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September 2003
Rev. 2.0
M41T81
SERIAL ACCESS RTC WITH ALARMS
FEATURES SUMMARY
s
2.0 TO 5.5V CLOCK OPERATING VOLTAGE
s
COUNTERS FOR TENTHS/HUNDREDTHS
OF SECONDS, SECONDS, MINUTES,
HOURS, DAY, DATE, MONTH, YEAR, and
CENTURY
s
AUTOMATIC SWITCH-OVER and DESELECT
CIRCUITRY
s
SERIAL INTERFACE SUPPORTS I
2
C BUS
(400KHz PROTOCOL)
s
PROGRAMMABLE ALARM and INTERRUPT
FUNCTION (valid even during Battery Back-up
Mode)
s
WATCHDOG TIMER
s
LOW OPERATING CURRENT OF 400A
s
BATTERY BACK-UP NOT RECOMMENDED
FOR 3.0V APPLICATIONS (CAPACITOR
BACK-UP ONLY)
s
BATTERY OR SUPER-CAP BACK-UP
s
OPERATING TEMPERATURE OF 40 TO
85C
s
ULTRA-LOW BATTERY SUPPLY CURRENT
OF 1
A
s
PACKAGE OPTIONS INCLUDE A 28-LEAD or
18-LEAD EMBEDDED CRYSTAL SOIC
Figure 1. 8-pin SOIC Package
Figure 2. 28-pin (300mil) SOIC Package*
Figure 3. 18-pin (300mil) SOIC Package*
8
1
SO8 (M)
SOX28 (MX)
EMBEDDED Crystal
1
18
SOX18 (MY)
EMBEDDED Crystal
M41T81
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TABLE OF CONTENTS
SUMMARY DESCRIPTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Figure 4. Logic Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Table 1. Signal Names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Figure 5. 8-pin SOIC (M) Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Figure 7. 28-pin, 300mil SOIC (MX) Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Figure 6. 18-pin, 300mil SOIC (MY) Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Figure 8. Block Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
MAXIMUM RATING. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Table 2. Absolute Maximum Ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
DC AND AC PARAMETERS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Table 3. Operating and AC Measurement Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Figure 9. AC Measurement I/O Waveform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Table 4. Capacitance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Table 5. DC Characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Table 6. Crystal Electrical Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
OPERATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2-Wire Bus Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Figure 10. Serial Bus Data Transfer Sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Figure 11. Acknowledgement Sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Figure 12. Bus Timing Requirements Sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Table 7. AC Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
READ Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Figure 13. Slave Address Location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Figure 14. READ Mode Sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Figure 15. Alternative READ Mode Sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
WRITE Mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Figure 16. WRITE Mode Sequence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Data Retention Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Figure 17. Power Down/Up Mode AC Waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Table 8. Power Down/Up AC Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Table 9. Power Down/Up Trip Points DC Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
CLOCK OPERATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
TIMEKEEPER Registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Table 10. TIMEKEEPER Register Map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Calibrating the Clock. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Setting Alarm Clock Registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Figure 18. Alarm Interrupt Reset Waveform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
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M41T81
Figure 19. Back-up Mode Alarm Waveform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Table 11. Alarm Repeat Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Watchdog Timer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Square Wave Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Table 12. Square Wave Output Frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Century Bit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Output Driver Pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Preferred Initial Power-on Default . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Table 13. Preferred Default Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Figure 20. Crystal Accuracy Across Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Figure 21. Clock Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
PACKAGE MECHANICAL INFORMATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
PART NUMBERING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
REVISION HISTORY. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
M41T81
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SUMMARY DESCRIPTION
The M41T81 Serial Access TIMEKEEPER
SRAM is a low power Serial RTC with a built-in
32.768 KHz oscillator (external crystal controlled).
Eight bytes of the SRAM (see Table 10, page 16)
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 M41T81 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
SRAM and clock operations can be supplied by a
small lithium button supply when a power failure
occurs. Functions available to the user include a
non-volatile, time-of-day clock/calendar, Alarm in-
terrupts, Watchdog Timer and programmable
Square Wave output. The eight clock address lo-
cations contain the century, year, month, date,
day, hour, minute, second and tenths/hundredths
of a second in 24 hour BCD format. Corrections for
28, 29 (leap year - valid until year 2100), 30 and 31
day months are made automatically.
The M41T81 is supplied in either an 8-pin SOIC or
an 18-pin (MY) or 28-pin (MX), 300mil SOIC pack-
age which includes an embedded 32kHz crystal.
The 8-pin and 28-pin, embedded crystal SOIC re-
quires only a user-supplied battery to provide non-
volatile operation.
Figure 4. Logic Diagram
Note: 1. For SO8 package only.
Table 1. Signal Names
Note: 1. For SO8 package only.
SCL
VCC
M41T81
VSS
SDA
IRQ/FT/OUT/SQW
VBAT
XI
(1)
XO
(1)
AI04613
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
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