User Manual

IOWA-LX-600 Half-size CPU Card
Page 140
NOTE:
The following discussion applies to DOS environment. IEI support is
contacted or the IEI website visited for specific drivers for more
sophisticated operating systems, e.g., Windows and Linux.
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,
Watchdog Timer either perform s a hardware re set (cold boot) or a Non -Maskable Interrupt
(NMI) to bring the system back to a know n 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-se cond is d ependent on the item “Watchdog
Timer unit select” in CMOS set up).
Table D-1: AH-6FH Sub-function
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 . While 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 Watchdo g Timer. However, the Wat chd og timer is disa bled
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 di sk I/O that can be very time-consuming.