_______________Detailed Description

Reset Output

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

known state. When the µP is in an unknown state, it should

be held in reset. The MAX690A/MAX692A/MAX802L/

MAX802M assert reset during power-up and prevent code

execution errors during power-down or brownout condi-

tions.

timer keeps RESET low for a time equal to the reset

pulse width; after this interval, RESET goes high (Figure

the reset threshold), RESET is triggered. Each time

RESET

is triggered, it stays low for the reset pulse width

the internal timer restarts the pulse. If a brownout con-

dition interrupts a previously initiated reset pulse, the

reset pulse continues for another 200ms. On power-

RESET

is also triggered by a watchdog timeout. If a

high or low is continuously applied to the WDI pin for

1.6sec, RESET pulses low. As long as RESET is assert-

ed, the watchdog timer remains clear. When RESET

comes high, the watchdog resumes timing and must be

serviced within 1.6sec. If WDI is tied high or low, a

RESET

The MAX805L active-high RESET output is the inverse

of the MAX690A/MAX692A/MAX802L/MAX802M RESET

1.1V. Some µPs, such as Intel’s 80C51, require an

active-high reset pulse.

Watchdog Input

The watchdog circuit monitors the µP’s activity. If the

µP does not toggle the watchdog input (WDI) within

1.6sec, a reset pulse is triggered. The internal 1.6sec

timer is cleared by either a reset pulse or by open cir-

cuiting the WDI input. As long as reset is asserted or

the WDI input is open circuited, the timer remains

cleared and does not count. As soon as reset is

released or WDI is driven high or low, the timer starts

counting. It can detect pulses as short as 50ns.

Power-Fail Comparator

The PFI input is compared to an internal 1.25V refer-

ence. If PFI is less than 1.25V, PFO goes low. The

power-fail comparator is intended for use as an under-

voltage detector to signal a failing power supply; it

Microprocessor Supervisory Circuits

6

_______________________________________________________________________________________

1.25V

0.8V

WATCHDOG

TIMER

PFO

RESET

(RESET)

RESET

GENERATOR

BATTERY-SWITCHOVER

CIRCUITRY

1.25V

3.5V

PFI

WDI

MAX690A

MAX692A

MAX802L

MAX802M

MAX805L

( ) ARE FOR MAX805L ONLY.

GND

Figure 1. Block Diagram

+5V

0V

0V

+5V

0V

RESET

+5V

0V

(RESET)

3.0V

3.0V

+5V

0V

PFO

( ) ARE FOR MAX805L ONLY.

Figure 2. Timing Diagram