Datasheet

lems, minimize external current leakage sources at this
pin, and locate the capacitor as close to OSC IN as pos-
sible. The sum of PC-board leakage plus OSC capacitor
leakage must be small compared to ±100nA.
Maximum V
CC
Fall Time
The V
CC
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 V
CC
fall rate
is just the inverse or 1A/100μF = 0.01V/μs. The V
CC
fall
rate decreases with time as V
CC
falls exponentially, 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 driving the watch-
dog input is set high at the beginning of the program, set
low at the beginning of every subroutine 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.
Figure 14. Watchdog Flow Diagram
START
SET
WDI
LOW
RETURN
END
SUBROUTINE
OR PROGRAM LOOP
SET WDI
HIGH
MAX691A/MAX693A/
MAX800L/MAX800M
Microprocessor Supervisory Circuits
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