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Notice: The information in this document is subject to change without notice

ASM690A / 692A

ASM802L / 802M

ASM805L

µP Power Supply Supervisor With Battery Backup Switch

rev 1.0

October 2003

Detailed Description

It is important to initialize a microprocessor to a known state

in response to specific events that could create code

execution errors and “lock-up”. The reset output of these

supervisory circuits send a reset pulse to the microprocessor

in response to power-up, power-down/power-loss or a

watchdog time-out.

RESET/RESET Timing

output is asserted in the appropriate logic state for the

200ms and is LOW for the ASM690A, ASM692A, ASM802

and HIGH for the ASM805L. Figure 1 shows the reset pin

timing.

the reset threshold resulting in a reset assertion for the

devices can provide a guaranteed logic-low output. To ensure

logic inputs connected to the ASM690A / ASM692A/ASM802

100k

Ω pull-down resistor at RESET is needed: the logic-high

Watchdog Timer

A Watchdog time-out reset occurs when a logic “1” or logic

“0” is continuously applied to the WDI pin for more than 1.6

seconds. After the duration of the reset interval, the watchdog

timer starts a new 1.6 second timing interval; the

microprocessor must service the watchdog input by changing

states or by floating the WDI pin before this interval is

finished. If the WDI pin is held either HIGH or LOW, a reset

pulse will be triggered every 1.8 seconds (the 1.6 second

Application Information

Microprocessor Interface

The ASM690 has logic-LOW RESET output while the

ASM805 has an inverted logic-HIGH RESET output.

Microprocessors with bidirectional reset pins can pose a

problem when the supervisory circuit and the microprocessor

output pins attempt to go to opposite logic states. The

problem can be resolved by placing a 4.7k

Ω resistor between

the RESET output and the microprocessor reset pin. This is

shown in Figure 2. Since the series resistor limits drive

capabilities, the reset signal to other devices should be

buffered.

Figure 1: RESET/RESET Timing

Figure 2: Interfacing with bi-directional

microprocessor reset inputs

RESET

RESET

GND

GND

ASM690A

Power Supply

Bi-directional I/O pin

RESET

Buffered

BUF

4.7K Ω