Applications Information

Selecting Reset/Watchdog

Timeout Capacitor

The reset timeout period is adjustable to accommodate

a variety of µP applications. Adjust the reset timeout

SRT and ground. Calculate the reset timeout capacitor

as folllows:

The watchdog timeout period is adjustable to accom-

modate a variety of µP applications. With this feature,

the watchdog timeout can be optimized for software

execution. The programmer can determine how often

the watchdog timer should be serviced. Adjust the

normal mode operation, calculate the watchdog time-

out capacitor as follows:

For the MAX6752 and MAX6753 windowed watchdog

follows:

capacitor. Ceramic capacitors are recommended.

Transient Immunity

In addition to issuing a reset to the µP during power-up,

power-down, and brownout conditions, these supervi-

sors are relatively immune to short-duration supply tran-

sients (glitches). The Maximum Transient Duration vs.

Reset Threshold Overdrive graph in the Typical

Operating Characteristics shows this relationship.

The area below the curves of the graph is the region

in which these devices typically do not generate a reset

pulse. This graph was generated using a falling pulse

(reset-threshold overdrive). As the magnitude of the tran-

sient increases (farther below the reset threshold), the

maximum allowable pulse width decreases. Typically, a

and lasts 50µs or less does not cause a reset pulse to be

issued.

µP Reset Circuits with Capacitor-Adjustable

Reset/Watchdog Timeout Delay

10

______________________________________________________________________________________

MAX6747

MAX6749

MAX6451

MAX6753

GND

N

RESET

RESET

5V

GND

3.3V

µP

100k

Ω

Figure 6. Interfacing to Other Voltage Levels

WDI

(a) FAST FAULT

(b) SLOW FAULT

WDI

RESET

(c) NORMAL OPERATION (NO PULSING, OUTPUT STAYS HIGH)

RESET

WDI

RESET