SP791 datasheet(16/19 Pages) SIPEX

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SP791DS/08

SP791 Low Power Microprocessor Supervisory with Battery Switch-Over

Figure 16. Watchdog Flow Diagram

Figure 15. Monitoring a Negative Voltage

NEGATIVE-GOING V

CC TRANSIENTS

The SP791 is relatively immune to short-dura-

tion negative-going VCC transients resulting

from power up, power down, and brownout con-

ditions. It is usually undesirable to reset the

µP

when VCC experiences only small glitches.

Typically, a VCC transient that goes 100mV be-

low the reset threshold and lasts for 40

µs or less

will not cause a reset pulse to be issued.

A 100nF bypass capacitor mounted close to the

VCC pin provides additional transient immunity.

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.

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 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 100

µF 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/100

µF =

0.01V/

µs. The VCC fall rate decreases with time

as VCC falls exponentially, which more than sat-

isfies the maximum fall-time requirement.

+5V

V–

PFO

+5V

VTRIP

GND

PFI

Vcc

PFO

5 - 1.25

1.25 - VTRIP

NOTE: VTRIP IS NEGATIVE.

Corporation

V–

START

SET

WDI

LOW

SUBROUTINE

OR PROGRAM LOOP

SET WDI

HIGH

RETURN

END

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