Date: 11/29/04

SP690A/692A Low Power Microprocessor Supervisory with Battery Switch-Over

© Copyright 2004 Sipex Corporation

9

THEORY OF OPERATION

The SP690A/692A/802L/802M/805L/805M

microprocessor (

µP) supervisory circuits

monitor the power supplied to digital circuits

such as microprocessors, 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. The operational features and benefits of

the SP690A/692A/802L/802M/805L/805M 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

SP690A/SP692A/SP802 assert reset during

power-up and prevent code execution errors

during power-down or brownout conditions.

On power-up, once V

guaranteed to be a logic low. As V

RESET remains low. When V

reset threshold, RESET will remain low for

200ms, Figure 9.

If a brownout condition

occurs and V

RESET is triggered.

Each time RESET is

triggered, it stays low for the reset pulse width

interval. If a brownout condition interrupts a

previously initiated reset pulse, the reset pulse

continues for another 200ms. On power-down,

once V

guaranteed to be logic low until V

below 1V.

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

clear. When RESET comes high, the watchdog

resumes timing and must be serviced within

1.6sec. If WDI is tied high or low, a RESET

pulse is triggered every 1.8sec (t

FEATURES

The SP690A/692A/802L/802M/805L/805M

provide four key functions:

1. A battery backup switching 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 +5V.

The parts differ in their reset-voltage threshold

levels and reset outputs. The SP690A/802L/

805L generate a reset when the supply voltage

drops below 4.65V. The SP692A/802M/805M

generate a reset below 4.40V.

The SP690A/692A/802L/802M/805L/805M

are ideally suited for applications in automotive

systems, intelligent instruments, and battery-

powered computers and controllers. All designs

into an environment where it is critical to

monitor the power supply to the

µP and it’s

related digital components will find the

SSP690A/692A/802L/802M/805L/805M ideal.

Figure 12. Typical Operating Circuit

GND

GND

RESET

NMI

I/O LINE

RESET

PFO

WDI

BUS

GND

Unregulated

Regulated +5V

0.1µF

PFI

DC

Lithium

Battery

3.6V

µP

CMOS

RAM