PIC876 datasheet(133/218 Pages) MICROCHIP

133 / 218 page

 2001 Microchip Technology Inc.

DS30292C-page 131

PIC16F87X

12.12 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.

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 (Watch-

dog Timer Wake-up). The TO bit in the STATUS regis-

ter will be cleared upon a Watchdog Timer time-out.

The WDT can be permanently disabled by clearing

configuration bit WDTE (Section 12.1).

WDT time-out period values may be found in the Elec-

trical Specifications section under parameter #31. Val-

ues for the WDT prescaler (actually a postscaler, but

shared with the Timer0 prescaler) may be assigned

using the OPTION_REG register.

FIGURE 12-10:

WATCHDOG TIMER BLOCK DIAGRAM

TABLE 12-7:

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 5-1)

To TMR0 (Figure 5-1)

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_REG register.

Address

Name

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

2007h

Config. bits

(1)

BODEN(1)

CP1

CP0

PWRTE(1)

WDTE

FOSC1

FOSC0

81h,181h

OPTION_REG

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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