4-Pin, Low-Power

µP Reset Circuits with Manual Reset

4

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Pin Description

Applications Information

Manual Reset Input

Many µP-based products require manual reset capabil-

ity, allowing the operator, a test technician, or external

logic circuitry to initiate a reset. A logic low on MR

asserts reset. Reset remains asserted while MR is low,

and for the reset active timeout period after MR returns

high. MR has an internal 20k

Ω pullup resistor, so it can

be left unconnected if not used. Connect a normally

open momentary switch from MR to GND to create a

manual reset function; external debounce circuitry is

not required.

Interfacing to µPs with

Bidirectional Reset Pins

Since the RESET output on the MAX6805 is open-drain,

this device interfaces easily with µPs that have bidirec-

tional 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

pullup resistor allows either device to assert reset

(Figure 1).

In addition to issuing a reset to the µP during power-up,

power-down, and brownout conditions, these devices

are relatively immune to short-duration, negative-going

Characteristics show the Maximum Transient Duration

vs. Reset Comparator Overdrive graph. The graph

shows the maximum pulse width that a negative-going

signal. As the amplitude of the transient increases, the

maximum allowable pulse width decreases.

Ensuring a Valid Reset Output

operating voltage of 0.7V, push/pull-structured reset

sinking (or sourcing) capabilities decrease drastically.

High-impedance CMOS-logic inputs connected to the

RESET pin can drift to indeterminate voltages. This

does not present a problem in most cases, since most

GND

MAX6805

RESET

MR

GND

RESET

MOTOROLA

68HCXX

µP

100k

Ω

Figure 1. Interfacing to µPs with Bidirectional Reset Pins

PIN

MAX6803

MAX6804

MAX6805

NAME

FUNCTION

1

1

GND

Ground

—

2

RESET

threshold, or while MR is asserted. RESET remains asserted for a reset

deasserted. RESET on the MAX6804 is push/pull. RESET on the MAX6805

is open-drain.

reset threshold or while MR is asserted, and RESET remains asserted for a

is deasserted. RESET on the MAX6803 is push/pull.

RESET

—

2

3

3

MR

Manual Reset Input. A logic low on MR asserts reset. Reset remains

Supply Voltage Input

4

4