SP791CN SIPEX [Sipex Corporation], SP791CN Datasheet - Page 16

• Manufacturer Part Number: SP791CN
• Description: Low Power Microprocessor Supervisory with Battery Switch-Over
• Manufacturer: SIPEX [Sipex Corporation]

NEGATIVE-GOING V
The SP791 is relatively immune to short-dura-
tion negative-going V
from power up, power down, and brownout con-
ditions. It is usually undesirable to reset the P
when V
Typically, a V
low the reset threshold and lasts for 40 s or less
will not cause a reset pulse to be issued.
A 100nF bypass capacitor mounted close to the
V
CONNECTING A TIMING CAPACITOR
TO THE SWT PIN
To prevent timing errors minimize external cur-
rent leakage sources at this pin, and locate the
capacitor as close to SWT as possible. The sum
of PC board leakage + SWT capacitor leakage
must be small compared to 100 nA.
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.
SP791DS/08
Figure 15. Monitoring a Negative Voltage
+5V
PFO
CC
0V
+5V
pin provides additional transient immunity.
CC
R2
R1
experiences only small glitches.
NOTE: V
V–
5 - 1.25
CC
R1
transient that goes 100mV be-
TRIP
SP791 Low Power Microprocessor Supervisory with Battery Switch-Over
=
IS NEGATIVE.
1.25 - V
V
CC
TRIP
R2
CC
PFI
TRIP
transients resulting
TRANSIENTS
GND
Vcc
Corporation
PFO
0V
V–
16
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
low at the beginning of the program, set high at
the beginning of every subroutine or loop, then
set low again when the program returns to the
beginning. If the program should "hang" in any
subroutine, the I/O is continually set high and
the watchdog timer is allowed to time out, caus-
ing a reset or interrupt to be issued.
MAXIMUM VCC FALL TIME
The V
delay of the battery switchover comparator and
should not exceed 0.03V/ s. A standard rule of
thumb for filter capacitance on most regulators
is on the order of 100 F per amp of current.
When the power supply is shut off or the main
battery is disconnected, the associated initial
V
0.01V/ s. The V
as V
isfies the maximum fall-time requirement.
Figure 16. Watchdog Flow Diagram
CC
CC
fall rate is just the inverse of 1A/100 F =
CC
falls exponentially, which more than sat-
fall time is limited by the propagation
OR PROGRAM LOOP
CC
SUBROUTINE
SET WDI
START
RETURN
HIGH
fall rate decreases with time
END
WDI
SET
LOW
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