CAT823 Datasheet, PDF (8/15 Pages) Catalyst Semiconductor – System Supervisory Voltage Reset with Watchdog and Manual Reset

CAT823, CAT824, CAT825
WATCHDOG TIMER
The CAT823 and CAT824 provide a Watchdog input
(WDI). The watchdog timer function forces R¯¯E¯S¯E¯T¯
(and RESET in the CAT824) signals active when the
WDI input does not have a transition from low-to-high
or high-to-low within 1.12 seconds. Timeout of the
watchdog starts when R¯¯E¯S¯E¯T¯ (RESET on the
CAT824) becomes inactive. If a transition occurs on
the WDI input pin prior to the watchdog time-out, the
watchdog timer is reset and begins to time-out again.
If the watchdog timer is allowed to time-out, then the
reset output(s) will go active for t_RP and once released
will repeat the watchdog timeout process.
Figure 3 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 4). 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.
Figure 3. Watchdog Timer
WDI
Supply
Voltage
V_CC
CAT823
MR
GND
RESET
PIC
µC
MCLR
ADDRESS DECODER
Figure 4. Watchdog Disable Circuit
RESET
GND
V_CC
CAT823
V_CC
Tristate
150kΩ
MR
WDI
110kΩ
µC
OUTPUT
Doc. No. MD-3027 Rev. D
8
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Characteristics subject to change without notice