SP706REN-L Datasheet, PDF (11/18 Pages) Sipex Corporation – Low Power Microprocessor Supervisory Circuits

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.
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
below 1.25V just before the +5V regulator drops
out. Use PFO to interrupt the µP so it can
prepare for an orderly power-down.
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/813M. Simply connect
WDO to MR.
Ensuring a Valid RESET Output Down to
V_CC = 0V
When V_CC falls below 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. It should be about 100KΩ, large enough
not to load RESET and small enough to pull
RESET to ground.
Monitoring Voltages Other Than the
Unregulated DC Input
Monitor voltages other than the unregulated DC
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)
between PFI and PFO. A capacitor between PFI
and GND will reduce the power-fail circuit's
Regulated +5V
Power Supply
Unregulated DC
Power Supply
VCC
0.1µF
µP ^RESET
RESET
INTERRUPT _PFO
VCC
R1
PFI
I/O LINE
NMI
GND
PFI
SP705
R2
WDO
SP706
SP813L
MR
SP813M
GND
PUSHBUTTON
SWITCH
to µP
+5V
+12V
MR
PFO
RESET
VCC
1MΩ
1%
PFI
130KΩ
1%
GND
Figure 16. Typical Operating Circuit
Figure 17. Monitoring Both +5V and +12V Power
Supplies
SP705DS/09
SP705 Low Power Microprocessor Supervisory Circuits
11
© Copyright 2000 Sipex Corporation