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Microprocessor Supervisory Circuits

Maxim Integrated 

Microprocessor Supervisory Circuits

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 associated initial

VCC fall rate is just the inverse or 1A/100µF = 0.01V/µs.

The VCC fall rate decreases with time as VCC falls expo-

nentially, which more than satisfies the maximum fall-time

requirement.

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

14 shows an example flow diagram where the I/O dri-

ving the watchdog input is set high at the beginning of

the program, set low at the beginning of every subrou-

tine or loop, then set high again when the program

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.

START

SET

WDI

LOW

SUBROUTINE

OR PROGRAM LOOP

SET WDI

HIGH

RETURN

END

Figure 14. Watchdog Flow Diagram

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