CAT705 Datasheet, PDF (8/14 Pages) Catalyst Semiconductor – μP Supervisory Circuits

CAT705, CAT706, CAT813
WATCHDOG TIMER
The CAT705, CAT706, & CAT813 provide a
Watchdog input (WDI). The watchdog timer function
controls the watchdog output (W¯¯D¯O¯ ) signal and forces
the W¯¯D¯O¯ to be low (active) when the WDI input does
not have a transition from low-to-high or high-to-low
within 1.6s typical. If a transition occurs on the WDI
input pin prior to the watchdog time-out, the watchdog
timer is restarted. The timing diagram is shown in
Figure 3. The watchdog timer starts as soon as reset
condition becomes inactive.
When the V_CC supply drops below the reset threshold,
the W¯¯D¯O¯ output becomes active and goes low
independently of the watchdog timing stage.
Figure 4 below shows a typical implementation of a
watchdog function. Any processor signal that repeats
dependant on the normal operation of the processor
or directed by the software operating on the processor
can be used to strobe the watchdog input. The most
reliable is a dedicated I/O output transitioned by a
specific software instruction.
The watchdog can be disabled by floating (or tri-
stating) the WDI input (see Figure 5). If the watchdog
is disabled the WDI pin will be pulled low for the first
7/8^th’s of the watchdog period (t_WD) and pulled high for
the last 1/8^th of the watchdog period. This pulling low
of the WDI input and then high is used to detect an
open or tri-state condition and will continue to repeat
until the WDI input is driven high or low.
For most efficient operation of devices with the
watchdog function the WDI input should be held low
the majority of the time and only strobed high as
required to reset the watchdog timer.
tWP
tWD
tWD
+5 V
WDI
0V
+5 V
WDO _{0 V}
+5 V
RESET
0V
+5 V
(RESET)
0V
RESET EXTERNALLY
TRIGGERED BY MR
(() Are for CAT813 Only)
Figure 3. Watchdog Timing Diagram
tWD
tRS
MR
WDO
V_CC
GND
CAT705
CAT706
CAT813
RESET/RESET
WDI
PFI
PFO
PIC
µC
MCLR
ADDRESS DECODER
Figure 4. Watchdog Timer Circuit
Doc. No. MD-3030 Rev. D
8
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Characteristics subject to change without notice