APX803/D

3-PIN MICROPROCESSOR RESET CIRCUIT

APX803

Document number: DS32132 Rev. 2 - 2

7 of 11

www.diodes.com

September 2010

© Diodes Incorporated

Timing Diagram

Vth

Vth

/RESET

RESET

Vcc

Vth

Functional Description

Microprocessors (µPs) and microcontrollers (µC) have a

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

The APX803/D drive the µP’s reset input to prevent

code-execution errors during power-up, power-down, or

brownout

conditions.

They

assert

a

reset

signal

preset threshold and keep it asserted for a fixed period

the APX803D this period is a minimum of 1ms while for

other APX803 variants it is at least 140ms. The

APX803/D have an open-drain output stage.

Ensuring a Valid Reset Output

keeps

RESET low for the reset timeout period; after

this interval, RESET goes high. If a brownout condition

threshold, the internal timer resets to zero, and RESET

above the reset threshold, and RESET remains low for

the reset timeout period.

RESET

output no longer sinks current — it becomes an open

circuit. Therefore, high-impedance CMOS logic inputs

connected to RESET can drift to undetermined voltages.

This presents no problem in most applications since

1V.

Interfacing to µP with Bidirectional Reset Pins

Since the RESET output on the APX803/D 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/D will connect to the supply voltage that is being

may use the APX803/D 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.