2001-2012 Microchip Technology Inc. DS21402D-page 5
TC32M
3.0 DETAILED DESCRIPTION
The TC32M provides three important functions to
safeguard stable processor operation: precision pro-
cessor monitor, Watchdog sanity timer and external
override Reset control.
3.1 Processor Monitor
The RS pin is immediately driven low any time V
DD
is
below the nominal threshold voltage. As a result, this
pin is LOW when power is initially applied, holding the
processor in its Reset state. RS
remains low for a
minimum of 500 msec after V
DD
is within tolerance to
allow the power supply and processor to stabilize.
3.2 Watchdog Timer
The processor drives the RS pin with an input/output
(I/O) line in series with an resistor voltage divider to
V
DD.
Pulling the bottom resistor of this divider low
results in an internal voltage change (strobe) sufficient
to reset the Watchdog Timer, but above the V
IL
input
threshold of the processor Reset
pin. The processor
must continuously apply strobes in this manner within
a set period to verify proper software execution. A
momentary Reset (500 msec minimum) is generated
by the TC32M if a hardware or software failure keeps
RS
from being strobed within the Watchdog Time-out
period. This action typically initiates the processor’s
power-up routine. If the interruption persists, new
Reset pulses are generated each time-out period until
RS is strobed. This time-out period is typically 700
msec.
FIGURE 3-1: Watchdog Strobe
The software routine that drives the RS strobe must be
in a section of the program that executes frequently
enough so the time between toggles is less than one
Watchdog Time-out period. The strobe signal can be
derived from microprocessor address, data and/or
control signals. Typical circuit examples are shown in
Figure .
3.3 Resistor Value Selection
The values of R1 and R2 must be chosen to ensure a
valid low strobe level (V
STL
) on RS when the processor
I/O line is low. The use of 10 k, ±5% tolerance
resistors are recommended. These values result in a
nominal strobe level of 2.5 on RS
(min/max of 2.13V/
3.08V, assuming V
DD
= 5.0V ±10%). Other resistor
values can be used, so long as the additive tolerances
of the power supply and resistor values result in a
strobe that falls within V
STH
and V
STL
under all additive
tolerance conditions.
V
ST
L
(Max)
t
ST
t
TD
RS
Note: t
TD
is the maximum elapsed time between strobes which
will keep the watchdog timer from forcing RS LOW.
(A Strobe is defined as a high-to-low transition from V
STH
to V
STL
).
V
STH
(Min)
V
STL
(Min)
