Disable the watchdog function by leaving WDI uncon-

nected or by three-stating the driver connected to WDI.

The watchdog input is internally driven low during the

first 7/8 of the watchdog timeout period and high for the

last 1/8 of the watchdog timeout period. When WDI is left

unconnected, this internal driver clears the 1.6s timer

every 1.4s. When WDI is three-stated or unconnected,

the maximum allowable leakage current is 10µA and the

maximum allowable load capacitance is 200pF.

Applications Information

Watchdog Input Current

The MAX6821/MAX6822/MAX6823/MAX6824 WDI inputs

are internally driven through a buffer and series resistor

unconnected, the watchdog timer is serviced within the

watchdog timeout period by a low-high-low pulse from

the counter chain. For minimum watchdog input current

(minimum overall power consumption), leave WDI low for

the majority of the watchdog timeout period, pulsing it

low-high-low once within the first 7/8 of the watchdog

timeout period to reset the watchdog timer. If WDI is

externally driven high for the majority of the timeout peri-

od, up to 160µA can flow into WDI.

Interfacing to µPs with

Bidirectional Reset Pins

Since the RESET output on the MAX6822 is open drain,

it interfaces easily with µPs that have bidirectional reset

pins, such as the Motorola 68HC11. Connecting the µP

supervisor’s RESET output directly to the microcon-

troller’s RESET pin with a single pullup resistor allows

either device to assert reset (see Figure 4).

These supervisors are relatively immune to short-dura-

usually do not require the entire system to shut down.

Low-Voltage SOT23 µP Supervisors with

Manual Reset and Watchdog Timer

6

_______________________________________________________________________________________

Figure 1. Functional Diagram

Figure 2. Reset Timing Diagram

1V

1V

RESET

GND

RESET

MAX6821–

MAX6825

WDI

(MAX6821– MAX6824)

1.26V

GND

RESET

(MAX6821/MAX6824/ MAX6825)

RESET (MAX6822

MR

(MAX6821/MAX6822/MAX6823,

MAX6825)

RESET

GENERATOR

WATCHDOG

TIMER

WATCHDOG

TRANSITION

DETECTOR