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MAX690C

Microprocessor Supervisory Circuits

Maxim Integrated 

MAX690–MAX695

Microprocessor Supervisory Circuits

1.3V Comparator and Power-Fail Warning

The power-fail input (PFI) is compared to an internal

1.3V reference. The power-fail output (PFO) goes low

when the voltage at PFI is less than 1.3V. Typically, PFI is

driven by an external voltage divider which senses either

the unregulated DC input to the system’s 5V regulator or

the regulated 5V output. The voltage divider ratio can be

chosen such that the voltage at PFI falls below 1.3V sev-

eral milliseconds before the +5V supply falls below 4.75V.

PFO is normally used to interrupt the microprocessor so

that data can be stored in RAM before VCC falls below

4.75V and the RESET output goes low (4.5V for MAX692/

MAX693).

The power-fail detector can also monitor the backup bat-

tery to warn of a low battery condition. To conserve bat-

tery power, the power-fail detector comparator is turned

off and PFO is forced when VCC is lower than VBATT

input voltage.

Watchdog Timer and Oscillator

The watchdog circuit monitors the activity of the micro-

processor. If the microprocessor does not toggle the

Watchdog Input (WDI) within the selected timeout period,

a 50ms* RESET pulse is generated. Since many systems

cannot service the watchdog timer immediately after a

reset, the MAX691/MAX693/MAX695 has a longer time-

out period after reset is issued. The normal timeout period

becomes effective following the first transition of WDI after

RESET has gone high. The watchdog timer is restarted

at the end of reset, whether the reset was caused by

lack of activity on WDI or by VCC falling below the reset

threshold. If WDI remains either high or low, reset pulses

will be issued every 1.6s. The watchdog monitor can be

deactivated by floating the watchdog input (WDI).

The watchdog output (WDO, MAX691/MAX693/MAX695

only) goes low if the watchdog timer times out and

remains low until set high by the next transition on the

watchdog input. WDO is also set high when VCC goes

below the reset threshold.

The watchdog timeout period is fixed at 1.6s and the

reset pulse width is fixed at 50ms* on the 8-pin MAX690/

MAX692/MAX694. The MAX691/MAX693/MAX695 allow

these times to be adjusted per Table 1. Figures 8 shows

various oscillator configurations.

The internal oscillator is enabled when OSC SEL is

floating. In this mode, OSC IN selects between the

1.6s and 100ms watchdog timeout periods. In either

case, immediately after a reset the timeout period 1.6s.

This gives the microprocessors time to reintialize the

system. If OSC IN is low, then the 100ms watchdog

period becomes effective after the first transition of WDI.

The software should be written such that the I/O port

driving WDI is left in its power-up reset state until the ini-

tialization routines are completed and the microprocessor

is able to toggle WDI at the minimum watchdog timeout

period of 70ms.

*200ms for MAX694

WATCHDOG INPUT

VCC 2.7V

+

-

-

+

1.0V

TRANSACTION

DETECTOR

FOR EACH TRANSITION

LOW LINE

(HI IF VCC < 4.65V)

HI IF WATCHDOG

INPUT IS FLOATING

PRESCALER

10.24 kHz FROM INTERNAL OSCILLATOR

OR EXTERNALLY SET FREQUENCY FROM

Q6

OSC IN PIN

WATCHDOG TIMEOUT SELECT

RESET

COUNTER

R Q10/12

WATCHDOG

COUNTER

Q11

Q13

R

Q15

WATCHDOG

TIMEOUT

SELECTOR

LOGIC

GOES HIGH AT THE

END OF WATCHDOG

TIMEOUT PERIOD

S

R

RESET

FLIP FLOP

Q

Q

SQ

LONG/SHORT LOW

FF

LINE

R

RS

WATCHDOG

FAULT FF

Q

Figure 7. Watchdog Timer Block Diagram

RESET RESET

WATCHDOG OUTPUT

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