Watchdog Input

(MAX690_/802_/804_/805_)

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 a transition (low-to-

high or high-to-low) at WDI. If WDI is tied high or low, a

–

R

—

E

—

S

—

E

—

T

–

As long as reset is asserted, the timer remains cleared

and does not count. As soon as reset is deasserted,

the timer starts counting. Unlike the 5V MAX690 family,

the watchdog function cannot be disabled.

Power-Fail Comparator

The PFI input is compared to an internal reference. If

–

P

—

F

—

O

–

goes low. The power-fail

comparator is intended for use as an undervoltage

detector to signal a failing power supply. However, the

comparator does not need to be dedicated to this

function because it is completely separate from the rest

of the circuitry.

The power-fail comparator turns off and

–

P

—

F

—

O

–

goes low

power-up. If the comparator is not used, connect PFI to

ground and leave

–

P

—

F

—

O

–

unconnected.

–

P

—

F

—

O

–

may be

connected to

–

M

—

R

–

on the MAX704_/MAX806_ so that a

low voltage on PFI will generate a reset (Figure 5b).

3.0V/3.3V Microprocessor Supervisory Circuits

_______________________________________________________________________________________

7

V

BATT

RESET

(RESET)

WDI

MR

BATTERY

SWITCHOVER

COMPARATOR

RESET

COMPARATOR

RESET

GENERATOR

WATCHDOG

TIMER

BATTERY

SWITCHOVER

CIRCUITRY

MAX690T/S/R

MAX704T/S/R

MAX802T/S/R

MAX804T/S/R

MAX805T/S/R

MAX806T/S/R

PFO

PFI

1.237V

1.237V

**

*

*

POWER-FAIL

COMPARATOR

* MAX690T/S/R, MAX802T/S/R, MAX804T/S/R, MAX805T/S/R ONLY

** MAX704T/S/R, MAX806T/S/R ONLY

( ) MAX804T/S/R, MAX805T/S/R ONLY

VBATT = 3.6V

3.0V OR 3.3V

RESET

PFO

(RESET)

3.0V OR 3.3V

3.0V OR 3.3V

0V

V

BATT = PFI = 3.6V

Figure 1. Block Diagram

Figure 2. Timing Diagram