Monitoring Voltages Other than VCC

voltage divider to PFI and adjusting the ratio appropri-

ately. If required, add hysteresis by connecting a resis-

tor (with a value approximately 10-times the sum of the

two resistors in the potential divider network) between

PFI and PFO. A capacitor between PFI and GND will

reduce the power-fail circuit’s sensitivity to high-fre-

quency noise on the line being monitored. RESET can

be asserted on other voltages in addition to the +5V

PFI drops below 2.50V (K, L, N suffix) or 1.70V (T suffix

or MAX816). Figure 11 shows the MAX814K/L/N/

MAX815K/L/N configured to assert RESET when the

+5V supply falls below the reset threshold, or when the

+12V supply falls below approximately 11V.

Monitoring a Negative Voltage

The power-fail comparator can also monitor a negative

supply rail (Figure 12). When the negative rail is good

(a negative voltage of large magnitude), PFO is low.

When the negative rail is degraded (a negative voltage

of lesser magnitude), PFO is high. By adding the resis-

tors and transistor as shown, a high PFO triggers reset.

As long as PFO remains high, the MAX814/MAX815/

MAX816 will keep reset asserted (RESET = low, RESET

= high). Note that this circuit’s accuracy depends on the

Watchdog Software Considerations

A way to help the watchdog timer keep closer tabs on

software execution involves setting and resetting the

watchdog input at different points in the program,

rather than pulsing the watchdog input high-low-high or

low-high-low. This technique avoids a stuck loop where

the watchdog timer continues to be reset within the

loop, keeping the watchdog from timing out. Figure 13

shows an example flow diagram where the I/O driving

the watchdog input is set low at the beginning of the

program, set high at the beginning of every subroutine,

then set low at the end of every subroutine. If the pro-

gram should hang in any subroutine, the I/O is continu-

ally set high and the watchdog timer is allowed to time

out, causing a reset to be issued.

±1% Accuracy, Low-Power, +3V and +5V

µP Supervisory Circuits

______________________________________________________________________________________

13

MAX814

MAX815

MAX816

RESET

R1

MAX814K/L/N

MAX815K/L/N

RESET

100k

1%

PARAMETER

MIN

TYP

MAX

UNIT

+12V Reset

Threshold at +25°C

10.57 11.00 11.45

V

340k

1%

+5V

+12V

PFI

GND

MR

PFO

TO

µP

Figure 10. RESET Valid to Ground Circuit

Figure 11. Monitoring Both +5V and +12V