September 2002

Specifications subject to change without notice,contact your sales representatives for the most recent information.

11/25

Ver 1.2

SM89516A 09/02

SyncMOS Technologies Inc.

SM89516A

3.Watch Dog Timer

The Watch Dog Timer (WDT) is a 16-bit free-running counter that generate reset signal if the counter overflows. The WDT is

useful for systems which are susceptible to noise, power glitches, or electronics discharge which causing software dead

loop or runaway. The WDT function can help user software recover from abnormal software condition. The WDT is different

from Timer0, Timer1 and Timer2 of general 8052. To prevent a WDT reset can be done by software periodically clearing the

WDT counter. User should check WDR bit of SCONF register whenever un-predicted reset happened

The purpose of the secure procedure is to prevent the WDTC value from being changed when system runaway.

There is a 250KHz RC oscillator embedded in chip. Set WDTE = “1” will enable the RC oscillator and the frequency is inde-

pendent to the system frequency.

To enable the WDT is done by setting 1 to the bit 7 (WDTE) of WDTC. After WDTE set to 1, the 16-bit counter starts to count

with the RC oscillator. It will generate a reset signal when overflows. The WDTE bit will be cleared to 0 automatically when

SM59264 been reset, either hardware reset or WDT reset.

To reset the WDT is done by setting 1 to the CLEAR bit of WDTC before the counter overflow. This will clear the content of

the 16-bit counter and let the counter re-start to count from the beginning.

3.1 Watch Dog Timer Registers:

Watch Dog Timer Registers - WDT Control Register (WDTC, $9F)

WDTE : Watch Dog Timer enable bit

CLEAR : Watch Dog Timer reset bit

PS[2:0] : Overflow period select bits

bit-7

bit-0

WDTE

Reserve

CLEAR

Unused

Unused

PS2

PS1

PS0

Read / Write:

R/W

-

R/W

-

-

R/W

R/W

R/W

Reset value:

0

*

0

*

*

0

0

0

PS [2:0]

Overflow Period (ms)

000

2.048

001

4.096

010

8.192

011

16.384

100

32.768

101

65.536

110

131.072

111

262.144