PIC16F818-ESLASHML datasheet(100/176 Pages) MICROCHIP

100 / 176 page

PIC16F818/819

DS39598E-page 98

 2004 Microchip Technology Inc.

12.12 Watchdog Timer (WDT)

For PIC16F818/819 devices, the WDT is driven by the

INTRC oscillator. When the WDT is enabled, the

INTRC (31.25 kHz) oscillator is enabled. The nominal

WDT period is 16 ms and has the same accuracy as

the INTRC oscillator.

During 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 (Watchdog

Timer wake-up). The TO bit in the Status register will be

cleared upon a Watchdog Timer time-out.

The WDT can be permanently disabled by clearing con-

figuration bit, WDTEN (see Section 12.1 “Configuration

Bits”).

WDT time-out period values may be found in

Section 15.0

“Electrical

Characteristics”

under

parameter #31. Values for the WDT prescaler (actually

a postscaler but shared with the Timer0 prescaler) may

be assigned using the OPTION_REG register.

FIGURE 12-8:

WATCHDOG TIMER BLOCK DIAGRAM

TABLE 12-5:

SUMMARY OF WATCHDOG TIMER REGISTERS

Note 1: The CLRWDT and SLEEP instructions

clear the WDT and the postscaler if

assigned to the WDT and prevent it from

timing out and generating a device Reset

condition.

2: 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 6-1)

To TMR0 (Figure 6-1)

Postscaler

INTRC

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_REG register.

31.25 kHz

Address

Name

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

81h,181h OPTION_REG

RBPU

INTEDG

T0CS

T0SE

PSA

PS2

PS1

PS0

2007h

Configuration bits(1)

LVP

BOREN

MCLRE

FOSC2

PWRTEN

WDTEN

FOSC1

FOSC0

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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