PIC16C925T-SSLASHCL datasheet(112/182 Pages) MICROCHIP

112 / 182 page

PIC16C925/926

DS39544A-page 110

Preliminary

 2001 Microchip Technology Inc.

12.7

Watchdog Timer (WDT)

The Watchdog Timer is a free running on-chip RC oscil-

lator, which 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/CLKOUT pins of the device has been stopped,

for example, by execution of a SLEEP instruction. Dur-

ing normal operation, a WDT time-out generates a

device RESET (Watchdog Timer Reset). If the device is

in SLEEP mode, a WDT time-out causes the device to

wake-up and continue with normal operation (Watch-

dog Timer Wake-up). The WDT can be permanently

disabled

clearing

configuration

bit

WDTE

(Section 12.1).

12.7.1

WDT PERIOD

The WDT has a nominal time-out period of 18 ms (with

no prescaler). The time-out periods vary with tempera-

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

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

prescaler with a division ratio of up to 1:128 can be

assigned to the WDT under software control, by writing

to the OPTION register. Thus, time-out periods up to

2.3 seconds can be realized.

The CLRWDT and SLEEP instructions clear the WDT

and the postscaler, if assigned to the WDT, prevent it

from timing out and generating a device RESET

condition.

The TO bit in the STATUS register will be cleared upon

a Watchdog Timer time-out.

12.7.2

WDT PROGRAMMING

CONSIDERATIONS

It should also be taken into account that under worst

case conditions (VDD = Min., Temperature = Max., and

Max. WDT prescaler) it may take several seconds

before a WDT time-out occurs.

FIGURE 12-12:

WATCHDOG TIMER BLOCK DIAGRAM

FIGURE 12-13:

SUMMARY OF WATCHDOG TIMER REGISTERS

Note:

When a CLRWDT instruction is executed

and the prescaler is assigned to the WDT,

the prescaler count will be cleared, but the

prescaler assignment is not changed.

From TMR0 Clock Source

(Figure 5-6)

To TMR0 (Figure 5-6)

Postscaler

WDT Timer

WDT

Enable bit

PSA

8 - to - 1 MUX

PS2:PS0

MUX

PSA

WDT

Time-out

Note: PSA and PS2:PS0 are bits in the OPTION register.

Address

Name

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

2007h

Config. bits

(1)

BOREN(1)

CP1

CP0

PWRTE(1)

WDTE

FOSC1

FOSC0

81h, 181h

OPTION

RBPU

INTEDG

T0CS

T0SE

PSA

PS2

PS1

PS0

Legend: Shaded cells are not used by the Watchdog Timer.

Note 1: See Register 12-1 for operation of these bits.

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