User manual

APC-3580 User Manual
127
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 software
bug. When the CPU stops working correctly, hardware on the board will either perform a hardware
reset (cold boot) to bring the system back to a known state.
A BIOS function call (INT 15H) is used to control the Watchdog Timer:
z
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).
You have to call sub-function 2 to set the time-out period of Watchdog Timer first. If the time-out value
is not zero, the Watchdog Timer will start counting 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 you set
the time-out value to be 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.
Notes: This function is applied by Winbond W83627HF chipset, if partners have further questions
about it, please refer to the original datasheets or contact with our customer service department.
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