Xycom 4115T, Руководство пользователя

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56 143926(A)
Appendix B – Watchdog Timer
The Watchdog Timer is a device to ensure that standalone systems can always
recover from abnormal conditions that cause the system to crash. These conditions
may result from an external EMI or a software bug. When the system stops working,
hardware on the board will perform 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:
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, Watchdog Timer must be periodically refreshed by calling sub-
function 2. However, 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.
Note
When exiting a program it is necessary to disable Watchdog Timer, otherwise the
system will reset.