W78L812A24FL(2006) Просмотр технического описания (PDF) - Winbond

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W78L812A24FL
(Rev.:2006)

8-BIT MICROCONTROLLER

Winbond 

W78LE812/W78L812A

5.6 Watchdog Timer

The Watchdog timer is a free-running timer which can be programmed by the user to serve as a

system monitor, a time-base generator or an event timer. It is basically a set of dividers that divide the

system clock. The divider output is selectable and determines the time-out interval. When the time-out

occurs, a system reset can also be caused if it is enabled. The main use of the Watchdog timer is as a

system monitor. This is important in real-time control applications. In case of power glitches or electro-

magnetic interference, the processor may begin to execute errant code. If this is left unchecked the

entire system may crash. The watchdog time-out selection will result in different time-out values

depending on the clock speed. The Watchdog timer will de disabled on reset. In general, software

should restart the Watchdog timer to put it into a known state. The control bits that support the

Watchdog timer are discussed below.

5.6.1 Watchdog Timer Control Register

Bit:

7

6

5

4

ENW CLRW WIDL

-

3

2

1

0

-

PS2 PS1 PS0

Mnemonic: WDTC

Address: 8FH

ENW : Enable watch-dog if set.

CLRW : Clear watch-dog timer and prescaler if set. This flag will be cleared automatically

WIDL : If this bit is set, watch-dog is enabled under IDLE mode. If cleared, watch-dog is disabled

under IDLE mode. Default is cleared.

PS2, PS1, PS0: Watch-dog prescaler timer select. Prescaler is selected when set PS2 − 0 as follows:

PS2 PS1 PS0

PRESCALER SELECT

00 0

2

01 0

4

00 1

8

01 1

16

10 0

32

10 1

64

11 0

128

11 1

256

The time-out period is obtained using the following equation:

1 × 214 × PRESCALER × 1000 × 12 mS

OSC

Before Watchdog time-out occurs, the program must clear the 14-bit timer by writing 1 to WDTC.6

(CLRW). After 1 is written to this bit, the 14-bit timer, prescaler and this bit will be reset on the next

instruction cycle. The Watchdog timer is cleared on reset.

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