5-Pin Microprocessor Supervisory Circuits With

Watchdog Timer and Manual Reset

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7

_______________Detailed Description

RESET Output

A microprocessor’s (µP’s) reset input starts the µP in a

known state. The MAX823/MAX824/MAX825 µP super-

visory circuits assert a reset to prevent code-execution

errors during power-up, power-down, and brownout

conditions. RESET is guaranteed to be a logic low for

old, an internal timer keeps RESET low for the specified

returns high (Figure 2).

reset threshold), RESET goes low. Each time RESET is

asserted it stays low for the reset timeout period. Any

timer restarts. RESET both sources and sinks current.

RESET on the MAX824/MAX825 is the inverse of

RESET.

Manual Reset Input (MAX823/MAX825)

Many µP-based products require manual reset capabili-

ty, allowing the operator, a test technician, or external

logic circuitry to initiate a reset. On the MAX823/

MAX825, a logic low on MR asserts reset. Reset remains

after it returns high. MR has an internal 52k

Ω pullup

resistor, so it can be left open if not used. This input can

be driven with CMOS logic levels or with open-drain/

collector outputs. Connect a normally open momentary

switch from MR to GND to create a manual-reset func-

tion; external debounce circuitry is not required. If MR is

driven from long cables or the device is used in a noisy

environment, connect a 0.1µF capacitor from MR to

GND to provide additional noise immunity.

Watchdog Input (MAX823/MAX824)

In the MAX823/MAX824, the watchdog circuit monitors

the µP’s activity. If the µP does not toggle the watchdog

1.6s timer is cleared by either a reset pulse or by tog-

gling WDI, which detects pulses as short as 50ns.

While reset is asserted, the timer remains cleared and

does not count. As soon as reset is released, the timer

starts counting (Figure 3).

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 uncon-

nected, the maximum allowable leakage current is

10µA and the maximum allowable load capacitance is

200pF.

Applications Information

Watchdog Input Current

The MAX823/MAX824 WDI inputs are internally driven

through a buffer and series resistor from the watchdog

counter (Figure 1). When WDI is left 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 period, up to 160µA

can flow into WDI.

1V

1V

RESET

GND

RESET

Figure 2. Reset Timing Diagram

WDI

RESET*

*RESET ON THE MAX824/MAX825 IS THE INVERSE OF RESET.

Figure 3. MAX823/MAX824 Watchdog Timing Relationship