Appendix F Watchdog Timer; Table F-1: Ah-6Fh Sub-Function - IEI Technology AESQ170-969 User Manual

Appendix F Watchdog Timer

NOTE:
The following discussion applies to DOS environment.
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 EMIs or a software bug. When the
CPU stops working correctly, Watchdog Timer either performs 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: Sets the Watchdog Timer's period.
BL: Time-out value (Its unit-second is dependent on the item
"Watchdog Timer unit select" in CMOS setup).
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 starts counting down. When
the timer value reaches zero, the system resets. To ensure that this reset
condition does not occur, calling sub-function 2 must periodically refresh the
Watchdog Timer. However, the watchdog timer is disabled if the time-out
value is set 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 resets.

Table F-1: AH-6FH Sub-function

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Appendix F Watchdog Timer