6. Software Utilities
PC-686BX(NLX)-LV, PC-686BX(NLX)-LVV
43
Watchdog Timer (WDT) Setup
Watchdog timers are widely used in industrial applications to monitor CPU activity.
The application software uses an appropriate timer setting to trigger the WDT. In a
normally functioning system, the system reloads the WDT before it times out.
Accordingly, the WDT time out never occurs in a normally functioning system.
However, if the system has a fault that results in the WDT not being reloaded, the
WDT times out and the system is automatically reset to recover from the fault.
The CPU board supports 16 different watchdog timer settings which can be specified
by software via an I/O port. Writing a value to I/O address 0441h disables the
watchdog timer. Writing a setting code (refer to the WDT setup table) to I/O address
0443h reloads the WDT.
The following shows an example application program for disabling the WDT and
reloading the WDT.
MOV DX,0441h REM Write a value to 0441h to disable the WDT
OUT DX,AX;
MOV AX,0001h REM Sets WDT timer = 28 seconds
MOV DX,0443h
OUT DX,AX REM Triggers the WDT with the timer setting value
Table 6.1. Timer Value Table
Value Timer Value Timer Value Timer Value Timer
0 30sec 4 22sec 8 14sec C 6sec
1 28sec 5 20sec 9 12sec D 4sec
2 26sec 6 18sec A 10sec E 2sec
3 24sec 7 16sec B 8sec F 0sec
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