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will perform hardware reset (cold boot) to bring the system back to a
known state.
Three I/O ports control the operation of WatchDog Timer.
443 (hex)
Write
Set WatchDog Time period
443 (hex)
Read
Enable the refresh the Watchdog Timer.
043/843 (hex) Read
Disable the Watchdog Timer.
Prior to enable the Watchdog Timer, user has to set the time-out
period. The resolution of the timer is 1 second and the range of the
timer is from 1 sec to 255 sec. You need to send the time-out value to
the I/O port – 443H, and then enable it by reading data from the same
I/O port – 443H. This will activate the timer that will eventually time
out and reset the CPU board. To ensure that this reset condition won’t
occur, the Watchdog Timer must be periodically refreshed by reading
the same I/O port 443H. This must be done within the time-out period
that is set by the software, please refer to the example program.
Finally, we have to disable the Watchdog timer by reading the I/O port
-- 843H or 043H. Otherwise the system could reset unconditionally.
A tolerance of at least 5% must be maintained to avoid unknown
routines in the operating system (DOS), such as disk I/O that can be
very time-consuming. Therefore if the time-out period has been set to
10 seconds, the I/O port 443H must be read within 7 seconds
.
Example assembly program:
TIMER_PORT = 443H
TIMER_START = 443H
TIMER_STOP = 843H
;;Initial Timer Counter
MOV DX, TIMER_PORT
MOV AL, 8
;;8 seconds
OUT DX, AL
MOV DX, TIMER_START
IN AL, DX
.
;;Start counter
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W_LOOP:
MOV DX, TIMER_STOP
IN AL, DX
MOV DX, TIMER_START
IN AL, DX
;;Restart counter
;;Add Your Appliaction Here
CMP EXIT_AP, 0
JNE W_LOOP
MOV DX, TIMER_STOP
IN AL, DX
;;Exit AP
