1/29
September 2005
M41T81S
Serial Access Real-Time Clock with Alarms
* Contact local ST sales office for availability.
Rev 4.0
FEATURES SUMMARY
COUNTERS FOR TENTHS/HUNDREDTHS
OF SECONDS, SECONDS, MINUTES,
HOURS, DAY, DATE, MONTH, YEAR, AND
CENTURY
32KHz CRYSTAL OSCILLATOR WITH
INTEGRATED LOAD CAPACITANCE
(12.5pF) WHICH PROVIDES EXCEPTIONAL
OSCILLATOR STABILITY AND HIGH
CRYSTAL SERIES RESISTANCE
OPERATION)
OSCILLATOR STOP DETECTION
(MONITORS CLOCK OPERATION)
SERIAL INTERFACE SUPPORTS I
2
C BUS
(400kHz PROTOCOL)
ULTRA-LOW BATTERY SUPPLY CURRENT
OF 0.6
A (typ)
2.0 TO 5.5V CLOCK OPERATING VOLTAGE
AUTOMATIC SWITCHOVER AND
DESELECT CIRCUITRY (FIXED
REFERENCE) WHICH PROVIDES FULL
OPERATION IN 3.0V APPLICATIONS)
V
CC
= 2.7 to 5.5V
2.5V
V
PFD
2.7V
POWER-DOWN TIME STAMP (HT BIT)
WHICH ALLOWS DETERMINATION OF
TIME ELAPSED IN BATTERY BACK-UP
BATTERY LOW FLAG
PROGRAMMABLE ALARM AND
INTERRUPT FUNCTION (valid even during
Battery Back-up Mode)
ACCURATE PROGRAMMABLE
WATCHDOG TIMER (from 62.5ms to 128s)
SOFTWARE CLOCK CALIBRATION (TO
COMPENSATE FOR CRYSTAL DEVIATION
DUE TO TEMPERATURE)
OPERATING TEMPERATURE OF 40 TO
85C
PACKAGE OPTIONS INCLUDE AN 8-LEAD
SOIC OR 18-LEAD EMBEDDED CRYSTAL
SOIC
Figure 1. Packages
8
1
SO8 (M)
8-pin SOIC
1
18
SOX18 (MY)*
18-pin (300mil) SOIC
with Embedded Crystal
M41T81S
2/29
TABLE OF CONTENTS
FEATURES SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Figure 1. Packages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
SUMMARY DESCRIPTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Figure 2. Logic Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Table 1. Signal Names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Figure 3. 8-pin SOIC (M) Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Figure 4. 18-pin, 300mil SOIC (MY) Connections. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Figure 5. Block Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
OPERATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2-Wire Bus Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Figure 6. Serial Bus Data Transfer Sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Figure 7. Acknowledgement Sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
READ Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Figure 8. Slave Address Location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Figure 9. READ Mode Sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Figure 10.Alternative READ Mode Sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
WRITE Mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Data Retention Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Figure 11.WRITE Mode Sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
CLOCK OPERATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Power-down Timestamp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
TIMEKEEPER Registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Table 2. TIMEKEEPER Register Map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Calibrating the Clock. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Figure 12.Crystal Accuracy Across Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Figure 13.Clock Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Setting Alarm Clock Registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Figure 14.Alarm Interrupt Reset Waveform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Figure 15.Back-up Mode Alarm Waveform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Table 3. Alarm Repeat Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Watchdog Timer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Square Wave Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Table 4. Square Wave Output Frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Century Bit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Battery Low Warning. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Oscillator Fail Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Oscillator Fail Interrupt Enable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Output Driver Pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Preferred Initial Power-on Default . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Table 5. Preferred Default Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
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M41T81S
MAXIMUM RATING. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Table 6. Absolute Maximum Ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
DC AND AC PARAMETERS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Table 7. Operating and AC Measurement Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Figure 16.AC Measurement I/O Waveform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Table 8. Capacitance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Table 9. DC Characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Table 10. Crystal Electrical Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Figure 17.Power Down/Up Mode AC Waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Table 11. Power Down/Up AC Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Table 12. Power Down/Up Trip Points DC Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Figure 18.Bus Timing Requirements Sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Table 13. AC Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
PACKAGE MECHANICAL INFORMATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Figure 19.SO8 8-lead Plastic Small Package Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Table 14. SO8 8-lead Plastic Small Outline (150 mils body width), Package Mech. Data . . . . . . 25
Figure 20.SOX18 18-lead Plastic Small Outline, 300mils, Embedded Crystal, Outline . . . . . . . . 26
Table 15. SOX18 18-lead Plastic Small Outline, 300mils, Embedded Crystal, Package Mech. . 26
PART NUMBERING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Table 16. Ordering Information Scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
REVISION HISTORY. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Table 17. Document Revision History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
M41T81S
4/29
SUMMARY DESCRIPTION
The M41T81S Serial Access TIMEKEEPER
SRAM is a low power Serial RTC with a built-in
32.768kHz oscillator (external crystal controlled).
Eight bytes of the SRAM (see
Table 2., page 12
)
are used for the clock/calendar function and are
configured in binary coded decimal (BCD) format.
An additional 12 bytes of SRAM provide status/
control of Alarm, Watchdog and Square Wave
functions. Addresses and data are transferred se-
rially via a two line, bi-directional I
2
C interface. The
built-in address register is incremented automati-
cally after each WRITE or READ data byte.
The M41T81S has a built-in power sense circuit
which detects power failures and automatically
switches to the battery supply when a power fail-
ure occurs. The energy needed to sustain the
clock operations can be supplied by a small lithium
button supply when a power failure occurs. Func-
tions available to the user include a non-volatile,
time-of-day clock/calendar, Alarm interrupts,
Watchdog Timer and programmable Square
Wave output. The eight clock address locations
contain the century, year, month, date, day, hour,
minute, second and tenths/hundredths of a sec-
ond in 24 hour BCD format. Corrections for 28, 29
(leap year - valid until year 2100), 30 and 31 day
months are made automatically.
The M41T81S is supplied in either an 8-pin SOIC
or an 18-pin (MY), 300mil SOIC package which in-
cludes an embedded 32kHz crystal.
The 18-pin, embedded crystal SOIC requires only
a user-supplied battery to provide non-volatile op-
eration.
Figure 2. Logic Diagram
Note: 1. For SO8 package only.
Table 1. Signal Names
Note: 1. For SO8 package only.
2. NC and NF pins should be tied to V
SS
.
SCL
VCC
M41T81S
VSS
SDA
IRQ/FT/OUT/SQW
VBAT
XI
(1)
XO
(1)
AI09160
XI
(1)
Oscillator Input
XO
(1)
Oscillator Output
IRQ/OUT/
FT/SQW
Interrupt / Output Driver / Frequency
Test / Square Wave (Open Drain)
SDA
Serial Data Input/Output
SCL
Serial Clock Input
V
BAT
Battery Supply Voltage
V
CC
Supply Voltage
V
SS
Ground
NC
(2)
No Connect
NF
(2)
No Function
5/29
M41T81S
Figure 3. 8-pin SOIC (M) Connections
Note: 1. Open Drain Output
Figure 4. 18-pin, 300mil SOIC (MY)
Connections
Note: 1. NC and NF pins should be tied to V
SS
. Pins 2 and 3 are
internally shorted together. Pins 17 and 16 are internally
shorted together.
2. Open Drain Output
Figure 5. Block Diagram
Note: 1. Open Drain Output
2. Square Wave function has the highest priority on IRQ/FT/OUT/SQW output.
2
3
4
5
6
8
7
1
IRQ/FT/OUT/SQW
(1)
SDA
V
BAT
SCL
V
SS
XO
XI
V
CC
M41T81S
AI09161
8
2
3
4
5
6
7
9
12
11
10
18
17
16
15
14
13
1
NF
(1)
NC
NC
NC
NC
NC
NC
SCL
SDA
V
SS
V
BAT
NF
(1)
NC
VCC
M41T81S
IRQ/FT/OUT/SQW
(2)
NF
(1)
NF
(1)
NC
AI09162
REAL TIME CLOCK
CALENDAR
RTC W/ALARM
& CALIBRATION
WATCHDOG
OSCILLATOR FAIL
CIRCUIT
SQUARE WAVE
FREQUENCY TEST
OUTPUT DRIVER
IRQ/FT/OUT/SQW
(1)
INTERNAL
POWER
SQWE
(2)
AFE
SDA
SCL
V
CC
OFIE
COMPARE
I
2
C
INTERFACE
32KHz
OSCILLATOR
V
BAT
CRYSTAL
V
SO
V
PFD
AI09163
WRITE
PROTECT
FT
OUT
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