Full-Featured µP Supervisory Circuit with

±1.5% Reset Accuracy

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9

The RESET output is active low and implemented with a

strong pulldown/relatively weak pullup structure. It is

≥ 1. It typically

sinks 3.2mA at 0.1V saturation voltage in its active state.

The RESET output is the inverse of the RESET output; it

both sources and sinks current and cannot be wire-OR

Manual Reset Input

Many µP-based products require manual-reset capabil-

ity to allow an operator or test technician to initiate a

reset. The Manual Reset (MR) input permits the genera-

tion of a reset in response to a logic low from a switch,

WDO, or external circuitry. Reset remains asserted

while MR is low, and for 200ms after MR returns high.

MR has an internal 50µA to 200µA pullup current, so it

can be left open if it is not used. MR can be driven with

TTL or CMOS-logic levels, or with open-drain/collector

outputs. Connect a normally open momentary switch

from MR to GND to create a manual-reset function;

external debounce circuitry is not required. If MR is dri-

ven from long cables or if the device is used in a noisy

environment, connect a 0.1µF capacitor from MR to

ground to provide additional noise immunity. As shown

in Figure 3, diode-ORed connections can be used to

allow manual resets from multiple sources. Figure 4

shows the reset timing.

Watchdog Timer

Watchdog Input

The watchdog circuit monitors the µP’s activity. If the

µP does not toggle the watchdog input (WDI) within

1.6s, WDO goes low. The internal 1.6s timer is cleared

and WDO returns high when reset is asserted or when

a transition (low-to-high or high-to-low) occurs at WDI

while RESET is high. As long as reset is asserted, the

timer remains cleared and does not count. As soon as

reset is released, the timer starts counting (Figure 5).

Supply current is typically reduced by 10µA when WDI

is at a valid logic level.

(MAX801)

(MAX808)

MAX807

*

*

OTHER

RESET

SOURCES

MANUAL RESET

MR

*DIODES NOT REQUIRED ON OPEN-DRAIN OUTPUTS.

Figure 3. Diode “OR” Connections Allow Multiple Reset

Sources to Connect to MR