R1 and R2 can have very high values to minimize cur-

rent consumption due to low leakage currents. Set R2

to some conveniently high value (500k

Ω, for example)

and calculate R1 based on the desired reset threshold

voltage, using the following formula:

Ω)

guarantee RESET IN threshold accuracy and timing

performance. The MAX6748 and MAX6749 can be con-

Dual-Voltage Monitoring

(MAX6750/MAX6751)

The MAX6750 and MAX6751 contain both factory-

trimmed threshold voltages and an adjustable reset

threshold input, allowing the monitoring of two voltages,

when either of the voltages falls below it respective

threshold voltages.

Manual Reset (MAX6746/MAX6747)

Many µP-based products require manual reset capabil-

ity, to allow an operator or external logic circuitry to initi-

ate a reset. The manual reset input (MR) can connect

directly to a switch without an external pullup resistor or

and, therefore, can be left unconnected if unused.

MR is designed to reject fast, falling transients (typically

100ns pulses) and it must be held low for a minimum of

1µs to assert the reset output. A 0.1µF capacitor from

MR to ground provides additional noise immunity. After

MR transitions from low to high, reset remains asserted

for the duration of the reset timeout period.

A manual reset option can easily be implemented

with the MAX6748–MAX6751 by connecting a normally

open momentary switch in parallel with R2 (Figure 3).

When the switch is closed, the voltage on RESET IN

goes to zero, initiating a reset. Similar to the MAX6746/

MAX6747 manual reset, reset remains asserted while

the voltage at RESET IN is zero and for the reset time-

out period after the switch is opened.

µP Reset Circuits with Capacitor-Adjustable

Reset/Watchdog Timeout Delay

8

_______________________________________________________________________________________

WDI

RESET

NORMAL MODE (WDS = GND)

OV

OV

Figure 4a. Watchdog Timing Diagram, WDS = GND

MAX6748

MAX6749

MAX6750

MAX6751

RESET IN

GND

R1

R2

Figure 3. Adding an External Manual Reset Function to the

MAX6748–MAX6751