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

12

THEORY OF OPERATION

The SP690T/S/R, SP802T/S/R, SP804T/S/R

and SP805T/S/R devices are microprocessor

(

µP) supervisory circuits that monitor the power

supplied to digital circuits such as microproces-

sors, microcontrollers, or memory. The series is

an ideal solution for portable, battery-powered

equipment that requires power supply monitoring.

Implementing this series will reduce the

number of components and overall complexity.

The watchdog functions of this product family

will continuously oversee the operational status

of a system.

These

µP supervisory circuits are not short-

circuit protected. Shorting V

excluding power-up transients such as charging

a decoupling capacitor - may potentially damage

these devices. Decouple both V

pins to ground by placing 0.1

µF capacitors as

close to the device as possible. The operational

features and benefits of the SP690T/S/R,

SP802T/S/R, SP804T/S/R and SP805T/S/R

devices are described in more detail below.

Reset Output

The microprocessor's (

µP's) reset input starts

the

µP in a known state. When the µP is in an

unknown state, it should be held in reset. The

SP690T/S/R, SP802T/S/R, SP804T/S/R and

SP805T/S/R devices assert reset during

power-up and prevent code execution errors

during power-down or brownout conditions.

RESET is guaranteed to be a logic LOW for 0V

< V

than 1V. Without a backup battery, RESET is

guaranteed valid for V

exceeds the reset threshold, an internal timer

keeps RESET low for the reset timeout period.

After this period, RESET goes HIGH, as seen in

Figure 19.

If a brownout condition occurs and V

below the reset threshold, RESET goes LOW.

Each time RESET is triggered, it stays low for

the reset timeout period. Any time V

below the reset threshold, the internal timer

restarts.

FEATURES

The SP690T/S/R, SP802T/S/R, SP804T/S/R

and SP805T/S/R devices provide four key

functions:

1. A battery backup switch for CMOS RAM,

CMOS microprocessors, or other logic.

2. A reset output during power-up, power-down

and brownout conditions.

3. A reset pulse if the optional watchdog timer

has not been toggled within a specified time.

4. A 1.25V threshold detector for power-fail

warning, low battery detection, or to monitor a

power supply other than 3.3V or 3.0V.

The SP690T/S/R, SP802T/S/R, SP804T/S/R

and SP805T/S/R devices differ in their reset-

voltage threshold levels and are ideally suited

for applications in automotive systems, intelligent

instruments, and battery-powered computers and

controllers. The series is a solid match for

designs where it is critical to monitor the

power supply to the

µP and it’s related digital

components.

Figure 20. Typical Operating Circuit

GND

GND

RESET

NMI

I/O LINE

pin 7*

PFO

WDI

BUS

GND

Unregulated

Regulated +3.3V or +3.0V

0.1

µF

PFI

DC

Lithium

Battery

3.6V

µP

CMOS

RAM

0.1

µF

RESET for the SP690T/S/R and the SP802T/S/R

RESET for the SP804T/S/R and the SP805T/S/R

*

SP690T/S/R

SP802T/S/R

SP804T/S/R

SP805T/S/R