Backup-Battery Replacement
The backup battery may be disconnected while V
CC
is
above the reset threshold, provided BATT is bypassed
with a 0.1µF capacitor to ground. No precautions are
necessary to avoid spurious reset pulses.
Negative-Going V
CC
Transients
While issuing resets to the µP during power-up, power-
down, and brownout conditions, these supervisors are
relatively immune to short-duration negative-going V
CC
transients (glitches). It is usually undesirable to reset
the µP when V
CC
experiences only small glitches.
The Typical Operating Characteristics show Maximum
Transient Duration vs. Reset Comparator Overdrive, for
which reset pulses are not generated. The graph was
produced using negative-going V
CC
pulses, starting at
5V and ending below the reset threshold by the magni-
tude indicated (reset comparator overdrive). The graph
shows the maximum pulse width that a negative-going
V
CC
transient may typically have without causing a
reset pulse to be issued. As the amplitude of the tran-
sient increases (i.e., goes farther below the reset
threshold), the maximum allowable pulse width
decreases.
Typically, a V
CC
transient that goes 40mV below the
reset threshold and lasts for 3µs or less will not cause a
reset pulse to be issued.
A 0.1µF bypass capacitor mounted close to the V
CC
pin provides additional transient immunity.
Watchdog Software Considerations
To help the watchdog timer keep a closer watch on soft-
ware execution, you can use the method of 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 16 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 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 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 exponen-
tially, which more than satisfies the maximum fall-time
requirement.
MAX807L/M/N
Full-Featured µP Supervisory Circuit with
±1.5% Reset Accuracy
______________________________________________________________________________________ 15
Figure 15. Adding Hysteresis to the Power-Fail Comparator
