Nova Natural NOVA-7820 VIA C3 Processor CPU Embedded Board User Manual

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Watchdog Timer is a device to ensure that standalone systems can always 

recover from catastrophic conditions that cause CPU crash which may be caused 

by external EMI or a software bug.  When the CPU stops working normally, 

hardware on the board will issue a time-out signal. 

 

A BIOS function call (INT 15H) is used to control Watchdog Timer: 

 

INT 15H: 

      

AH – 6FH 
Sub-function: 

AL  –  2 : Set the Watchdog Timer’s period 

BL: Time-out value(the time unit--second, is               

dependent on the item “Watchdog Timer unit      

(selected in BIOS 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, Watchdog Timer will start counting down.  When 

the timer value reaches zero, the system will reset.  To ensure reset condition 

does not occur, Watchdog Timer must be periodically refreshed by calling sub-

function 2.  However the 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. 

 

 

; INITIAL TIMER PERIOD COUNTER 

; 

 

        MOV     AX, 6F02H              ;setting the time-out value 

        MOV     BL, 30                     ;time-out value is 48 seconds 

        INT       15H 

; 

; ADD YOUR APPLICATION PROGRAM HERE 

; 

        CMP      EXIT_AP, 1            ;is your application over? 

        JNE       W_LOOP               ;No, restart your application 

 

         MOV    AX, 6F02H              ;disable Watchdog Timer 

         MOV    BL, 0                       ; 

         INT      15H 

; 

; EXIT