ICP Electronics WAFER-5821, Руководство пользователя

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This will enable and activate the countdown timer
which will eventually time out and reset the CPU to
ensure that this reset condition does not occur, the
Watch-Dog Timer must be periodically refreshed by
reading the same I/O port 043/843H and 443H. This
must be done within the time out period that is
selected by software, please refer to the example
program.
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 Watchdog Timer, otherwise the system
will reset.
Example 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