SP705_06 datasheet(11/18 Pages) SIPEX

11 / 18 page

OCT 17-06 RevB

SP705 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

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 100K

Ω,largeenough

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

PFI

PFO

VCC

+5V

GND

RESET

µP

+12V

Ω

130K

Ω

Figure 17. Monitoring Both +5V and +12V Power

Supplies

GND

RESET

INTERRUPT

I/O LINE

VCC

RESET

PFO

PFI

Unregulated DC

Power Supply

Regulated +5V

Power Supply

VCC

0.1

µF

PFI

µP

NMI

WDO

PUSHBUTTON

SWITCH

SP705

SP706

SP813L

SP813M

Figure 16. Typical Operating Circuit

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