m41t81s STMicroelectronics, m41t81s Datasheet - Page 6

• Manufacturer Part Number: m41t81s
• Description: Serial Access Real-time Clock With Alarms
• Manufacturer: STMicroelectronics

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Summary description
The M41T81S is a low power Serial RTC with a built-in 32.768kHz oscillator (external crystal
controlled). Eight bytes of the SRAM (see
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 serially via a two line, bi-directional I
interface. The built-in address register is incremented automatically 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 failure occurs. The energy
needed to sustain the 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 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 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 M41T81S is supplied in either an 8-pin SOIC or an 18-pin (MY), 300mil SOIC package
which includes an embedded 32kHz crystal.
The 18-pin, embedded crystal SOIC requires only a user-supplied battery to provide non-
volatile operation.
Figure 1.
1. For SO8 package only
Logic diagram
XO
SDA
XI
SCL
(1)
(1)
V CC
M41T81S
V SS
V BAT
Table 2: Clock register map on page
IRQ/FT/OUT/SQW
AI09160
15) are used
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C