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1996 Microchip Technology Inc.

DS30412C-page 103

PIC17C4X

14.3

Watchdog Timer (WDT)

The Watchdog Timer’s function is to recover from soft-

ware malfunction. The WDT uses an internal free run-

ning on-chip RC oscillator for its clock source. This

does not require any external components. This RC

oscillator is separate from the RC oscillator of the

OSC1/CLKIN pin. That means that the WDT will run,

even if the clock on the OSC1/CLKIN and OSC2/CLK-

OUT pins of the device has been stopped, for example,

by execution of a SLEEP instruction. During normal

operation and SLEEP mode, a WDT time-out gener-

ates a device RESET. The WDT can be permanently

disabled

programming

the

conguration

bits

WDTPS1:WDTPS0 as '00' (Section 14.1).

Under normal operation, the WDT must be cleared on

a regular interval. This time is less the minimum WDT

overow time. Not clearing the WDT in this time frame

will cause the WDT to overow and reset the device.

14.3.1

WDT PERIOD

The WDT has a nominal time-out period of 12 ms, (with

postscaler = 1). The time-out periods vary with temper-

ature, VDD and process variations from part to part (see

DC specs). If longer time-out periods are desired, a

postscaler with a division ratio of up to 1:256 can be

assigned to the WDT. Thus, typical time-out periods up

to 3.0 seconds can be realized.

The CLRWDT and SLEEP instructions clear the WDT

and the postscaler (if assigned to the WDT) and pre-

vent it from timing out thus generating a device RESET

condition.

The TO bit in the CPUSTA register will be cleared upon

a WDT time-out.

14.3.2

CLEARING THE WDT AND POSTSCALER

The WDT and postscaler are cleared when:

The device is in the reset state

A SLEEP instruction is executed

A CLRWDT instruction is executed

Wake-up from SLEEP by an interrupt

The WDT counter/postscaler will start counting on the

rst edge after the device exits the reset state.

14.3.3

WDT PROGRAMMING CONSIDERATIONS

It should also be taken in account that under worst case

conditions (VDD = Min., Temperature = Max., max.

WDT postscaler) it may take several seconds before a

WDT time-out occurs.

The WDT and postscaler is the Power-up Timer during

the Power-on Reset sequence.

14.3.4

WDT AS NORMAL TIMER

When the WDT is selected as a normal timer, the clock

source is the device clock. Neither the WDT nor the

postscaler are directly readable or writable. The over-

ow time is 65536 TOSC cycles. On overow, the TO bit

is cleared (device is not reset). The CLRWDT instruction

can be used to set the TO bit. This allows the WDT to

be a simple overow timer. When in sleep, the WDT

does not increment.

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