Monitoring an Additional Power Supply

These µP supervisors can monitor either positive or

negative supplies using a resistor voltage-divider to

PFI. PFO can be used to generate an interrupt to the µP

or cause reset to assert (Figure 3).

Monitoring a Negative Voltage

The power-fail comparator can be used to monitor a

negative supply voltage using the circuit shown in

Figure 3. When the negative supply is valid, PFO is low.

When the negative supply voltage drops, PFO goes

high. The circuit’s accuracy is affected by the PFI

The MAX6715–MAX6729 supervisors are relatively

(glitches). It is usually undesirable to reset the µP when

Operating Characteristics show Maximum Transient

Duration vs. Reset Threshold Overdrive, for which reset

pulses are not generated. The graph was produced

and ending below the reset threshold by the magnitude

indicated (reset threshold overdrive). The graph shows

transient may typically have without causing a reset

pulse to be issued. As the amplitude of the transient

increases (i.e., goes farther below the reset threshold),

the maximum allowable pulse width decreases. A 0.1µF

vides additional transient immunity.

Watchdog Software Considerations

Setting and resetting the watchdog input at different

points in the program, rather than “pulsing” the watch-

dog input high-low-high or low-high-low, helps the

watchdog timer to closely monitor software execution.

This technique avoids a “stuck” loop where the watch-

dog timer continues to be reset within the loop, keeping

the watchdog from timing out. Figure 6 shows an exam-

ple flow diagram where the I/O driving the watchdog

input is set high at the beginning of the program, set low

at the beginning of every subroutine or loop, then set

high again when the program returns to the beginning. If

the program should “hang” in any subroutine, the I/O is

continually set low and the watchdog timer is allowed to

time out, causing a reset or interrupt to be issued.

Chip Information

TRANSISTOR COUNT: 1072

PROCESS: BiCMOS

Dual/Triple Ultra-Low-Voltage SOT23 µP

Supervisory Circuits

12

______________________________________________________________________________________

MAX6728/

MAX6729

R1

R3

R2

PFI

GND

PFO

A

R2 = 200k

Ω (FOR > 2.5µA R2 CURRENT)

PFO

Figure 5. Adding Hysteresis to Power-Fail for Push-Pull PFO

START

SET WDI

HIGH

PROGRAM

CODE

SUBROUTINE OR

PROGRAM LOOP

SET WDI LOW

RETURN

SUBROUTINE

COMPLETED

HANG IN

SUBROUTINE

Figure 6. Watchdog Flow Diagram