Watchdog Input
The watchdog timer can also initiate a reset.
Refer to the Watchdog Input section.
The watchdog circuit monitors the µP's activity.
If the µP does not toggle the watchdog input
(WDI) within 1.6sec, a reset pulse is triggered.
The internal 1.6sec timer is cleared by either a
reset pulse or by a transition (LOW-to-HIGH or
HIGH-to-LOW) at WDI. If WDI is tied HIGH
or LOW, a RESET pulse is triggered every
1.8sec (tWD plus tRS).
TheSP804T/S/RandSP805T/S/Ractive-HIGH
RESET output is open drain and the inverse
of the SP690T/S/R and SP802T/S/R RESET
outputs.
RESET is also triggered by a watchdog timeout.
If WDI remains either high or low for a period
that exceeds the watchdog timeout period (1.6
sec), RESET pulses low for 200mS. As long as
RESET is asserted, the watchdog timer remains
cleared. When RESET comes high, the watch-
dog resumes timing and must be serviced within
1.6sec. If WDI is tied high or low, a RESET
pulse is triggered every 1.8sec (tWD plus tRS).
As long as reset is asserted, the timer remains
cleared and does not count. As soon as reset is
de-asserted, the timer starts counting. Unlike
the 5V SP690A series, the watchdog function
cannot be disabled.
Power-Fail Comparator
Reset Threshold
The power-fail comparator can be used as an
under-voltage detector to signal the failing of a
powersupply(it iscompletelyseparatefromthe
rest of the circuitry and does not need to be
dedicated to this function). The PFI input is
comparedtoaninternal1.25V. IfPFIislessthan
VPFT, PFO goes low.
The SP690T and SP805T devices are designed
for 3.3V systems with a ±5% power-supply
tolerance and a 10% system tolerance. Except
for watchdog faults, reset will not assert as long
as the power supply remains above 3.15V (3.3V
- 5%). Reset is guaranteed to assert before the
power supply falls below 3.0V.
The power-fail comparator turns off and PFO
goes LOW when VCC falls below VSW on
power-down. The power-fail comparator turns
on as VCC crosses VSW on power-up. If the
comparator is not used, connect PFI to ground
and leave PFO unconnected.
The SP690S and SP805S devices are designed
for 3.3V ±10% power supplies. Except for
watchdog faults, they are guaranteed not to
assert reset as long as the supply remains above
3.0V (3.3V - 10%). Reset is guaranteed to
assertbeforethepowersupplyfailsbelow2.85V
(VCC - 14%).
Backup-Battery Switchover
In the event of a brownout or power failure, it
may be necessary to preserve the contents of
RAM. With a backup battery installed at
VBATTERY, the devices automatically switch
RAM to backup power when VCC fails.
The SP690R and SP805R devices are optimized
formonitoring3.0V±10%powersupplies. Reset
will not occur until VCC falls below 2.7V (3.0V
- 10%), but is guaranteed to occur before the
supply falls below 2.55V (3.0V - 15%).
This family of µP supervisors (designed for
3.3V and 3V systems) doesn't always connect
VBATTERY to VOUT when VBATTERY is greater
than VCC. VBATTERY connects to VOUT (through
a 15Ω switch) when VCC is below VSW and
The SP802T/S/R and SP804T/S/R devices are
respectively similar to the SP690T/S/R and
SP805T/S/R devices with tightened reset
and power-fail threshold tolerances.
VBATTERY is greater than VCC
.
SP690T/S/R JAN 30-06 SP690T/S/R, 802/T/S/R, 804T/S/R, 805T/S/R Low Power Microprocessor Supervisory
© 2006 Sipex Corporation
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EXAR [ EXAR CORPORATION ]
