ALUDRA user manual
Issue A
114
Appendix C - Watchdog Timer
The following discussion applies to DOS environment. Please contact Eurotech
support or visit the Eurotech website for information on drivers used with more
sophisticated operating systems, e.g., Windows and Linux. See
for contact details.
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 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).
Table: 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 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.
When exiting a program it is necessary to disable the Watchdog Timer, otherwise
the system resets.
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