CAT823, CAT824, CAT825

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6

Watchdog Timer

The CAT823 and CAT824 provide a Watchdog input

(WDI). The watchdog timer function forces RESET (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

RESET (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

repeat the watchdog timeout process.

Figure 6 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 7). If the watchdog is disabled the

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 6. Watchdog Timer

CAT823

PIC

μC

DECODER

ADDRESS

GND

WDI

Supply

Voltage

RESET

MR

MCLR

Figure 7. Watchdog Disable Circuit

mC

OUTPUT

Tristate

150 kW

CAT823

GND

WDI

MR

RESET

110 kW

Figure 8. Timing Diagram – Strobe Input

WDI

INVALID

STROBE

VALID

STROBE

MIN.

MAX.

INDETERMINATE

STROBE

RESET