Appendix A. Watchdog Timer - ICP Electronics ROCKY-3786EV User Manual

Appendix A. Watchdog Timer

The Watchdog Timer is provided to ensure that standalone
systems can always recover from catastrophic conditions that
cause the CPU to crash. These conditions may result from an
external EMI or a software bug. When the system stops working
correctly, hardware on the board will perform hardware reset
(cold boot) to bring the system back to a known state.
Three I/O ports control the operation of Watch-Dog
Timer.
443 (hex) Write
443 (hex) Read
043/843
Read
(hex)
Prior to enable the Watch-Dog Timer, user has to set the time-
out period. The resolution of the timer is 1 second and the
range of the timer is from 1 sec to 255 sec. The user need to
send the time-out value to the I/O port – 443H, and then
enabling it by reading data from the same I/O port – 443H. This
will activate the timer that will eventually time out and reset
the CPU board. To ensure that this reset condition won't occur,
the Watch-Dog Timer must be periodically refreshed by reading
the same I/O port 443H. This must be done within the time-out
period, please refer to the example program. Finally, we have
to disable the Watch-Dog timer by reading the I/O port -- 843H
or 043H. Otherwise the system could reset unconditionally.
A tolerance of at least 5% must be maintained to avoid
unknown routines in the operating system (DOS), such as disk
I/O that can be very time-consuming. Therefore if the time-out
period has been set to 10 seconds, the I/O port 443H must be
read within 7 seconds.
Set Watchdog Time period
Enable the refresh the Watchdog
Timer.
Disable the Watch-Dog Timer.
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