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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.