PIC16F871-E/L【PDF 4/143 页】IC MCU CMOS 20MHZ 2K FLSH 44PLCC

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2003 Microchip Technology Inc.

DS30569B-page 99

PIC16F870/871

11.12 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/CLKI pin. That means that the WDT will run,

even if the clock on the OSC1/CLKI and OSC2/CLKO

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

(Watchdog Timer Wake-up). The TO bit in the STATUS

time-out.

The WDT can be permanently disabled by clearing

configuration bit WDTEN (Section 11.1).

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 11-10:

WATCHDOG TIMER BLOCK DIAGRAM

TABLE 11-7:

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)

BOREN(1)

CP1

CP0

PWRTEN(1) WDTEN

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 11-1 for operation of these bits.

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