (Note 5) 2-WIRE DIGITAL OUTPUT SDA Output Low Voltage VOL ISINK = 4mA 0.4 V VIH VIL VHYS 0 < VIN < VCC 0.05 x VCC -10 10 10 0.7 x VCC 0.3 x VCC V V V nA pF SYMBOL VCC ICC ITK VCC = +2.0V VCC = +5.0V VCC = +2.0V VCC = +5.0V 0.225 1.2 CONDITIONS MIN 2 TYP MAX 5.5 30 110 0.630 1.7 UNITS V A A
AC ELECTRICAL CHARACTERISTICS
(VCC = +2.0V to +5.5V, TA = TMIN to TMAX, unless otherwise noted. Typical values are at VCC = +3.3V, TA = +25C.) (Notes 1, 6)
PARAMETER OSCILLATOR X1 to Ground Capacitance X2 to Ground Capacitance FAST I2C-BUS-COMPATIBLE TIMING SCL Clock Frequency Bus Free Time Between STOP and START Condition (Note 4) fSCL tBUF 0 1.3 400 kHz s 25 25 pF pF SYMBOL CONDITIONS MIN TYP MAX UNITS
2
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I2C-Compatible RTC in a TDFN
AC ELECTRICAL CHARACTERISTICS (continued)
(VCC = +2.0V to +5.5V, TA = TMIN to TMAX, unless otherwise noted. Typical values are at VCC = +3.3V, TA = +25C.) (Notes 1, 6)
PARAMETER Hold Time After (Repeated) START Condition (After this Period, the First Clock Is Generated) Repeated START Condition Setup Time STOP Condition Setup Time Data Hold Time (Note 7) Data Setup Time SCL Low Period SCL High Period Minimum SCL/SDA Rise Time (Note 8) Maximum SCL/SDA Rise Time (Note 8) Minimum SCL/SDA Fall Time (Receiving) (Notes 8, 9) Maximum SCL/SDA Fall Time (Receiving) (Notes 8, 9) Minimum SDA Fall Time (Transmitting) (Notes 8, 9) Maximum SDA Fall Time (Transmitting) (Notes 8, 9) Pulse Width of Spike Suppressed Capacitive Load for Each Bus Line SYMBOL CONDITIONS MIN TYP MAX UNITS
MAX6900
tHD:STA
0.6
s
tSU:STA tSU:STO tHD:DAT tSU:DAT tLOW tHIGH tr tr tf tf tf tf tSP CB
0.6 0.6 0 100 1.3 0.6 20 + 0.1CB 300 20 + 0.1CB 300 20 + 0.1CB 250 50 400 0.9
s s s ns s s ns ns ns ns ns ns ns pF
Note 1: Note 2: Note 3: Note 4: Note 5: Note 6: Note 7: Note 8: Note 9:
All parameters are 100% tested at TA = +25C. Limits over temperature are guaranteed by design and not production tested. ICC is specified with SCL = 400kHz and SDA = 400kHz. ITK is specified with SCL = Logic High (4.7k pullup resistor) and SDA = Logic High (4.7k pullup resistor); I2C-compatible bus inactive. MAX6900 I/O pins do not obstruct the SDA and SCL lines if VCC is switched off. Guaranteed by design. Not subject to production testing. All values referred to VIH min and VIL max levels. The MAX6900 internally provides a hold time of at least 300ns for the SDA signal (referred to the VIH min of the SCL signal) in order to bridge the undefined region of the falling edge of SCL. CB = total capacitance of one bus line in pF. The maximum tf for the SDA and SCL bus lines is specified at 300ns. The maximum fall time for the SDA output stage tf is specified at 250ns. This allows series protection resistors to be connected between the SDA/SCL pins and the SDA/SCL bus lines without exceeding the maximum specified tf.
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I2C-Compatible RTC in a TDFN MAX6900
Typical Operating Characteristics
(TA = +25C, unless otherwise noted.)
TIMEKEEPING CURRENT vs. VCC
1.6 1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 VCC (V)
MAX6900 toc01
TIMEKEEPING CURRENT (A)
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
Pin Description
PIN 1 2 3 4 5 6 -- NAME VCC X1 X2 GND SCL SDA PAD FUNCTION Power Supply 32.768kHz External Crystal 32.768kHz External Crystal Ground I2C-Bus-Compatible Clock Input I2C-Bus-Compatible Data Input/Output Ground
Detailed Description
The MAX6900 contains eight timekeeping registers, burst address registers, a control register, an on-chip 32.768kHz oscillator circuit, and a serial 2-wire, I2Ccompatible interface. There are also 31 bytes, 8 bits wide of SRAM on board. Time and calendar data are stored in the registers in a binary-coded decimal (BCD) format. Figure 1 shows an I2C-bus-compatible timing diagram. Figure 2 shows the MAX6900 functional diagram.
Real-Time Clock
The RTC provides seconds, minutes, hours, day, date, month, and year information. The end of the month is automatically adjusted for months with fewer than 31
BIT 6 (A6) BIT 0 LSB (R/W) ACKNOWLEDGE (A) STOP CONDITION (P)
PROTOCOL
START CONDITION (S)
BIT 7 MSB (A7)
tLOW tSU:STA
tHIGH 1/fSCL
SCL
tBUF SDA
tr
tf
tHD:STA
tHD:DAT
tHD:DAT
tSU:STO
Figure 1. Detailed I2C-Bus Timing Diagrams 4 _______________________________________________________________________________________
I2C-Compatible RTC in a TDFN MAX6900
X1 X2
1Hz OSCILLATOR 32.768kHz DIVIDER
SECONDS MINUTES HOURS DATE
VCC GND CONTROL LOGIC
MONTH DAY YEAR
SCL SDA
CONTROL I2C BUS INTERFACE ADDRESS REGISTER CENTURY CLOCK BURST 31 X 8 SRAM
Figure 2. Functional Diagram
days, including corrections for leap year up to the year 2100.
Applications Information
I2C-Bus-Compatible Interface
The I2C-bus-compatible serial interface allows bidirectional, 2-wire communication between multiple ICs. The two lines are SDA and SCL. Connect both lines to a positive supply through individual pullup resistors. A device on the I 2C-compatible bus that generates a message is called a transmitter and a device that receives the message is a receiver. The device that controls the message is the master and the devices that are controlled by the master are called slaves (Figure 3). The word message refers to data in the form of three 8-bit bytes for a Single Read or Write. The first byte is the Slave ID byte, the second byte is the Address/Command byte, and the third is the data. Data transfer can only be initiated when the bus is not busy (both SDA and SCL are high). A high-to-low transition of SDA while SCL is high is defined as the Start (S) condition; low-to-high transition of the data line while SCL is high is defined as the Stop (P) condition (Figure 4).
Crystal Oscillator
The MAX6900 uses an external, standard 12.5pF load watch crystal. No other external components are required for this timekeeping oscillator. Power-up oscillator start-time is dependent mainly upon applied VCC and ambient temperature. The MAX6900, because of its low timekeeping current, exhibits a typical startup time between 5s to 10s.
I2C-Compatible Interface
Interfacing the MAX6900 with a microprocessor or other I2C master is made easier by using the serial, I2Cbus-compatible or other I2C master interface. Only 2 wires are required to communicate with the clock and SRAM: SCL (serial clock) and SDA (data line). Data is transferred to and from the MAX6900 over the I/O data line, SDA. The MAX6900 uses 7-bit slave ID addressing. The MAX6900 does not respond to general call address commands.
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I2C-Compatible RTC in a TDFN MAX6900
SDA SCL MASTER TRANSMITTER/ RECEIVER SLAVE TRANSMITTER/ RECEIVER MASTER TRANSMITTER/ RECEIVER
SLAVE RECEIVER
MASTER TRANSMITTER
Figure 3. I2C Bus System Configuration
SDA
SDA
SCL S START CONDITION P STOP CONDITION
SCL
Figure 4. I2C Bus Start and Stop Conditions
SDA
SCL
DATA LINE STABLE; DATA VALID
CHANGE OF DATA ALLOWED
Figure 5. I2C Bus Bit Transfer
After the Start condition occurs, 1 bit of data is transferred for each clock pulse. The data on SDA must remain stable during the high portion of the clock pulse as changes in data during this time are interpreted as a control signal (Figure 5). Any time a start condition occurs, the Slave ID must follow immediately, regardless of completion of the previous data transfer. Before any data is transmitted on the I2C-bus-compatible serial interface, the device that is expected to respond is addressed first. The first byte sent after the start (S) procedure is the Address byte or 7-bit Slave
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ID. The MAX6900 acts as a slave transmitter/receiver. Therefore, SCL is only an input clock signal and SDA is a bidirectional data line. The Slave Address for the MAX6900 is shown in Figure 6.
1 BIT 7
0
1
0
0
0
0
RD/W BIT 0
Figure 6. I2C Bus Slave Address or 7-Bit Slave ID
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I2C-Compatible RTC in a TDFN MAX6900
DATA OUTPUT BY TRANSMITTER
D7
D6
D0
NOT ACKNOWLEDGE DATA OUTPUT BY RECEIVER
ACKNOWLEDGE
CLK9
SCL FROM MASTER S START CONDITION
1 CLK1
2 CLK2
8 CLK8
9
CLOCK PULSE FOR ACKNOWLEDGMENT
Figure 7. I2C Bus Acknowledge
An unlimited number of data bytes between the start and stop conditions can be sent between the transmitter and receiver. Each 8-bit byte is followed by an acknowledge bit. Also, a master receiver must generate an acknowledge after each byte it receives that has been clocked out of the slave transmitter. The device that acknowledges must pull down the SDA line during the acknowledge clock pulse (Figure 7), so that the SDA line is stable low during the high period of the acknowledge clock pulse (setup and hold times must also be met). A master receiver must signal an end of data to the transmitter by not generating an acknowledge on the last byte that has been clocked out of the slave. In this case, the transmitter must leave SDA high to enable the master to generate a stop condition. Any time a stop condition is received before the current byte of data transfer is complete, the last incomplete byte is ignored. The second byte of data sent after the start condition is the Address/Command byte (Figure 8). Each data transfer is initiated by an Address/Command byte. The MSB (bit 7) must be a logic 1. When the MSB is zero, Writes to the MAX6900 are disabled. Bit 6 specifies clock/calendar data if logic 0 or RAM data if logic 1 (Tables 1 and 2). Bits 1 through 5 specify the designated registers to be input or output. The LSB (bit 0) specifies a Write operation (input) if logic 0 or Read operation (output) if logic 1. The Command byte is always input starting with the MSB (bit 7).
A7 1
A6 RAM /CLK
A5 A5
A4 A4
A3 A3
A2 A2
A1 A1 RD
A0
/W
Figure 8. Address/Command Byte
Reading from the Timekeeping Registers
The timekeeping registers (Seconds, Minutes, Hours, Date, Month, Day, Year, and Century) read either with a Single Read or a Burst Read. Since the clock runs continuously and a Read takes a finite amount of time, it is possible that the clock counters could change during a Read operation, thereby reporting inaccurate timekeeping data. In the MAX6900, the clock counter data is buffered by a latch. Clock counter data is latched by the I2C-bus-compatible read command (on the falling edge of SCL when the Slave Acknowledge bit is sent after the Address/Command byte has been sent by the master to read a timekeeping register). Collision-detection circuitry ensures that this does not happen coincident with a seconds counter update to ensure accurate time data is being read. This avoids time data changes during a Read operation. The clock counters continue to count and keep accurate time during the Read operation. When using a Single Read to read each of the timekeeping registers individually, perform error checking
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I2C-Compatible RTC in a TDFN MAX6900
Table 1. Register Address Definition
FUNCTION CLOCK SEC A7 A6 A5 A4 A3 A2 A1 A0 1 0 0 0 0 0 0 RD /W VALUE 00-59 *POR STATE D7 D6 7 0 0 D5 D4 D3 D2 D1 1 SEC 0 0 0 0 0 D0
10 SEC 0
MIN
1
0
0
0
0
0
1
RD /W
00-59 *POR STATE
0 0 0
10 MIN 0 0 0
1 MIN 0 0 0
HR
1
0
0
0
0
1
0
RD /W
00-23 01-12 *POR STATE 01-28/29 01-30 01-31 *POR STATE
12/24 0 1/0 0 0
10 HR 10 A/P HR 0/1 0 0 0
1 HR
0
0
0
DATE
1
0
0
0
0
1
1
RD /W
0 0
0 0
10 DATE 0 0 0
1 DATE 0 0 1
MONTH
1
0
0
0
1
0
0
RD /W
01-12 *POR STATE
0 0
0 0
0 10M 0 0 0
1 MONTH 0 0 1
DAY
1
0
0
0
1
0
1
RD /W
01-07 *POR STATE
0 0
0 0
0 0
0 0
0 0
WEEK DAY 0 0 1
YEAR
1
0
0
0
1
1
0
RD /W
00-99 *POR STATE 0
10 YEAR 1 1 1 0
1 YEAR 0 0 0
CONTROL
1
0
0
0
1
1
1
RD /W *POR STATE
WP 0
0 0
0 0
0 0
0 0
0 0
0 0
0 0
CENTURY
1
0
0
1
0
0
1
RD /W
00-99 *POR STATE 0
1000 YEAR 0 0 1 1
100 YEAR 0 0 1
RESERVED
1
0
0
1
0
1
1
RD /W *POR STATE
0 0
0 0
0 0
0 0
0 0
0 1
0 1
0 1
CLOCK BURST
1
0
1
1
1
1
1
RD /W
8
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I2C-Compatible RTC in a TDFN MAX6900
Table 1. Register Address Definition (continued)
FUNCTION CLOCK RAM RAM 0 1 1 0 0 0 0 0 RD /W RAM DATA 0 x x x x x x x x A7 A6 A5 A4 A3 A2 A1 A0 VALUE D7 D6 D5 D4 D3 D2 D1 D0
RAM 30
1
1
1
1
1
1
0
RD /W
RAM DATA 30
x
x
x
x
x
x
x
x
RAM BURST
1
1
1
1
1
1
1
RD /W
Note: POR STATE defines power-on reset state of register contents.
Table 2. Hex Register Address Definition
HEX REGISTER ADDRESS/DESCRIPTION WRITE ADDRESS/ COMMAND BYTE (HEX) 80 82 84 86 88 8A 8C 8E 92 96 BE C0 C2 C4 C6 C8 CA CC CE READ ADDRESS/ COMMAND BYTE (HEX) 81 83 85 87 89 8B 8D 8F 93 97 BF C1 C3 C5 C7 C9 CB CD CF
DESCRIPTION
POR CONTENTS
Seconds Minutes Hours Date Month Day Year Control Century Reserved Clock Burst RAM 0 RAM 1 RAM 2 RAM 3 RAM 4 RAM 5 RAM 6 RAM 7
00 00 00 01 01 01 70 00 19 07 N/A Indeterminate Indeterminate Indeterminate Indeterminate Indeterminate Indeterminate Indeterminate Indeterminate
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I2C-Compatible RTC in a TDFN MAX6900
Table 2. Hex Register Address Definition (continued)
HEX REGISTER ADDRESS/DESCRIPTION WRITE ADDRESS/ COMMAND BYTE (HEX) D0 D2 D4 D6 D8 DA DC DE E0 E2 E4 E6 E8 EA EC EE F0 F2 F4 F6 F8 FA FC FE READ ADDRESS/ COMMAND BYTE (HEX) D1 D3 D5 D7 D9 DB DD DF E1 E3 E5 E7 E9 EB ED EF F1 F3 F5 F7 F9 FB FD FF
DESCRIPTION
POR CONTENTS
RAM 8 RAM 9 RAM 10 RAM 11 RAM 12 RAM 13 RAM 14 RAM 15 RAM 16 RAM 17 RAM 18 RAM 19 RAM 20 RAM 21 RAM 22 RAM 23 RAM 24 RAM 25 RAM 26 RAM 27 RAM 28 RAM 29 RAM 30 RAM Burst
Indeterminate Indeterminate Indeterminate Indeterminate Indeterminate Indeterminate Indeterminate Indeterminate Indeterminate Indeterminate Indeterminate Indeterminate Indeterminate Indeterminate Indeterminate Indeterminate Indeterminate Indeterminate Indeterminate Indeterminate Indeterminate Indeterminate Indeterminate N/A
on the receiving end. The potential for errors occurs when the seconds counter increments before all the other registers are read out. For example, suppose a carry of 13:59:59 to 14:00:00 occurs during Single Read operations of the timekeeping registers. The net data could become 14:59:59, which is erroneous realtime data. To prevent this with Single Read operations, read the Seconds register first (initial seconds) and store this value for future comparison. When the remaining timekeeping registers have been read out, read the Seconds register again (final seconds). If the initial seconds value is 59, check that the final seconds value is still 59; if not, repeat the entire Single Read process for the timekeeping registers. A comparison of
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the initial seconds value with the final seconds value can indicate if there was a bus delay problem in reading the timekeeping data (difference should always be 1s or less). Using a 100kHz bus speed, sequential Single Reads take under 2.5ms to read all seven of the timekeeping registers plus a second read of the Seconds register. The most accurate way to read the timekeeping registers is a Burst Read. In the Burst Read mode, the main timekeeping registers (Seconds, Minutes, Hours, Date, Month, Day, Year) and the Control register are read sequentially. All of the main timekeeping registers and the Control register must be read out as a group of eight registers, with 8 bytes each, for proper execution
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SINGLE WRITE BIT 7.................................. BIT 0 BIT 7..........................BIT 0 8_BIT DATA AS P ACK BIT 7-BIT SLAVE ID ADDR. 0 AS 1 R 0 AS ACK BIT ADDRESS/COMMAND BYTE BIT 7.....................................BIT ACK BIT
S
Table 3. Data Transfer Summary
SINGLE READ ACK BIT BIT 7.........................BIT 0 ACK BIT S 7-BIT SLAVE ID 1 AS 8_BIT DATA AS ADDR. 1 R 1 AS ADDRESS/COMMAND BYTE BIT 7...............................BIT 0 ACK BIT BIT 7...................BIT 0 ACK BIT /AM P
BIT 7..........................BIT 0 0
S
7-BIT SLAVE ID
BURST WRITE ACK BIT AS 1 11111 0 AS FIRST 8_BIT DATA AS ADDRESS/COMMAND BYTE BIT 7.........................BIT 0 ACK BIT BIT 7...............................BIT 0 ACK BIT BIT 7............................BIT 0 ACK BIT LAST 8_BIT DATA AS P
BIT 7............................BIT 0 0
S
7-BIT SLAVE ID
BURST READ ADDRESS/COMMAND BYTE BIT 7............................... BIT 0 ACK BIT 1 11111 1 AS S 7-BIT SLAVE ID 1 AS BIT 7...............................BIT 0 ACK BIT FIRST 8_BIT DATA AM
BIT 7............................ BIT 0 ACK BIT 0 AS
S
7-BIT SLAVE ID
BIT 7.........................BIT 0 ACK BIT LAST 8_BIT DATA /AM P
MAX6900
______________________________________________________________________________________
SLAVE ID: 1010000 ADDR: 5-BIT RAM OR REGISTER ADDRESS R: RAM/REGISTER SELECTION BIT. R = 0 WHEN REGISTER IS SELECTED, R = 1 WHEN RAM IS SELECTED S: 2-WIRE BUS START CONDITION BY MASTER P: 2-WIRE BUS STOP CONDITION BY MASTER AS: ACKNOWLEDGE BY SLAVE AM: ACKNOWLEDGE BY MASTER /AM: NO ACKNOWLEDGE BY MASTER
I2C-Compatible RTC in a TDFN
11
I2C-Compatible RTC in a TDFN MAX6900
of the Burst Read function. The seven timekeeping registers are latched upon the receipt of the Burst Read command. The worst-case error that can occur between the actual time and the read time is 1s, assuming the entire Burst Read is done in less than 1s. burst write function. All seven timekeeping registers are simultaneously loaded into the clock counters by the Stop bit at the end of the I2C-bus-compatible Write operation. The worst-case error that can occur between the actual time and the write time update is 1s, assuming the entire Burst Write is done in less than 1s. Note: After writing to any time or date register, no read or write operations are allowed for 2.5ms.
Writing to the Timekeeping Registers
The time and date may be set by writing to the timekeeping registers (Seconds, Minutes, Hours, Date, Month, Day, Year, and Century). To avoid changing the current time by an incomplete Write operation, the current time value is buffered from being written directly to the clock counters. Current time data is loaded into this buffer at the falling edge of SCL, on the Slave Acknowledge bit, before the data input byte or bytes are sent to the MAX6900. The clock counters continue to count. The new data replaces the current contents of this input buffer. The time update data is loaded into the clock counters by the Stop bit at the end of the I2C- bus-compatible Write operation. Collision-detection circuitry ensures that this does not happen coincident with a seconds counter update to ensure accurate time data is being written. This avoids time data changes during a Write operation. An incomplete Write operation aborts the time update procedure and the contents of the input buffer are discarded. The clock counters reflect the new time data beginning with the first 1s clock cycle after the Stop bit. When using single Write operations to write to each of the timekeeping registers, error checking is needed. If the Seconds register is the one to be updated, update it first and then read it back and store its value as the initial seconds. Update the remaining timekeeping registers and then read the Seconds register again (final seconds). If initial seconds was 59, ensure that it is still 59. If initial seconds was not 59, ensure that final seconds is within 1s of initial seconds. If the Seconds register is not to be written to, then read the Seconds register first and save it as initial seconds. Write to the required timekeeping registers and then read the Seconds register again (final seconds). If initial seconds was 59, ensure it is still 59. If initial seconds was not 59, ensure that final seconds is within 1s of initial seconds. The burst write mode is the most accurate way to write to the timekeeping registers, although both single Writes and Burst Writes are possible. In Burst Write, the main timekeeping registers (Seconds, Minutes, Hours, Date, Month, Day, Year) and the control register are written to sequentially. All the main timekeeping registers and the Control register must be written to as a group of eight registers, with 8 bytes each, for proper execution of the
Write Protect Bit
Bit 7 of the Control register is the Write Protect bit. The lower 7 bits (bits 0 to 6) are forced to zero and always read a zero when read. Before any Write operation to the clock or RAM, bit 7 must be zero. When high, the Write Protect bit prevents a Write operation to any other register.
AM-PM/12Hr-24Hr Mode
Bit 7 of the Hours register is defined as the 12hr or 24hr Mode Select bit. When high, the 12hr mode is selected. In the 12hr mode, bit 5 is the AM/PM bit with logic high being PM. In the 24hr mode, bit 5 is the second 10hr bit (20hr to 23hr).
Clock Burst Mode
Addressing the Clock Burst register (BEh for write, or BFh for read) specifies burst mode operation. In this mode, the first eight clock/calendar registers can be consecutively read or written starting with bit 7 of Address/Command 81h (Read) or 80h (Write). If the Write Protect bit is set high when a write clock/calendar burst mode is specified, no data transfer occurs to any of the eight clock/calendar registers or the Control register. When writing to the clock registers in the burst mode, the first eight registers must be written in order for the data to be transferred.
RAM
The static RAM is 31 bytes addressed consecutively in the RAM address space. Even Address/Commands (C0h-FCh) are used for Writes, and odd Address/ Commands (C1h-FDh) are used for Reads. The contents of the RAM are static and remain valid for VCC down to 2V.
RAM Burst Mode
Addressing the RAM Burst register (FEh for Write, or FFh for Read) specifies burst mode operation. In this mode, the 31 RAM locations can be consecutively read or written starting with bit 7 of Address/Command C1h (Read) or C0h (Write). When writing to RAM in burst mode, it is not necessary to write all 31 bytes for the data to transfer. Each byte that is written to is transferred to RAM. If the Write Protect bit is set high when a
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I2C-Compatible RTC in a TDFN
RAM burst mode is specified, no data transfer occurs to any of the RAM locations. Burst writes of data greater than 31 bytes could cause erroneous data in the MAX6900.
MAX6900
Rf MAX6900
Power-On Reset (POR)
The MAX6900 contains an integral POR circuit that ensures all registers are reset to a known state on power-up. Once VCC rises above 1.6V (typ), the POR circuit releases the registers for normal operation. When VCC drops to less than 1.6V (typ), the MAX6900 resets all register contents to the POR defaults.
Rd
Cg 25pF
Cd 25pF
Oscillator Startup
The MAX6900 oscillator typically takes 5s to 10s to begin oscillating. To ensure the oscillator is operating correctly, the software should validate proper timekeeping. This is accomplished by reading the Seconds register. Any reading of 1s or more from the POR is a validation of proper startup.
X1
X2 EXTERNAL CRYSTAL
Figure 9. Oscillator Functional Schematic
tal with a specified load capacitance (CL) of 12.5pF. See Table 4 for a list of crystal parameters. Table 5 lists some crystal manufacturers and part numbers for their surface-mount 32.768kHz watch crystals that require 12.5pF. In addition, these manufacturers offer other package options depending upon the specific application considerations.
Reserved Register
This is reserved for factory testing ONLY. Do not write to this register. If inadvertent Writes are done to this register, cycle power on the MAX6900.
Crystal Selection
Connect a 32.768kHz watch crystal directly to the MAX6900 through pin 2 and pin 3 (X1, X2). Use a crys-
Frequency Stability Overtemperature
Timekeeping accuracy of the MAX6900 is dependent on the frequency stability of the external crystal. To
Table 4. Quartz Crystal Parameters
PARAMETER Frequency Equivalent Series Resistance (ESR) Parallel Load Capacitance Q Factor SYMBOL f Rs CL Q 40 11.2 40,000 12.5 MIN TYP 32.768 60 13.7 60,000 MAX UNITS kHz k pF
Table 5. Crystal Manufacturers
MANUFACTURER Abracon Corporation Caliber Electronics ECS INC International Fox Electronics M-tron Raltron SaRonix MANUFACTURER PART NO. ABS25-32.768-12.5-BAWS2A-32.768KHz ECS-.327-12.5-17 FSM327 SX2010/SX2020 RSE-32.768-12.5-C-T 32S12A TEMP. RANGE -40C to +85C -20C to +70C -10C to +60C -40C to +85C -20C to +75C -10C to +60C -40C to +85C CL (pF) 12.5 12.5 12.5 12.5 12.5 12.5 12.5 +25C FREQUENCY TOLERANCE (ppm) 20 20 20 20 20 20 20
______________________________________________________________________________________
13
I2C-Compatible RTC in a TDFN MAX6900
TEMPERATURE (C)
-50 -40 -30 -20 -10 0 0 10 20 25 30 40 50 60 70 80 90
-50
Assuming 20ppm initial crystal tolerance (53s initial accuracy); total worst-case timekeeping error at the end of 1 month = 66.96s - 53s = -119.96s or about 2 minutes (assumes negligible parasitic layout capacitance).
f (ppm)
-100
Power-Supply Considerations
For most applications, a 0.1F capacitor from VCC to GND provides adequate bypassing for the MAX6900. Because the MAX6900's supply current is well under 1A, a series resistor can be added to the supply to reject extremely harsh noise.
-150
-200
-250 TYPICAL TEMPERATURE CHARACTERISITICS (k = -0.035ppm/C2, TO = +25C)
PC Board Layout Considerations
When designing the PC board, keep the crystal as close to the X1 and X2 pins of the MAX6900 as possible (Figure 11). Keep the trace lengths short and small to avoid introducing excessive capacitive loading and preventing unwanted noise pickup. Place a guard ring around the crystal and tie the ring to ground to help isolate the crystal from unwanted noise pickup. Keep all signals away from the crystal and the X1 and X2 pins to prevent noise coupling. Finally, an additional local ground plane on an adjacent PC board layer can be added under the crystal to shield it from unwanted pickup from traces on other layers of the board. This plane should be isolated from the regular PC board ground plane and tied to the GND pin of the MAX6900. This plane needs to be no larger than the perimeter of the guard ring. Ensure that this ground plane does not contribute to significant capacitance between ground and the traces that run from X1 and X2 to the crystal.
Figure 10. Typical Temperature Curve for 32.768kHz Watch Crystal
determine frequency stability, use the parabolic curve in Figure 10 and the following equations: f = f k (T0 - T )2 where: f = change in frequency from +20C. f = nominal crystal frequency. k = parabolic curvature constant (-0.035 0.005ppm/C2 for 32.768kHz watch crystals). T0 = turnover temperature (+25C 5C for 32.768kHz watch crystals). T = temperature of interest (C). For example: What is the worst-case change in oscillator frequency from +25C to +45C ambient? f (worst case) = 32,768 (-0.04 / 1 10e6) (20 -45)2 = -0.8192Hz After 1 month, that translates to:
hr min t = (31days) x 24 x 60 hr day s -0.8192Hz x 60 x = 66.96s min 32768Hz
Chip Information
TRANSISTOR COUNT: 19,307 PROCESS: CMOS
14
______________________________________________________________________________________
I2C-Compatible RTC in a TDFN
Figure 11. Recommended Board Layout
; ;;; ;;;
GROUND PLANE VIA CONNECTION GUARD RING VCC PLANE VIA CONNECTION
MAX6900
*
0.1F SM CAP
*
MAX6900
*
*
**
**
*
*
*
GROUND PLANE VIA CONNECTION
SM WATCH CRYSTAL
*
**
**
*
GROUND PLANE VIA CONNECTION
*LAYER 1 TRACE
** LAYER 2 LOCAL GROUND PLANE CONNECT ONLY TO PIN 4 GROUND PLANE VIA
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15
I2C-Compatible RTC in a TDFN MAX6900
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information go to www.maxim-ic.com/packages.)
6, 8, &10L, QFN THIN.EPS
1 2
L D A A2
PIN 1 ID
D2
1
N
1
b
PIN 1 INDEX AREA
C0.35 [(N/2)-1] x e REF. e
E
DETAIL A
E2
A1
k
C L
C L
L e A e
L
SEMICONDUCTOR
PROPRIETARY INFORMATION TITLE:
DALLAS
PACKAGE OUTLINE, 6, 8 & 10L, TDFN, EXPOSED PAD, 3x3x0.80 mm
NUMBER OF LEADS SHOWN ARE FOR REFERENCE ONLY
APPROVAL DOCUMENT CONTROL NO. REV.
21-0137
D
16
______________________________________________________________________________________
I2C-Compatible RTC in a TDFN MAX6900
Package Information (continued)
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information go to www.maxim-ic.com/packages.)
COMMON DIMENSIONS SYMBOL A D E A1 L k A2 MIN. 0.70 2.90 2.90 0.00 0.20 MAX. 0.80 3.10 3.10 0.05 0.40
0.25 MIN. 0.20 REF.
PACKAGE VARIATIONS PKG. CODE T633-1 T833-1 T1033-1 N 6 8 10 D2 1.50-0.10 1.50-0.10 1.50-0.10 E2 2.30-0.10 2.30-0.10 2.30-0.10 e 0.95 BSC 0.65 BSC 0.50 BSC JEDEC SPEC MO229 / WEEA MO229 / WEEC MO229 / WEED-3 b 0.40-0.05 0.30-0.05 0.25-0.05 [(N/2)-1] x e 1.90 REF 1.95 REF 2.00 REF
SEMICONDUCTOR
PROPRIETARY INFORMATION TITLE:
DALLAS
PACKAGE OUTLINE, 6, 8 & 10L, TDFN, EXPOSED PAD, 3x3x0.80 mm
APPROVAL DOCUMENT CONTROL NO. REV.
2 2
21-0137
D
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 17 (c) 2003 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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