User`s manual

64 PCN-6351/6351L User's Manual
A.1 Programming the Watchdog Timer
Jumper J3 controls the watchdog settings. The default configuration
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 normally.
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.