W78LE812

- 10 -

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

−0 as follows:

PS2 PS1 PS0

PRESCALER SELECT

0

0

0

2

0

0

1

4

0

1

0

8

0

1

1

16

1

0

0

32

1

0

1

64

1

1

0

128

1

1

1

256

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

1

2

1000 12

14

OSC

PRESCALER

×

×

×

×

mS

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.

OSC

1/12

PRESCALER

14-BIT TIMER

CLEAR

CLRW

EXTERNAL

RESET

INTERNAL

RESET

WIDL

IDLE

ENW

Watchdog Timer Block Diagram

Typical Watch-Dog time-out period when OSC = 20 MHz

PS2 PS1 PS0

WATCHDOG TIME-OUT PERIOD

0

0

0

19.66 mS

0

0

1

39.32 mS

0

1

0

78.64 mS

0

1

1

157.28 mS

1

0

0

314.57 mS

1

0

1

629.14 mS

1

1

0

1.25 s

1

1

1

2.50 s