User`s guide

30 • Installations
2.5 Watchdog Timer Configuration
The function of the watchdog timer is to reset the system automatically. It
contains a one-second/minute resolution down counter, CRF2 of logical
device 8, and two Watchdog control registers, WDT_CTRL0 and
WDT_CTRL1 of logical device 8. We can uses compatible PNP protocol to
access configuration registers for setting up watchdog timer configuration.
To grogram configuration registers, the following configuration sequence
must be followed:
(1) Enter the extended function mode by writing 87h to the location
3F0h twice.
(2) Configure the configuration registers
(3) Exit the extended function mode by writing 0AAh to the location
3F0h.
The following example is written in Intel 8086 assembly language. It will
reset the system in 15 seconds. We can use both keyboard interrupt and
mouse interrupt to cause the watchdog to reload and start to count down
from the value of CRF2.
MODEL SMALL
DATA
CODE
STARTUP
BEGIN:
;-----------------------------------------------------------------------
;Enter the extended function mode, interruptible double-write
;-----------------------------------------------------------------------
MOV DX,3F0H
MOV AL,87H
OUT DX,AL
OUT DX,AL
;----------------------------------------------------------------------
;Configurate logical device 7, configuration register CRE2
;----------------------------------------------------------------------
MOV DX,3F0H
MOV AL,07H
OUT DX,AL ;point to logical Device Number Reg.
MOV DX,3F1H
MOV AL,07H ;select device 7