Date: 5/25/04

SP691A/693A/800L/800M Low Power Microprocessor Supervisor with Battery Switch-Over © Copyright 2004 Sipex Corporation

19

Figure 30. Watchdog Flow Diagram

START

SET

WDI

LOW

SUBROUTINE

OR PROGRAM LOOP

SET WDI

HIGH

RETURN

END

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 30 shows an example flow diagram

where the I/O driving the watchdog input is set

high at the beginning of the program, set low at

the beginning of every subrouting 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.

Maximum V

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 100

µF per amp of current.

When the power supply is shut off or the main

battery is disconnected, the associated initial

µF =

0.01V/

satisfies the maximum fall-time requirement.