4-Pin µP Voltage Monitors

with Manual Reset Input

_______________________________________________________________________________________

5

______________________________________________________________ Pin Description

_______________ Detailed Description

Reset Output

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

known state. These µP supervisory circuits assert reset

to prevent code execution errors during power-up,

power-down, or brownout conditions.

timer keeps RESET low for the reset timeout period;

after this interval, RESET goes high.

below the reset threshold, the internal timer resets to

zero, and RESET goes low. The internal timer starts

remains low for the reset timeout period.

The manual reset input (MR) can also initiate a reset.

See the

Manual Reset Input section.

The MAX812 has an active-high RESET output that is

the inverse of the MAX811’s RESET output.

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,

returns high. This input has an internal 20k

Ω pull-up

resistor, so it can be left open if it is not used. MR can

be driven with TTL or CMOS-logic levels, or with open-

drain/collector outputs. Connect a normally open

momentary switch from MR to GND to create a manual-

reset function; external debounce circuitry is not

required. If MR is driven from long cables or if the

device is used in a noisy environment, connecting a

0.1µF capacitor from MR to ground provides additional

noise immunity.

Reset Threshold Accuracy

The MAX811/MAX812 are ideal for systems using a 5V

±5% or 3V ±5% power supply with ICs specified for 5V

±10% or 3V ±10%, respectively. They are designed to

meet worst-case specifications over temperature. The

reset is guaranteed to assert after the power supply

falls out of regulation, but before power drops below

the minimum specified operating voltage range for the

system ICs. The thresholds are pre-trimmed and exhibit

tight distribution, reducing the range over which an

undesirable reset may occur.

Manual Reset Input. A logic low on MR asserts reset. Reset remains asserted as long as MR is

low and for 180ms after MR returns high. This active-low input has an internal 20k

Ω pull-up

resistor. It can be driven from a TTL or CMOS-logic line, or shorted to ground with a switch.

Leave open if unused.

3

3

+5V, +3.3V, or +3V Supply Voltage

4

4

tions are terminated.

2

—

conditions are terminated.

—

2

Ground

1

1

FUNCTION

PIN

MR

RESET

RESET

GND

NAME

MAX811

MAX812