Detailed Description

The MAX7391 replaces ceramic resonators, crystals,

and supervisory functions for microcontrollers in 3.3V

and 5V applications.

The MAX7391 provides a clock source, reset, and

power-fail functions. The power-fail output provides

early warning of power failure. The power-fail threshold

is configurable to detect either an external voltage or

The clock output can be switched between normal and

half-speed operation. This functionality allows the

microcontroller to operate at reduced power and may

be used to extend the time available to perform house-

keeping tasks, such as writing data to flash, during a

power failure. Connecting the power-fail output (PFO)

to the SPEED input reduces clock speed automatically

during power-fail events.

The integrated reset provides the power-supply monitor-

ing functions necessary to ensure correct microcontroller

operation. The reset circuit has built-in power-supply

transient immunity and provides both power-on reset

and power-fail or brownout reset functionality. Two stan-

dard factory-trimmed reset levels are available.

A power-fail function is provided for power-supply volt-

age monitoring and can provide advance notice of an

impending power failure. The power-fail input monitors

external power-supply voltages through an external

Clock Output (CLOCK)

The push-pull clock output (CLOCK) drives a ground-

connected 1k

Ω load or a positive supply connected

500

Ω load to within 300mV of either supply rail. CLOCK

remains stable over the full operating voltage range and

does not generate short output cycles during either

power-on or power-off. A typical startup characteristic is

shown in the Typical Operating Characteristics section.

The clock output frequency is reduced by a factor of

two by taking SPEED low. This functionality allows the

microcontroller to operate at reduced power and may

be used to extend the time available to perform house-

keeping tasks, such as writing data to flash during

power failure.

Reset

The reset function drives the microcontroller reset input

to prevent operation in the cases of the initial power-on

setting, low power-supply voltages, and the failed

watchdog operations. Three reset output versions are

available: push-pull RST, push-pull RST, and open-drain

Power-On Reset (POR)

The internal power-on reset (POR) circuit detects the

Upon completion of the reset timeout, the reset output

Speed-Switching Clock Generator

with Power Fail

6

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PIN

NAME

FUNCTION

1

PFI

section for more details.

bypass capacitor as close to the device as possible.

3

N.C.

No Connection

4

GND

Ground

5

PFO

Power-Fail Output. Open-drain output asserts when the voltage being monitored drops below the power-fail

threshold voltage.

6

SPEED

Clock-Speed Select Input. Connect SPEED high for the factory-trimmed clock output frequency. Connect

SPEED low to reduce the clock output frequency by half.

7

RST/RST

Reset Output. Reset output is available in one of three configurations: push-pull RST, push-pull RST,

or open-drain RST. The reset output occurs if any combination of the following conditions occurs: reset

reset output asserts when WDI does not receive a rising or falling edge within the watchdog timeout period.

8

CLOCK

Clock Output

Pin Description