E.1 Programming the watchdog timer
Jumper J3 controls the watchdog settings. The default configura- tion of the timer is enabled via a system reset.
To enable the watchdog timer, you must write a program which writes 1 to I/O port address 443 (hex) at regular intervals. The first time your program reads the port, it enables the watchdog timer. After that, your program must write 1 to the port at time interval of less than 1.6 seconds, otherwise the watchdog timer will activate and reset the CPU or generate an interrupt on IRQ11. When you want to disable the watchdog timer, your program should write 0 to I/O port 443.
If CPU processing comes to a standstill because of EMI or a software bug, your program's signals to I/O port 443 to the timer will be interrupted. The timer will then automatically reset the CPU or invoke an IRQ, and data processing will continue normal- ly.
You must write your program so that it writes 1 to I/O port 443 at an interval shorter than the timer's preset interval. The timer's intervals have a tolerance of ± 30%, so you should program an instruction that will refresh the timer about every second.
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