STFPC320 datasheet(24/78 Pages) STMICROELECTRONICS

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Real-time clock (RTC) operation

STFPC320

24/78

6.3

Watchdog timer

The watchdog timer can be used to detect an out of control microprocessor. The user

programs the watchdog timer by setting the desired amount of time-out into the Watchdog

RB0 select the resolution where:

●

000 = 1/16 second (16Hz)

●

001 = 1/4 second (4Hz)

●

010 = 1 second (1Hz)

●

011 = 4 seconds (1/4Hz)

●

100 = 1 minute (1/60Hz)

Note:

Invalid combinations (101, 110, and 111) do NOT enable a watchdog time-out. Setting the

BMB4-BMB0 = 0 with any combination of RB2-RB0, other than 000, will result in an

immediate watchdog time-out. The amount of time-out is then determined to be the

multiplication of the five-bit multiplier value with the resolution. (For example: writing

00001110 in the Watchdog register = 3*1 or 3 seconds). If the processor does not reset the

timer within the specified period, the STFPC320 generates a watchdog output pulse on the

IRQ_N/SQW pin.

The watchdog timer can only be reset by having the microprocessor perform a WRITE of the

Watchdog register. The time-out period then starts over. Should the watchdog timer time-

out, any value may be written to the Watchdog Register in order to clear the IRQ_N/SQW

pin. A value of 00h will disable the watchdog function until it is again programmed to a new

value. A READ of the Flags Register will reset the Watchdog flag (Bit D7; Register 0Fh). The

watchdog function is automatically disabled upon power-up, and the Watchdog Register is

cleared.

6.4

Real-time clock (RTC)

The RTC keeps track of the date and time. Once the date and time are set, the clock works

when the STFPC320 is in normal operation and standby operation. The wake-up alarm

feature is included in the RTC module. The accuracy of the RTC is approximately 10 ppm

(±25 secs/month).

The wakeup alarm is programmed to wake up once the date and time set are met. This

feature is present in normal and standby mode of operation. Only one date and time is

available for setting.

The real-time clock (RTC) uses an external 32.768 kHz quartz crystal to maintain an

accurate internal representation of the second, minute, hour, day, date, month, and year.

The RTC has leap-year correction. The clock also corrects for months having fewer than 31

days.

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