APX809/810

3-PIN MICROPROCESSOR RESET CIRCUITS

APX809/810 Rev. 3

4 of 9

MARCH 2008

www.diodes.com

Typical Application Circuit

APX8XX

GND

RESET

(RESET)

V

CC

Microprocessor

V

CC

GND

RESET

INPUT

V

CC

100nF

Functional Description

A microprocessor’s (µP’s) reset input starts the µP in a known

state. The APX809/810 assert reset to prevent code-execution

errors during power-up, power-down, or brownout conditions.

declines below a preset threshold, keeping it asserted for at least

APX809/810 have a push-pull 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.

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

applications where

pull down resistor to

currents to flow to ground, holding

not critical; 100k are large enough not to load

small enough to pull

Benefits of Highly Accurate Reset Threshold

Most µP supervisor ICs has reset threshold voltages between

5% and 10% below the value of nominal supply voltages. This

ensures a reset will not occur within 5% of the nominal supply,

but will occur when the supply is 10% below nominal. When using

ICs rated at only the nominal supply ±5%, this leaves a zone of

uncertainty where the supply is between 5% and 10% low, and

where the reset may or may not be asserted.