M41ST87YMX6F datasheet(35/52 Pages) STMICROELECTRONICS

35 / 52 page

M41ST87Y, M41ST87W

Clock operation

Doc ID 9497 Rev 8

35/52

Figure 25.

Backup mode alarm waveform

3.4

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

RB1-RB0 select the resolution, where 00=1/16 second, 01=1/4 second, 10=1 second, and

11=4 seconds. 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

Note:

The accuracy of the timer is within ± the selected resolution.

If the processor does not reset the timer within the specified period, the M41ST87Y/W sets

the WDF (watchdog flag) and generates either a watchdog interrupt or a microprocessor

reset.

The most significant bit of the watchdog register is the watchdog steering bit (WDS). When

set to a '0,' the watchdog will activate the IRQ/OUT pin when timed-out. When WDS is set to

a '1,' the watchdog will output a negative pulse on the RST pin for trec. The watchdog

when the WDS bit is set to a '1.'

The watchdog timer can be reset by two methods: 1) a transition (high-to-low or low-to-high)

can be applied to the watchdog input pin (WDI) or 2) the microprocessor can perform a

WRITE of the watchdog register. The time-out period then starts over.

Note:

The WDI pin should be tied to VSS if not used and is only available in the SOX28 (MX)

package.

In order to perform a software reset of the watchdog timer, the original time-out period can

be written into the watchdog register, effectively restarting the count-down cycle.

AI07087

VCC

IRQ/OUT

VPFD

ABE, AFE Bits in Interrupt Register

AF bit in Flags Register

HIGH-Z

VSO

HIGH-Z

trec

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