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M48T08 M48T18
64 Kbit (8Kb x 8) TIMEKEEPER(R) SRAM
INTEGRATED ULTRA LOW POWER SRAM, REAL TIME CLOCK, POWER-FAIL CONTROL CIRCUIT and BATTERY BYTEWIDETM RAM-LIKE CLOCK ACCESS BCD CODED YEAR, MONTH, DAY, DATE, HOURS, MINUTES and SECONDS TYPICAL CLOCK ACCURACY of 1 MINUTE a MONTH, at 25C AUTOMATIC POWER-FAIL CHIP DESELECT and WRITE PROTECTION WRITE PROTECT VOLTAGES (VPFD = Power-fail Deselect Voltage): - M48T08: 4.5V VPFD 4.75V - M48T18: 4.2V VPFD 4.5V SOFTWARE CONTROLLED CLOCK CALIBRATION for HIGH ACCURACY APPLICATIONS SELF-CONTAINED BATTERY and CRYSTAL in the CAPHAT DIP PACKAGE PACKAGING INCLUDES a 28-LEAD SOIC and SNAPHAT(R) TOP (to be Ordered Separately) SOIC PACKAGE PROVIDES DIRECT CONNECTION for a SNAPHAT TOP which CONTAINS the BATTERY and CRYSTAL PIN and FUNCTION COMPATIBLE with DS1643 and JEDEC STANDARD 8K x 8 SRAMs
SNAPHAT (SH) Battery/Crystal
28
28 1
1
SOH28 (MH)
PCDIP28 (PC) Battery/Crystal CAPHAT
Figure 1. Logic Diagram
VCC
13 A0-A12
8 DQ0-DQ7
Table 1. Signal Names
A0-A12 DQ0-DQ7 INT E1 E2 G W VCC VSS May 1999 Address Inputs
W E1 M48T08 M48T18 INT
Data Inputs / Outputs Power Fail Interrupt (Open Drain) Chip Enable 1 Chip Enable 2 Output Enable Write Enable Supply Voltage Ground 1/19
E2 G
VSS
AI01020
M48T08, M48T18
Figure 2A. DIP Pin Connections
INT A12 A7 A6 A5 A4 A3 A2 A1 A0 DQ0 DQ1 DQ2 VSS 28 1 27 2 26 3 25 4 24 5 23 6 7 M48T08 22 8 M48T18 21 20 9 19 10 18 11 17 12 13 16 14 15
AI01182
Figure 2B. SOIC Pin Connections
VCC W E2 A8 A9 A11 G A10 E1 DQ7 DQ6 DQ5 DQ4 DQ3 INT A12 A7 A6 A5 A4 A3 A2 A1 A0 DQ0 DQ1 DQ2 VSS 1 28 27 2 26 3 25 4 24 5 23 6 22 7 M48T18 21 8 20 9 19 10 18 11 17 12 16 13 15 14
AI01021B
VCC W E2 A8 A9 A11 G A10 E1 DQ7 DQ6 DQ5 DQ4 DQ3
Table 2. Absolute Maximum Ratings (1)
Symbol TA TSTG TSLD
(2)
Parameter Ambient Operating Temperature Storage Temperature (VCC Off, Oscillator Off) Lead Solder Temperature for 10 seconds Input or Output Voltages Supply Voltage Output Current Power Dissipation
Value 0 to 70 -40 to 85 260 -0.3 to 7 -0.3 to 7 20 1
Unit C C C V V mA W
VIO VCC IO PD
Notes: 1. Stresses greater than those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. This is a stress rating only and functional operation of the device at these or any other conditions above those indicated in the operational section of this specification is not implied. Exposure to the absolute maximum rating conditions for extended periods of time may affect reliability. 2. Soldering temperature not to exceed 260C for 10 seconds (total thermal budget not to exceed 150C for longer than 30 seconds).
CAUTION: Negative undershoots below -0.3 volts are not allowed on any pin while in the Battery Back-up mode. CAUTION: Do NOT wave solder SOIC to avoid damaging SNAPHAT sockets.
Table 3. Operating Modes
Mode Deselect Deselect Write Read Read Deselect Deselect VSO to VPFD (min) VSO 4.75V to 5.5V or 4.5V to 5.5V VCC E1 VIH X VIL VIL VIL X X E2 X VIL VIH VIH VIH X X G X X X VIL VIH X X W X X VIL VIH VIH X X DQ0-DQ7 High Z High Z DIN DOUT High Z High Z High Z Power Standby Standby Active Active Active CMOS Standby Battery Back-up Mode
Notes: 1. X = VIH or VIL; VSO = Battery Back-up Switchover Voltage.
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M48T08, M48T18
Figure 3. Block Diagram
OSCILLATOR AND CLOCK CHAIN 32,768 Hz CRYSTAL POWER
8 x 8 BiPORT SRAM ARRAY
A0-A12
8184 x 8 SRAM ARRAY LITHIUM CELL VOLTAGE SENSE AND SWITCHING CIRCUITRY VPFD
DQ0-DQ7
E1 E2 W G
VCC
INT
VSS
AI01333
DESCRIPTION The M48T08/18 TIMEKEEPER(R) RAM is an 8K x 8 non-volatile static RAM and real time clock which is pin and functional compatible with the DS1643. The monolithic chip is available in two special packages to provide a highly integrated battery backed-up memory and real time clock solution. The M48T08/18 is a non-volatile pin and function equivalent to any JEDEC standard 8K x 8 SRAM. It also easily fits into many ROM, EPROM, and EEPROM sockets, providing the non-volatility of PROMs without any requirement for special write timing or limitations on the number of writes that can be performed. The 28 pin 600mil DIP CAPHATTM houses the M48T08/18 silicon with a quartz crystal and a long life lithium button cell in a single package. The 28 pin 330mil SOIC provides sockets with gold plated contacts at both ends for direct connection to a separate SNAPHAT housing containing the battery and crystal. The unique design allows the SNAPHAT battery package to be mounted on top of the SOIC package after the completion of the surface mount process. Insertion of the SNAPHAT housing after reflow prevents potential battery and crystal damage due to the high temperatures required for device surface-mounting. The SNAPHAT housing is keyed to prevent reverse insertion.
Table 4. AC Measurement Conditions
Input Rise and Fall Times Input Pulse Voltages Input and Output Timing Ref. Voltages 5ns 0 to 3V 1.5V
Note that Output Hi-Z is defined as the point where data is no longer driven.
Figure 4. AC Testing Load Circuit
5V
1.8k DEVICE UNDER TEST 1k
OUT
CL = 100pF
CL includes JIG capacitance
AI01019
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M48T08, M48T18
Table 5. Capacitance (1, 2) (TA = 25 C, f = 1 MHz )
Symbol CIN CIO
(3)
Parameter Input Capacitance Input / Output Capacitance
Test Condition VIN = 0V VOUT = 0V
Min
Max 10 10
Unit pF pF
Notes: 1. Effective capacitance measured with power supply at 5V. 2. Sampled only, not 100% tested. 3. Outputs deselected.
Table 6. DC Characteristics (TA = 0 to 70C; VCC = 4.75V to 5.5V or 4.5V to 5.5V)
Symbol ILI
(1) (1)
Parameter Input Leakage Current Output Leakage Current Supply Current Supply Current (Standby) TTL Supply Current (Standby) CMOS Input Low Voltage Input High Voltage Output Low Voltage Output Low Voltage (INT)
(4)
Test Condition 0V VIN VCC 0V VOUT VCC Outputs open E1 = VIH, E2 = VIL E1 = VCC - 0.2V, E2 = VSS + 0.2V
Min
Max 1 5 80 3 3
Unit A A mA mA mA V V V V V
ILO
ICC ICC1
(2)
ICC2 (2) VIL(3) VIH VOL VOH
Notes: 1. 2. 3. 4.
-0.3 2.2 IOL = 2.1mA IOL = 0.5mA IOH = -1mA 2.4
0.8 VCC + 0.3 0.4 0.4
Output High Voltage
Outputs Deselected. Measured with Control Bits set as follows: R = '1'; W, ST, FT = '0'. Negative spikes of -1V allowed for up to 10ns once per Cycle. The INT pin is Open Drain.
Table 7. Power Down/Up Trip Points DC Characteristics (1) (TA = 0 to 70C)
Symbol VPFD VPFD VSO tDR(2) Parameter Power-fail Deselect Voltage (M48T08) Power-fail Deselect Voltage (M48T18) Battery Back-up Switchover Voltage Expected Data Retention Time 10 Min 4.5 4.2 Typ 4.6 4.3 3.0 Max 4.75 4.5 Unit V V V YEARS
Notes: 1. All voltages referenced to VSS. 2. At 25C
DESCRIPTION (cont'd) The SOIC and battery/crystal packages are shipped separately in plastic anti-static tubes or in Tape & Reel form. For the 28 lead SOIC, the battery/crystal package (i.e. SNAPHAT) part number is "M4T28-BR12SH1".
As Figure 3 shows, the static memory array and the quartz controlled clock oscillator of the M48T08/18 are integrated on one silicon chip. The two circuits are interconnected at the upper eight memory locations to provide user accessible BYTEWIDETM clock information in the bytes with addresses 1FF8h-1FFFh.
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M48T08, M48T18
Table 8. Power Down/Up Mode AC Characteristics (TA = 0 to 70C)
Symbol tPD tF (1) tFB
(2)
Parameter E1 or W at VIH or E2 at VIL before Power Down VPFD (max) to VPFD (min) VCC Fall Time VPFD (min) to VSO VCC Fall Time VPFD(min) to VPFD (max) VCC Rise Time VSO to VPFD (min) VCC Rise Time E1 or W at VIH or E2 at VIL after Power Up INT Low to Auto Deselect VPFD (max) to INT High
Min 0 300 10 0 1 1 10
Max
Unit s s s s s ms
tR tRB tREC tPFX tPFH
(3)
40 120
s s
Notes: 1. VPFD (max) to VPFD (min) fall time of less than tF may result in deselection/write protection not occurring until 200 s after VCC passes VPFD (min). 2. VPFD (min) to VSO fall time of less than tFB may cause corruption of RAM data. 3. INT may go high anytime after VCC exceeds VPFD (min) and is guaranteed to go high tPFH after VCC exceeds VPFD (max).
Figure 5. Power Down/Up Mode AC Waveforms
VCC VPFD (max) VPFD (min) VSO tF tPD tFB tPFX INT tREC INPUTS
RECOGNIZED
tDR tRB
tR
tPFH
DON'T CARE
NOTE
RECOGNIZED
HIGH-Z OUTPUTS VALID
(PER CONTROL INPUT)
VALID
(PER CONTROL INPUT)
AI00566
Note: Inputs may or may not be recognized at this time. Caution should be taken to keep E1 high or E2 low as VCC rises past VPFD(min). Some systems may perform inadvertent write cycles after VCC rises above VPFD(min) but before normal system operations begin. Even though a power on reset is being applied to the processor, a reset condition may not occur until after the system clock is runn ing.
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M48T08, M48T18
Table 9. Read Mode AC Characteristics (TA = 0 to 70C; VCC = 4.75V to 5.5V or 4.5V to 5.5V)
M48T08 / M48T18 Symbol Parameter Min tAVAV tAVQV tE1LQV tE2HQV tGLQV tE1LQX tE2HQX tGLQX tE1HQZ tE2LQZ tGHQZ tAXQX Read Cycle Time Address Valid to Output Valid Chip Enable 1 Low to Output Valid Chip Enable 2 High to Output Valid Output Enable Low to Output Valid Chip Enable 1 Low to Output Transition Chip Enable 2 High to Output Transition Output Enable Low to Output Transition Chip Enable 1 High to Output Hi-Z Chip Enable 2 Low to Output Hi-Z Output Enable High to Output Hi-Z Address Transition to Output Transition 5 10 10 5 50 50 40 5 100 100 100 100 50 10 10 5 75 75 60 -100 Max Min 150 150 150 150 75 -150 Max ns ns ns ns ns ns ns ns ns ns ns ns Unit
Figure 6. Read Mode AC Waveforms
tAVAV A0-A12 tAVQV tE1LQV E1 tE1LQX tE2HQV E2 tE2HQX tGLQV G tGLQX DQ0-DQ7 VALID
AI00962
VALID tAXQX tE1HQZ
tE2LQZ
tGHQZ
Note: Write Enable (W) = High.
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M48T08, M48T18
Table 10. Write Mode AC Characteristics (TA = 0 to 70C; VCC = 4.75V to 5.5V or 4.5V to 5.5V)
M48T08 / M48T18 Symbol Parameter Min tAVAV tAVWL tAVE1L tAVE2H tWLWH tE1LE1H tE2HE2L tWHAX tE1HAX tE2LAX tDVWH tDVE1H tDVE2L tWHDX tE1HDX tE2LDX tWLQZ tAVWH tAVE1H tAVE2L tWHQX Write Cycle Time Address Valid to Write Enable Low Address Valid to Chip Enable 1 Low Address Valid to Chip Enable 2 High Write Enable Pulse Width Chip Enable 1 Low to Chip Enable 1 High Chip Enable 2 High to Chip Enable 2 Low Write Enable High to Address Transition Chip Enable 1 High to Address Transition Chip Enable 2 Low to Address Transition Input Valid to Write Enable High Input Valid to Chip Enable 1 High Input Valid to Chip Enable 2 Low Write Enable High to Input Transition Chip Enable 1 High to Input Transition Chip Enable 2 Low to Input Transition Write Enable Low to Output Hi-Z Address Valid to Write Enable High Address Valid to Chip Enable 1 High Address Valid to Chip Enable 2 Low Write Enable High to Output Transition 80 80 80 10 100 0 0 0 80 80 80 10 10 10 50 50 50 5 5 5 50 130 130 130 10 -100 Max Min 150 0 0 0 100 130 130 10 10 10 70 70 70 5 5 5 70 -150 Max ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns Unit
DESCRIPTION (cont'd) The clock locations contain the year, month, date, day, hour, minute, and second in 24 hour BCD format. Corrections for 28, 29 (leap year), 30, and 31 day months are made automatically. Byte 1FF8h is the clock control register. This byte controls user access to the clock information and also stores the clock calibration setting.
The eight clock bytes are not the actual clock counters themselves; they are memory locations consisting of BiPORTTM read/write memory cells. The M48T08/18 includes a clock control circuit which updates the clock bytes with current information once per second. The information can be accessed by the user in the same manner as any other location in the static memory array.
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Figure 7. Write Enable Controlled, Write AC Waveforms
tAVAV A0-A12 VALID tAVWH tAVE1L E1 tAVE2H E2 tWLWH tAVWL W tWLQZ tWHDX DQ0-DQ7 DATA INPUT tDVWH
AI00963
tWHAX
tWHQX
Figure 8. Chip Enable Controlled, Write AC Waveforms
tAVAV A0-A12 VALID tAVE1H tAVE1L E1 tE1LE1H tE1HAX
tAVE2L tAVE2H E2 tAVWL W tE1HDX tE2LDX DQ0-DQ7 DATA INPUT tDVE1H tDVE2L tE2HE2L tE2LAX
AI00964B
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M48T08, M48T18
DESCRIPTION (cont'd) The M48T08/18 also has its own Power-fail Detect circuit. The control circuitry constantly monitors the single 5V supply for an out of tolerance condition. When VCC is out of tolerance, the circuit write protects the SRAM, providing a high degree of data security in the midst of unpredictable system operation brought on by low VCC. As VCC falls below approximately 3V, the control circuitry connects the battery which maintains data and clock operation until valid power returns. READ MODE The M48T08/18 is in the Read Mode whenever W (Write Enable) is high, E1 (Chip Enable 1) is low, and E2 (Chip Enable 2) is high. The device architecture allows ripple-through access of data from eight of 65,536 locations in the static storage array. Thus, the unique address specified by the 13 Address Inputs defines which one of the 8,192 bytes of data is to be accessed. Valid data will be available at the Data I/O pins within Address Access time (tAVQV) after the last address input signal is stable, providing that the E1, E2, and G access times are also satisfied. If the E1, E2 and G access times are not met, valid data will be available after the latter of the Chip Enable Access times (tE1LQV or tE2HQV) or Output Enable Access time (tGLQV). The state of the eight three-state Data I/O signals is controlled by E1, E2 and G. If the outputs are activated before tAVQV, the data lines will be driven to an indeterminate state until tAVQV. If the Address Inputs are changed while E1, E2 and G remain active, output data will remain valid for Output Data Hold time (tAXQX) but will go indeterminate until the next Address Access. WRITE MODE The M48T08/18 is in the Write Mode whenever W, E1, and E2 are active. The start of a write is referenced from the latter occurring falling edge of W or E1, or the rising edge of E2. A write is terminated by the earlier rising edge of W or E1, or the falling edge of E2. The addresses must be held valid throughout the cycle. E1 or W must return high or E2 low for a minimum of tE1HAX or tE2LAX from Chip Enable or tWHAX from Write Enable prior to the initiation of another read or write cycle. Data-in must be valid tDVWH prior to the end of write and remain valid for tWHDX afterward. G should be kept high during write cycles to avoid bus contention; although, if the output bus has been activated by a low on E1 and G and a high on E2, a low on W will disable the outputs tWLQZ after W falls.
Table 11. Register Map
Address D7 1FFFh 1FFEh 1FFDh 1FFCh 1FFBh 1FFAh 1FF9h 1FF8h 0 0 0 0 0 ST W R D6 D5 D4 Data D3 D2 Year 10 M. 10 Date 0 0 0 Month Date Day Hours Minutes Seconds Calibration D1 D0 Function/Range BCD Format Year Month Date Day Hour Minutes Seconds Control 00-99 01-12 01-31 01-07 00-23 00-59 00-59
10 Years 0 0 FT 0 0
10 Hours 10 Minutes 10 Seconds S
Keys: S = SIGN Bit FT = FREQUENCY TEST Bit (Set to '0' for normal clock operation) R = READ Bit W = WRITE Bit ST = STOP Bit 0 = Must be set to '0'
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M48T08, M48T18
DATA RETENTION MODE With valid VCC applied, the M48T08/18 operates as a conventional BYTEWIDE static RAM. Should the supply voltage decay, the RAM will automatically power-fail deselect, write protecting itself when VCC falls within the VPFD(max), VPFD(min) window. All outputs become high impedance, and all inputs are treated as "don't care." Note: A power failure during a write cycle may corrupt data at the currently addressed location, but does not jeopardize the rest of the RAM's content. At voltages below VPFD(min), the user can be assured the memory will be in a write protected state, provided the VCC fall time is not less than tF. The M48T08/18 may respond to transient noise spikes on VCC that reach into the deselect window during the time the device is sampling VCC. Therefore, decoupling of the power supply lines is recommended. When VCC drops below VSO, the control circuit switches power to the internal battery which preserves data and powers the clock. The internal button cell will maintain data in the M48T08/18 for an accumulated period of at least 10 years when VCC is less than VSO. As system power returns and VCC rises above VSO, the battery is disconnected, and the power supply is switched to external VCC. Write protection continues until VCC reaches VPFD (min) plus tREC (min). E1 should be kept high or E2 low as VCC rises past VPFD(min) to prevent inadvertent write cycles prior to system stabilization. Normal RAM operation can resume tREC after VCC exceeds VPFD(max). For more information on Battery Storage Life refer to the Applicatio Noyte ANxxx. POWER FAIL INTERRUPT PIN The M48T08/18 continuously monitors VCC. When VCC falls to the power-fail detect trip point, an interrupt is immediately generated. An internal clock provides a delay of between 10s and 40s before automatically deselecting the M48T08/18. The INT pin is an open drain output and requires an external pull up resistor, even if the interrupt output function is not being used. SYSTEM BATTERY LIFE The useful life of the battery in the M48T08/18 is expected to ultimately come to an end for one of two reasons: either because it has been discharged while providing current to the RAM and clock in the battery back-up mode, or because the effects of aging render the cell useless before it can
Figure 9. Predicted Battery Storage Life versus Temperature
AI01024
50 40 30 t50% (AVERAGE) 20
t1% 10
YEARS
8 6 5 4 3 2
1 20 30 40 50 60 70 80 90
TEMPERATURE (Degrees Celsius)
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M48T08, M48T18
actually be completely discharged. The two effects are virtually unrelated, allowing discharge or Capacity Consumption, and the effects of aging or Storage Life, to be treated as two independent but simultaneous mechanisms. The earlier occurring failure mechanism defines the battery system life of the M48T08/18. Cell Storage Life Storage life is primarily a function of temperature. Figure 9 illustrates the approximate storage life of the M48T08/18 battery over temperature. The results in Figure 9 are derived from temperature accelerated life test studies performed at SGSTHOMSON. For the purpose of the testing, a cell failure is defined as the inability of a cell stabilized at 25C to produce a 2.4V closed circuit voltage across a 250 k load resistor. The two lines, t1% and t50%, represent different failure rate distributions for the cell's storage life. At 70C, for example, the t1% line indicates that an M48T08/18 has a 1% chance of having a battery failure 11 years into its life while the t50% shows the part has a 50% chance of failure at the 20 year mark. The t1% line represents the practical onset of wear out and can be considered the worst case Storage Life for the cell. The t50% can be considered the normal or average life. Calculating Storage Life The following formula can be used to predict storage life:
1 {[(TA1/TT)/SL1]+[(TA2/TT)/SL2]+...+[(TAN/TT)/SLN]}
battery at room temperature. At this rate, the capacity consumption life is 50E-3/520E-9 = 96,153 hours or about 11 years. Capacity consumption life can be extended by applying VCC or turning off the clock oscillator prior to system power down. Calculating Capacity Life The RAM and TIMEKEEPER load remains relatively constant over the operating temperature range. Thus, worst case cell capacity life is essentially a function of one variable, VCC duty cycle. For example, if the oscillator runs 100% of the time with VCC applied 60% of the time, the capacity consumption life is 10/(1-0.6), or 25 years. Estimated System Life Since either storage life or capacity consumption can end the battery's life, the system life is marked by which ever occurs first. In the above example, this would be 25 years. Reference for System Life Each M48T08/18 is marked with a nine digit manufacturing date code in the form of H99XXYYZZ. For example, H995B9431 is: H = fabricated in Carrollton, TX 9 = assembled in Muar, Malaysia, 9 = tested in Muar, Malaysia, 5B = lot designator, 9431 = assembled in the year 1994, work week 31. CLOCK OPERATIONS Reading the Clock Updates to the TIMEKEEPER registers should be halted before clock data is read to prevent reading data in transition. Because the BiPORT TIMEKEEPER cells in the RAM array are only data registers, and not the actual clock counters, updating the registers can be halted without disturbing the clock itself. Updating is halted when a '1' is written to the READ bit, the seventh bit in the control register. As long as a '1' remains in that position, updating is halted. After a halt is issued, the registers reflect the count; that is, the day, date, and the time that were current at the moment the halt command was issued. All of the TIMEKEEPER registers are updated simultaneously. A halt will not interrupt an update in progress. Updating is within a second after the bit is reset to a '0'. Setting the Clock The eighth bit of the control register is the WRITE bit. Setting the WRITE bit to a '1', like the READ bit, halts updates to the TIMEKEEPER registers. The user can then load them with the correct day, date, and time data in 24 hour BCD format (on Table 11).
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where, - TA1, TA2, TAN = time at ambient temperature 1, 2, etc. - TT = total time = TA1+TA2+...+TAN - SL1, SL2, SLN = storage life at temperature 1, 2, etc. For example, an M48T08/18 is exposed to temperatures of 55C or less for 8322 hrs/yr, and temperatures greater than 60C but less than 70C for the remaining 438 hrs/yr. Reading predicted t1% values from Figure 9, - SL1 = 41 yrs, SL2 = 11.4 yrs - TT = 8760 hrs/yr - TA1 = 8322 hrs/yr, TA2 = 438 hrs/yr Predicted storage life
1 {[(8322/8760)/41]+[(431/8760)/11.4]}
or 36 years. Cell Capacity Life The M48T08/18 internal cell has a rated capacity of 50mAh. The device places a nominal RAM and TIMEKEEPER load of less than 520nA on the
M48T08, M48T18
Setting the Clock (cont'd) Resetting the WRITE bit to a '0' then transfers the values of all time registers (1FF9h-1FFFh) to the actual TIMEKEEPER counters and allows normal operation to resume. The FT bit and the bits marked as '0' in Table 11 must be written to '0' to allow for normal TIMEKEEPER and RAM operation. See the Application Note AN923 "TIMEKEEPER rolling into the 21st century" for information on Century Rollover. Stopping and Starting the Oscillator The oscillator may be stopped at any time. If the device is going to spend a significant amount of time on the shelf, the oscillator can be turned off to minimize current drain on the battery. The STOP bit is the MSB of the seconds register. Setting it to a '1' stops the oscillator. The M48T08/18 is shipped from STMicroelectronics with the STOP bit set to a '1'. When reset to a '0', the M48T08/18 oscillator starts within 1 second. Calibrating the Clock The M48T08/18 is driven by a quartz controlled oscillator with a nominal frequency of 32,768 Hz. A typical M48T08/18 is accurate within 1 minute per month at 25C without calibration. The devices are tested not to exceed 35 PPM (parts per million) oscillator frequency error at 25C, which equates to about 1.53 minutes per month. The oscillation rate of any crystal changes with temperature. Figure 11 shows the frequency error that can be expected at various temperatures. Most clock chips compensate for crystal frequency and temperature shift error with cumbersome trim capacitors. The M48T08/18 design, however, employs periodic counter correction. The calibration circuit adds or subtracts counts from the oscillator divider circuit at the divide by 256 stage, as shown in Figure 10. The number of times pulses are blanked (subtracted, negative calibration) or split (added, positive calibration) depends upon the value loaded into the five bit Calibration byte found in the Control Register. Adding counts speeds the clock up, subtracting counts slows the clock down. The Calibration byte occupies the five lower order bits in the Control register. This byte can be set to represent any value between 0 and 31 in binary form. The sixth bit is a sign bit; '1' indicates positive calibration, '0' indicates negative calibration. Calibration occurs within a 64 minute cycle. The first 62 minutes in the cycle may, once per minute, have one second either shortened by 128 or lengthened by 256 oscillator cycles. If a binary '1' is loaded into the register, only the first 2 minutes in the 64 minute cycle will be modified; if a binary 6 is loaded, the first 12 will be affected, and so on. Therefore, each calibration step has the effect of adding 512 or subtracting 256 oscillator cycles for every 125,829,120 actual oscillator cycles; that is +4.068 or -2.034 PPM of adjustment per calibration step in the calibration register. Assuming that the oscillator is in fact running at exactly 32,768 Hz, each of the 31 increments in the Calibration byte would represent +10.7 or - 5.35 seconds per month which corresponds to a total range of +5.5 or - 2.75 minutes per month.
Figure 10. Clock Calibration
NORMAL
POSITIVE CALIBRATION
NEGATIVE CALIBRATION
AI00594B
12/19
M48T08, M48T18
Two methods are available for ascertaining how much calibration a given M48T08/18 may require. The first involves simply setting the clock, letting it run for a month and comparing it to a known accurate reference (like WWV broadcasts). While that may seem crude, it allows the designer to give the end user the ability to calibrate his clock as his environment may require, even after the final product is packaged in a non-user serviceable enclosure. All the designer has to do is provide a simple utility that accesses the Calibration byte. The utility could even be menu driven and made foolproof. The second approach is better suited to a manufacturing environment, and involves the use of some test equipment. When the Frequency Test (FT) bit, the seventh-most significant bit in the Day Register, is set to a '1', and the oscillator is running at 32,768 Hz, the LSB (DQ0) of the Seconds Register will toggle at 512 Hz. Any deviation from 512 Hz indicates the degree and direction of oscillator Figure 11. Crystal Accuracy Across Temperature frequency shift at the test temperature. For example, a reading of 512.01024 Hz would indicate a +20 PPM oscillator frequency error, requiring a -10 (WR001010) to be loaded into the Calibration Byte for correction. Note that setting or changing the Calibration Byte does not affect the Frequency test output frequency. The device must be selected and addresses must stable at Address 1FF9h when reading the 512 Hz on DQ0. The FT bit must be set using the same method used to set the clock, using the Write bit. The LSB of the Seconds Register is monitored by holding the M48T08/18 in an extended read of the Seconds Register, without having the Read bit set. The FT bit MUST be reset to '0' for normal clock operations to resume. For more information on calibration, see the Application Note AN924 "TIMEKEEPER Calibration".
ppm 20
0
-20
-40 F = -0.038 ppm (T - T )2 10% 0 F C2 T0 = 25 C -80
-60
-100 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 C
AI02124
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M48T08, M48T18
POWER SUPPLY DECOUPLING and UNDERSHOOT PROTECTION ICC transients, including those produced by output switching, can produce voltage fluctuations, resulting in spikes on the VCC bus. These transients can be reduced if capacitors are used to store energy, which stabilizes the VCC bus. The energy stored in the bypass capacitors will be released as low going spikes are generated or energy will be absorbed when overshoots occur. A ceramic bypass capacitor value of 0.1F (as shown in Figure 12) is recommended in order to provide the needed filtering. In addition to transients that are caused by normal SRAM operation, power cycling can generate negative voltage spikes on VCC that drive it to values below VSS by as much as one Volt. These negative spikes can cause data corruption in the SRAM while in battery backup mode. To protect from these voltage spikes, it is recommeded to connect a schottky diode from VCC to VSS (cathode connected to VCC, anode to VSS). Schottky diode 1N5817 is recommended for through hole and MBRS120T3 is recommended for surface mount. Figure 12. Supply Voltage Protection
VCC VCC
0.1F
DEVICE
VSS
AI02169
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M48T08, M48T18
ORDERING INFORMATION SCHEME Example: M48T18 -100 MH 1 TR
Supply Voltage and Write Protect Voltage 08 (1) 18 VCC = 4.75V to 5.5V VPFD = 4.5V to 4.75V VCC = 4.5V to 5.5V VPFD = 4.2V to 4.5V
Speed -100 100ns -150 150ns PC
Package PCDIP28
Temp. Range 1 0 to 70 C
Shipping Method for SOIC blank Tubes TR Tape & Reel
MH (2) SOH28
Notes: 1. The M48T08 part is offered with the PCDIP28 (i.e. CAPHAT) package only. 2. The SOIC package (SOH28) requires the battery/crystal package (SNAPHAT) which is ordered separately under the part number "M4T28-BR12SH1" in plastic tube or "M4T28-BR12SH1TR" in Tape & Reel form. Caution: Do not place the SNAPHAT battery/crystal package "M4T18-BR12SH1" in conductive foam since this will drain the lithium button-cell battery.
For a list of available options (Speed, Package, etc...) or for further information on any aspect of this device, please contact the STMicroelectronics Sales Office nearest to you.
15/19
M48T08, M48T18
PCDIP28 - 28 pin Plastic DIP, battery CAPHAT
Symb Typ A A1 A2 B B1 C D E e1 e3 eA L N mm Min 8.89 0.38 8.38 0.38 1.14 0.20 39.37 17.83 2.29 29.72 15.24 3.05 28 Max 9.65 0.76 8.89 0.53 1.78 0.31 39.88 18.34 2.79 36.32 16.00 3.81 Typ inches Min 0.350 0.015 0.330 0.015 0.045 0.008 1.550 0.702 0.090 1.170 0.600 0.120 28 Max 0.380 0.030 0.350 0.021 0.070 0.012 1.570 0.722 0.110 1.430 0.630 0.150
A2
A
A1 B1 B e3 D
N
L eA
C
e1
E
1 PCDIP
Drawing is not to scale.
16/19
M48T08, M48T18
SOH28 - 28 lead Plastic Small Outline, battery SNAPHAT
Symb Typ A A1 A2 B C D E e eB H L N CP 1.27 0.05 2.34 0.36 0.15 17.71 8.23 - 3.20 11.51 0.41 0 28 0.10 mm Min Max 3.05 0.36 2.69 0.51 0.32 18.49 8.89 - 3.61 12.70 1.27 8 0.050 0.002 0.092 0.014 0.006 0.697 0.324 - 0.126 0.453 0.016 0 28 0.004 Typ inches Min Max 0.120 0.014 0.106 0.020 0.012 0.728 0.350 - 0.142 0.500 0.050 8
A2 B e
A C eB CP
D
N
E
H A1 L
1 SOH-A
Drawing is not to scale.
17/19
M48T08, M48T18
SH - SNAPHAT Housing for 28 lead Plastic Small Outline
Symb Typ A A1 A2 A3 B D E eA eB L 0.46 21.21 14.22 15.55 3.20 2.03 6.73 6.48 mm Min Max 9.78 7.24 6.99 0.38 0.56 21.84 14.99 15.95 3.61 2.29 0.018 0.835 0.560 0.612 0.126 0.080 0.265 0.255 Typ inches Min Max 0.385 0.285 0.275 0.015 0.022 0.860 0.590 0.628 0.142 0.090
A1
A2 A A3
eA D
B eB
L
E
SHTK-A
Drawing is not to scale.
18/19
M48T08, M48T18
Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics.
(c) 1999 STMicroelectronics - All Rights Reserved
(R) TIMEKEEPER and SNAPHAT are registered trademarks of STMicroelectronics TM CAPHAT, BYTEWIDE and BiPORT are trademarks of STMicroelectronics
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19/19

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