APX803

3-PIN MICROPROCESSOR RESET CIRCUIT

APX803 Rev. 1

5 of 10

MARCH 2010

Document number: DS32132 Rev. 1 – 2

www.diodes.com

Functional Description

Microprocessors (µPs) and microcontrollers (µC) have a reset

input to ensure that it starts up in a known state. The APX803

drive the µP’s reset input to prevent code-execution errors during

power-up, power-down, or brownout conditions. They assert a

has risen above the reset threshold. The APX803 has an

open-drain output stage.

Ensuring a Valid Reset Output

exceeds the reset threshold, an internal timer keeps

RESET

low for the reset timeout period; after this interval,

RESET

goes high.

goes low. Any time

zero, and

low for the reset timeout period.

longer sinks current — it becomes an open circuit. Therefore,

high-impedance CMOS logic inputs connected to

drift to undetermined voltages.

This presents no problem in most applications since most µP and

Interfacing to µP with Bidirectional Reset Pins

Since the RESET output on the APX803 is open drain, this

device interfaces easily with

μP/µC that have bidirectional reset

pins, such as the Motorola 68HC11.

Connecting the

μP supervisor’s RESET output directly to the

microcontroller’s (

μC’s) RESET pin with a single pull-up resistor

allows either device to assert reset.

Supervising and monitoring Multiple Supplies

Generally, the pull-up resistor connected to the APX803 will

connect to the supply voltage that is being monitored at the IC’s

open-drain output to level-shift from the monitored supply to reset

the µP powered by a different supply voltage or monitor multiple

supplies that will be fed into 1 µC/µP reset input.

Block Diagram

Delay Circuit

Driver

RESET

/ RESET