Datasheet

STM809/810/811/812
6/18
OPERATION
Reset Output
The STM809/810/811/812 MICROPROCESSOR
RESET CIRCUIT asserts a reset signal to the
MCU whenever V
CC
goes below the reset thresh-
old (V
RST
), or when the push-button reset input
(MR
) is taken low (see Figure 15., page 11). RST
(active high for STM810/812) is guaranteed valid
down to V
CC
=1V (0° to 70°C).
During power-up, once V
CC
exceeds the reset
threshold an internal timer keeps RST
low for the
reset time-out period, t
rec
. After this interval, RST
returns high.
If V
CC
drops below the reset threshold, RST goes
low. Each time RST
is asserted, it stays low for at
least the reset time-out period. Any time V
CC
goes
below the reset threshold, the internal timer clears.
The reset timer starts when V
CC
returns above the
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-
serted while MR
is low, and for t
rec
after it returns
high. The MR
input has an internal 20kΩ 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.
Negative-Going V
CC
Transients
The STM809/810/811/812 are relatively immune
to negative-going V
CC
transients (glitches). Figure
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
pulse applied to V
CC
, starting at 0.5V above the
actual reset threshold and ending below it by the
magnitude indicated (comparator overdrive). The
graph indicates the maximum pulse width a nega-
tive V
CC
transient can have without causing a re-
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-
cally, a V
CC
transient that goes 100mV below the
reset threshold and lasts 20µs or less will not
cause a reset pulse. A 0.1µF bypass capacitor
mounted as close as possible to the V
CC
pin pro-
vides additional transient immunity.
Valid /RST Output Down to V
CC
= 0V
When V
CC
falls below 1V, the RST (STM809/811)
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
100KΩ resistor is recommended.
Note: The same situation applies for the active-
high RST of the STM810/812. A 100KΩ pull-up re-
sistor to V
CC
should be used if RST must remain
valid for V
CC
< 1.0V.