Comtrol DM ATS-XPe - Manual (Page 80)

80 Appendix C. Default System Values
Changing the Watch-Dog Timer
Three I/O ports control the Watch-Dog Timer and are accessed using the
addresses defined in the following table.
Prior to enabling the Watch-Dog Timer, the user has to define the time interval to
be used. The timer interval is defined by writing a value to address 443H. This
value is within the range from 01 (hex) to FF (hex) and defines an interval
between 1 second to 255 seconds, respectively. The following table illustrates this
correlation:
The Watch-Dog Timer is activated by reading the value at address 443H. To
ensure that a reset condition does not occur, the timer must be periodically reset
to restart the countdown at the beginning of the defined interval before the time
out period has expired. This is achieved by first disabling the timer by reading
address 843H and then re-enabling it by reading the value at 443H before the
timer reaches zero. Refer to the example of the assembly program below.
A tolerance of at least 5% must be maintained to avoid unknown routines within
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.
Note: When exiting a program it is necessary to disable the Watch-Dog Timer,
otherwise the system will reset.
Example of Assembly Program
TIMER_PORT = 443H
TIMER_START = 443H
TIMER_STOP = 843H
;;INITIAL TIME PERIOD COUNTER
MOV DX, TIME_PORT
MOV AL, 8:;;8 SECONDS
OUT DX,AL
;;ADD YOUR APPLICATION HERE
MOV DX, TIMER_START
IN AL, DX.;;START COUNTER
;;ADD YOUR APPLICATION HERE
W_LOOP:
MOV DX, TIMER_STOP
IN AL, DX
Hex Address Read/Write Description
443H Write Set Watch-Dog Time period
443H Read Enable and refresh the Watch-Dog Timer.
843H Read Disable the Watch-Dog Timer.
Hex Value Time Interval
01 1 sec
02 2 sec
03 3 sec
04 4 sec
.
.
. .
.
.
FF 255 sec