Description
The M41ST84W serial real-time clock is built in a low power CMOS SRAM process. It has a 64-byte memory space with 44 bytes of NVRAM and 20 memory-mapped RTC registers (see Table 3 on page 18). The RTC registers are configured in binary coded decimal (BCD) format.
A built-in, low power 32.768 kHz oscillator (external crystal controlled) provides the time base for the timekeeping and calendar functions.
The basic clock/calendar functions are handled by the first eight RTC registers, while the other twelve bytes provide status/control for the alarm, watchdog, and square wave functions.
Addresses and data are transferred serially via the two line, bidirectional I2C interface. The built-in address register is incremented automatically after each WRITE or READ data byte. The M41ST84W 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 SRAM and clock operations can be supplied by a small lithium button cell supply when a power failure occurs. Functions available to the user include a nonvolatile, time-of-day clock/calendar, alarm interrupts, watchdog timer and programmable square wave output. Other features include a power-on reset as well as an additional input (RSTIN) which can also generate an output reset (RST). 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 M41ST84W is supplied in a 16-lead SOIC package.

Features
■ Automatic battery switchover and deselect
   – Power-fail deselect, VPFD = 2.60 V (nom)
   – Switchover, VSO = 2.50 V (nom)
■ 400 kHz I2C serial interface
■ 3.0/3.3 V operating voltage
   – VCC = 2.7 to 3.6 V
■ Ultra-low battery supply current of 500 nA (max)
■ RoHS compliance
   Lead-free components are compliant with the RoHS directive

Serial RTC features
■ 400 kHz I2C
■ 44 bytes of general purpose NVRAM
■ Counters for:
   – Seconds, minutes, hours, day, date, month, and year
   – Century
   – 10ths/100ths of seconds
   – Clock calibration register allows compensation for crystal variations over temperature
■ Programmable alarm with interrupt
   – Functions during battery backup mode
■ Power-down timestamp (HT bit)
■ 2.5 to 5.5 V oscillator operating voltage
■ 32 KHz oscillator with integrated load capacitance (12.5 pF)
■ Battery low flag

Microprocessor supervisory features
■ Programmable watchdog timer
   – 62.5 ms to 128 s time-out period
■ Power-on reset/low voltage detect output
■ PFI/PFO with 1.25 V reference

Other features
■ Programmable squarewave generator (1 Hz to 32 KHz)
■ –40°C to +85°C operation
■ Packaged in a 16-lead SOIC