Ensuring a Valid RESET Output Down to
VCC = 0V
Power-Fail Comparator
The power-fail comparator can be used for
various purposes because its output and
noninverting input are not internally connected.
The inverting input is internally connected to a
1.25V reference.
When VCC falls below 1.1V, the SP705/706/707/
708 RESET output no longer sinks current, it
becomes an open circuit. High-impedance
CMOS logic inputs can drift to undetermined
voltages if left undriven. If a pull-down resistor
is added to the RESET pin, any stray charge or
leakage currents will be shunted to ground,
holding RESET LOW. The resistor value is not
critical. Itshouldbeabout100KΩ,largeenough
not to load RESET and small enough to pull
RESET to ground.
To build an early-warning circuit for power
failure, connect the PFI pin to a voltage divider
as shown in Figure 16. Choose the voltage
divider ratio so that the voltage at PFI falls
below1.25Vjustbeforethe+5Vregulatordrops
out. Use PFO to interrupt the µP so it can
prepare for an orderly power-down.
Monitoring Voltages Other Than the
Unregulated DC Input
Manual Reset
The manual-reset input (MR) allows RESET to
be triggered by a pushbutton switch. The switch
is effectively debounced by the 140ms mini-
mum RESET pulse width. MR is TTL/CMOS
logic compatible, so it can be driven by an
external logic line. MR can be used to force a
watchdog timeout to generate a RESET pulse
in the SP705/706/813L. Simply connect
WDO to MR.
MonitorvoltagesotherthantheunregulatedDC
by connecting a voltage divider to PFI and
adjusting the ratio appropriately. If required,
add hysteresis by connecting a resistor (with a
value approximately 10 times the sum of the
two resistors in the potential divider network)
betweenPFIandPFO. AcapacitorbetweenPFI
and GND will reduce the power-fail circuit's
Regulated +5V
Power Supply
+12V
+5V
V
Unregulated DC
Power Supply
0.1µF
VCC
1MΩ
1%
CC
MR
VCC
R
1
2
PFI
RESET
RESET
µP
PFI
MR
INTERRUPT
PFO
PFI
PFO
130KΩ
1%
I/O LINE
NMI
R
SP705
SP706
SP813L
WDO
RESET
to µP
GND
GND
GND
PUSHBUTTON
SWITCH
Figure 16. Typical Operating Circuit
Figure 17. Monitoring Both +5V and +12V Power
Supplies
June 2008 Rev C
SP705 Low Power Microprocessor Supervisory Circuits
© 2008 Exar Corporation
11
EXAR [ EXAR CORPORATION ]
