Datasheet

2002-2012 Microchip Technology Inc. Preliminary DS21370D-page 9
TC1232
4.3 Watchdog Timer
When the ST input is not stimulated for a preset time
period, the watchdog timer function forces RST and
RST
signals to the active state. The preset time period
is determined by the TD
inputs to be 150 ms with TD
connected to ground, 600 ms with TD floating or
1200 ms with TD connected to V
CC
(typ.). The
watchdog timer starts timing-out from the set time
period as soon as RST and RST are inactive. If a high-
to-low transition occurs on the ST
input pin prior to
time-out, the watchdog timer is reset and begins to
time-out again. If the watchdog timer is allowed to time-
out, the RST and RST
signals are driven to the active
state for 250 ms, minimum (Figure 4-7).
The software routine that strobes ST is critical. The
code must be in a section of software that is executed
frequently enough so the time between toggles is less
than the watchdog time-out period. One common
technique controls the microprocessor I/O line from two
sections of the program. The software might set the I/O
line high while operating in the Foreground mode and
set it low while in the Background or Interrupt modes. If
both modes do not execute correctly, the watchdog
timer issues reset pulses.
t
TD
is the maximum elapsed time between ST high-to-
low transitions (ST
is activated by falling edges only),
which will keep the watchdog timer from forcing the
reset outputs active for a time of t
RST
. t
TD
is a function
of the voltage at the TD pin, as tabulated below:
TABLE 4-1: WATCHDOG TIMER
PERIODS
Figure 4-7 shows a block diagram for using the
TC1232 with a PICmicro
®
MCU and the Watchdog
input.
FIGURE 4-7: Watchdog Timer.
Figure 4-8 shows the expected reset output pin wave-
forms depending on the period of the ST
pin falling
edge and the state of the TD input pin.
FIGURE 4-8: Strobe Input.
4.4 Supply Monitor Noise Sensitivity
The TC1232 is optimized for fast response to negative-
going changes in V
DD
. Systems with an inordinate
amount of electrical noise on V
DD
(such as systems
using relays) may require a 0.01 µF or 0.1 µF bypass
capacitor to reduce detection sensitivity. This capacitor
should be installed as close to the TC1232 as possible
to keep the capacitor lead length short.
Condition
t
TD
Min Typ Max
TD pin = 0V 62.5 ms 150 ms 250 ms
TD pin = Open 250 ms 600 ms 1000 ms
TD pin = VCC 500 ms 1200 ms 2000 ms
V
CC
TOL GND
RST
ST
RESET
+5V
TC1232
TD
0.1
10 k
I/O
PICmicro
®
3-Terminal
Regulator
+5V
µF
MCU
(example:
MCP1700)
t
ST
ST
t
TD
RST (when t
TD
t
TD
(min))
“H”
RST
(when t
TD
(min) t
TD
t
TD
(max))
(when t
TD
t
TD
(max))
RST
RST (when t
TD
t
TD
(min))
“L”
RST
(when t
TD
(min) t
TD
t
TD
(max))
(when t
TD
t
TD
(max))
RST