STM809/810/811/812

6/18

OPERATION

Reset Output

The STM809/810/811/812 MICROPROCESSOR

RESET CIRCUIT asserts a reset signal to the

(MR) is taken low (see Figure 15., page 11). RST

(active high for STM810/812) is guaranteed valid

threshold an internal timer keeps RST low for the

returns high.

low. Each time RST is asserted, it stays low for at

below the reset threshold, the internal timer clears.

reset threshold. The active-low reset (RST) and

active-high reset (RST) both source and sink cur-

rent.

Push-Button Reset Input (STM811/812)

A logic low on MR asserts RST. RST remains as-

high. The MR input has an internal 20k

Ω pull-up

resistor, allowing it to be left open if not used. This

input can be driven with TTL/CMOS-logic levels or

with open-drain/collector outputs. Connect a nor-

mally open push-button switch from MR to GND to

create a manual reset function; external debounce

circuitry is not required. If the device is used in a

noisy environment, connect a 0.1µF capacitor

from MR to GND to provide additional noise immu-

nity.

The STM809/810/811/812 are relatively immune

13., page 9 shows typical transient duration ver-

sus reset comparator overdrive (for which the

STM809/810/811/812 will NOT generate a reset

pulse). The graph was generated using a negative

actual reset threshold and ending below it by the

magnitude indicated (comparator overdrive). The

graph indicates the maximum pulse width a nega-

set pulse. As the magnitude of the transient

increases (further below the threshold), the maxi-

mum allowable pulse width decreases. Any com-

bination of duration and overdrive which lies under

the curve will NOT generate a reset signal. Typi-

reset threshold and lasts 20µs or less will not

cause a reset pulse. A 0.1µF bypass capacitor

vides additional transient immunity.

output no longer sinks current, but becomes an

open circuit. In most systems this is not a problem,

as most MCUs do not operate below 1V. However,

in applications where RST output must be valid

down to 0V, a pull-down resistor may be added to

hold the RST output low. This resistor must be

large enough to not load the RST output, and still

be small enough to pull the output to ground. A

100K

Ω resistor is recommended.

Note: The same situation applies for the active-

high RST of the STM810/812. A 100K

Ω pull-up re-