
FM811/812
4
REV. 1.5.1 7/15/02
P
Circuit Description
Microprocessor Reset
The RESET pin is asserted whenever V
reset threshold voltage or if MR (manual reset) is forced low.
The reset pin remains asserted for a period of 40ms after
V
CC
has risen above the reset threshold voltage or MR has
returned high. The reset function ensures the microprocessor
is properly reset and powers up into a known condition after
a power failure. RESET will remain valid with V
as 1.4V.
CC
falls below the
CC
as low
V
CC
Transients
The FM811/ FM812 are relatively immune to negative-going
V
CC
glitches below the reset threshold. Typically, a negative-
going transient 125mV below the reset threshold with a
duration of 50ms (25ms for FM81_R/S/T) or less will not
cause an unwanted reset.
Manual Reset Function
Momentarily connecting the Manual Reset input pin (MR) to
GND forces a reset. Typically a Push-button switch is used
in most of the applications to force a manual reset. See
Typical Applications diagram. The reset will remain asserted
as long as MR is held low and for one reset time-out period
(140ms min.) after MR goes high. Note that this input cannot
be driven by a CMOS or TTL logic. This pin should be left
floating if manual reset feature is unused.
Interfacing to Bidirectional Reset Pins
The FM811/ FM812 can interface with μPs with
bidirectional reset pins by connecting a 4.7k
series with the FM811/ FM812 output and the μP reset pin.
resistor in
RESET Valid to 0V
A resistor can be added from the RESET pin to ground to
ensure the RESET output remains low with V
A 100k
resistor connected from RESET to ground is
recommended. The size of the resistor should be large
enough to not load the RESET output and small enough to
pull-down any stray leakage currents.
CC
down to 0V.
V
TH
V
CC
tR
MR
RESET
tR
FM811
μ
P
100k
V
CC
V
CC
RESET
RESET
V
CC