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Detailed Description

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

known state. The MAX803/MAX809/MAX810 assert

reset to prevent code-execution errors during power-

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

below a preset threshold, keeping it asserted for at

old. The MAX803 uses an open-drain output, and the

MAX809/MAX810 have a push-pull output stage.

Connect a pull-up resistor on the MAX803’s RESET out-

put to any supply between 0 and 6V.

Applications Information

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

power-down, and brownout conditions, the MAX803/

MAX809/MAX810 are relatively immune to short-duration

Figure 1 shows typical transient duration vs. reset com-

parator overdrive, for which the MAX803/MAX809/

MAX810 do not generate a reset pulse. The graph was

starting 0.5V above the actual reset threshold and end-

ing below it by the magnitude indicated (reset compara-

tor overdrive). The graph indicates the maximum pulse

causing a reset pulse. As the magnitude of the transient

increases (goes farther below the reset threshold), the

maximum allowable pulse width decreases. Typically, for

100mV below the reset threshold and lasts 20µs or less

will not cause a reset pulse. A 0.1µF bypass capacitor

additional transient immunity.

Ensuring a Valid Reset Output

no longer sinks current—it becomes an open circuit.

Therefore, high-impedance CMOS logic inputs con-

nected to RESET can drift to undetermined voltages.

This presents no problem in most applications since

below 1V. However, in applications where RESET must

be valid down to 0V, adding a pull-down resistor to

RESET causes any stray leakage currents to flow to

3-Pin Microprocessor Reset Circuits

R1

100k

GND

MAX809

RESET

2

RESET

(MAX803/

MAX809)

is below the reset threshold, and for at

reset threshold.

1

GND

Ground

PIN

NAME

FUNCTION

RESET

(MAX810)

is below the reset threshold, and for at

reset threshold.

_____________________Pin Description

400

320

0

1

100

1000

160

80

240

10

MAX8_ _J/L/M

MAX8_ _R/S/T/Z

Figure 1. Maximum Transient Duration Without Causing a

Reset Pulse vs. Reset Comparator Overdrive

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Supply Voltage (+5V, +3.3V, +3.0V, or

+2.5V)