PIC12C67X datasheet(65/129 Pages) MICROCHIP

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DS30561B-page 65

PIC12C67X

9.7

Watchdog Timer (WDT)

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

oscillator, which does not require any external compo-

nents. This RC oscillator is separate from the RC oscil-

lator 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 9.1).

9.7.1

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, and prevent

it from timing out early and generating a premature

device RESET condition.

The TO bit in the STATUS register will be cleared upon

a Watchdog Timer time-out.

9.7.2

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.

See Example 7-1 and Example 7-2 for changing pres-

caler between WDT and Timer0.

Note:

When the prescaler is assigned to the

WDT, always execute a CLRWDT instruction

before changing the prescale value, other-

wise a WDT reset may occur.

FIGURE 9-15: WATCHDOG TIMER BLOCK DIAGRAM

TABLE 9-8:

SUMMARY OF WATCHDOG TIMER REGISTERS

Address

Name

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

2007h

Config. bits(1)

MCLRE

CP1

CP0

PWRTE

WDTE

FOSC2

FOSC1

FOSC0

81h

OPTION

GPPU

INTEDG

T0CS

T0SE

PSA

PS2

PS1

PS0

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

Note 1:

See Register 9-1 for operation of these bits. Not all CP0 and CP1 bits are shown.

From TMR0 Clock Source

(Figure 7-5)

To TMR0 (Figure 7-5)

Postscaler

WDT Timer

WDT

Enable Bit

PSA

8 - to - 1 MUX

PS<2:0>

MUX

PSA

WDT

Time-out

Note: PSA and PS<2:0> are bits in the OPTION register.

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