SP691A_05 Datasheet, PDF (19/24 Pages) Sipex Corporation – Low Power Microprocessor Supervisory with Battery Switch-Over

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_CC 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 of 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.
START
SET
WDI
LOW
SUBROUTINE
OR PROGRAM LOOP
SET WDI
HIGH
RETURN
END
Figure 30. Watchdog Flow Diagram
Date: 4/18/05
SP691A/693A/800L/800M Low Power Microprocessor Supervisor with Battery Switch-Over © Copyright 2005 Sipex Corporation
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