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Appendix A Watchdog Timer
The Watchdog Timer is provided to ensure that standalone systems can always
recover from catastrophic conditions that cause the CPU to crash. This condition
may have occurred by external EMI or a software bug. When the CPU stops
working correctly, hardware on the board will either perform a hardware reset
(cold boot) or a Non-Maskable Interrupt (NMI) to bring the system back to a
known state.
A BIOS function call (INT 15H) is used to control the Watchdog Timer:
INT 15H:
AH – 6FH
Sub-function:
AL – 2 : Set the Watchdog Timer’s period
BL : Time-out value(Its unit--second or minute, is
dependent on the item “Watchdog Timer unit
select” in CMOS setup).
The sub-function 2 needs to be called to set the time-out period of Watchdog
Timer first. If the time-out value is not zero, the Watchdog Timer will start to
count down. While the timer value reaches zero, the system will reset. To
ensure that this reset condition does not occur, the Watchdog Timer must be
periodically refreshed by calling sub-function 2. However the Watchdog timer will
be disabled if the user set the time-out value to zero.
A tolerance of at least 10% must be maintained to avoid unknown
routines within the operating system (DOS), such as disk I/O that can be
very time-consuming.
Note: when exiting a program, it is necessary to disable the Watchdog Timer;
otherwise the system will reset.
Example program:
; INITIAL TIMER PERIOD COUNTER
;
W_LOOP:
MOV AX, 6F02H ;setting the time-out value
MOV BL, 30 ;time-out value is 48 seconds
INT 15H
;
; ADD YOUR APPLICATION PROGRAM HERE
;
CMP EXIT_AP, 1 ;is your application over?
JNE W_LOOP ;No, restart your application
MOV AX, 6F02H ;disable Watchdog Timer
MOV BL, 0 ;
INT 15H
;
; EXIT
;