M41ST85W datasheet(6/43 Pages) STMICROELECTRONICS

6 / 43 page

Description

M41ST85W

6/43

DocID7531 Rev 12

Description

The M41ST85W is a combination serial real-time clock, microprocessor supervisor, and

NVRAM supervisor. It is built in a low-power CMOS SRAM process and has a 64-byte

memory space with 44 bytes of NVRAM and 20 memory-mapped RTC registers (see Table

on page 20

). The RTC registers are configured in binary coded decimal (BCD) format.

The M41ST85W combines a 400 kHz I²C serial RTC with an automatic backup battery

switchover circuit for powering an external LPSRAM as well as the internal RTC. When

power begins to fail, the switchover automatically connects to the backup battery to keep the

RTC and external LPSRAM alive in the absence of system power. Access to the LPSRAM is

also cut off via a chip-enable gate function, thereby write-protecting the memory. A

programmable watchdog and power-on reset/low voltage detect function are the key

elements in the microprocessor supervisor section.

The real-time clock includes a built-in 32.768 kHz oscillator (crystal-controlled), which

provides the time base for the timekeeping and calendar functions. Eight of the 20 clock

registers provide the basic clock/calendar functions while the other 12 bytes provide

status/control for the alarm, watchdog, and squarewave functions.

RTC 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 M41ST85W has a built-in power sense circuit which detects power failures and

automatically switches to the backup battery when a power failure occurs. During an

outage, the power to sustain the SRAM and clock operations is typically supplied by a small

lithium button-cell battery as is the case when using the SNAPHAT® package option.

Functions available to the user include a non-volatile, time-of-day clock/calendar, alarm

interrupts, watchdog timer, and programmable squarewave generator. Other features

include a power-on reset as well as two additional debounce reset inputs (RSTIN1 and

RSTIN2) which can also generate an output reset (RST).

The eight registers for basic clock/calendar functions 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.

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