CyberResearch CPEX CEL-24-X Manuel d'utilisateur télécharger pdf (Page 65)

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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, Watchdog Timer 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 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, calling sub-function 2 must periodically refresh the

Wa t c h d o g T i m e r. H o w e v e r, t h e Wa t c h d o g 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 the Watchdog Timer,

otherwise the system will reset.

Example program:

; INITIAL TIMER PERIOD COUNTER

;

W_LOOP:

1 2 ... 60 61 62 63 64 65 66 67 68 69 70 ... 91 92

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CyberResearch CPEX CEL-24-X Manuel d'utilisateur Page 65