MAX791M(1995) 查看數據表（PDF） - Maxim Integrated

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MAX791M
(Rev.:1995)

Microprocessor Supervisory Circuit

Maxim Integrated 

Microprocessor Supervisory Circuit

START

SET

WDI

LOW

SUBROUTINE

OR PROGRAM LOOP,

SET WDI

HIGH

RETURN

END

Figure 17. Watchdog Flow Diagram

Watchdog Software Considerations

A way to help the watchdog timer keep a closer watch

on software execution involves setting and resetting the

watchdog input at different points in the program,

rather than “pulsing” the watchdog input high-low-high

or low-high-low. This technique avoids a “stuck” loop

where the watchdog timer continues to be reset within

the loop, keeping the watchdog from timing out.

Figure 17 shows an example flow diagram where the

I/O driving the watchdog input is set high at the begin-

ning of the program, set low at the beginning of every

subroutine or loop, then set high again when the pro-

gram returns to the beginning. If the program should

“hang” in any subroutine, the I/O is continually set low

and the watchdog timer is allowed to time out, causing

a reset or interrupt to be issued.

Maximum VCC Fall Time

The VCC fall time is limited by the propagation delay of

the battery switchover comparator and should not

exceed 0.03V/µs. A standard rule of thumb for filter

capacitance on most regulators is on the order of

100µF per amp of current. When the power supply is

shut off or the main battery is disconnected, the associ-

ated initial VCC fall rate is just the inverse or 1A / 100µF

= 0.01V/µs. The VCC fall rate decreases with time as

VCC falls exponentially, which more than satisfies the

maximum fall-time requirement.

______________________________________________________________________________________ 17

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