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M41T81S：Serial real-time clock (RTC) with alarm

The M41T81S is a low-power serial real-time clock (RTC) with a built-in 32.768 kHz oscillator (external crystal controlled). Eight bytes of the SRAM 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 serially via a two line, bidirectional I²C 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 300 mil SOIC package which includes an embedded 32 KHz crystal.

The 18-pin, embedded crystal SOIC requires only a user-supplied battery to provide non-volatile operation.

Key Features

• Counters for tenths/hundredths of seconds, seconds, minutes, hours, day, date, month, year, and century
• 32 KHz crystal oscillator with integrated load capacitance (12.5 pf) which provides exceptional oscillator stability and high crystal series resistance operation)
• Oscillator stop detection (monitors clock operation)
• Serial interface supports I2 C bus (400 kHz protocol)
• Ultra-low battery supply current of 0.6 μA (typ)
• 2.0 to 5.5 V clock operating voltage
• Automatic switchover and deselect circuitry (fixed reference) which provides full operation in 3.0 V applications)
• VCC = 2.7 to 5.5 V
• 2.5 V ≤ VPFD ≤ 2.7 V
• Power-down time-stamp (HT bit) which allows determination of time elapsed in battery backup
• Battery low flag
• Programmable alarm and interrupt function (valid even during battery backup mode)
• Accurate programmable watchdog timer (from 62.5 ms to 128 s)
• Software clock calibration (to compensate for crystal deviation due to temperature)
• Operating temperature of –40 to 85 °C
• Package options include an 8-lead SOIC or 18-lead embedded crystal SOIC

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