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Appendix A. Watch-Dog Timer
The WatchDog Timer is a device to ensure that standalone systems
can always recover from abnormal conditions that cause the system to
crash. These conditions may result from an external EMI or a software
bug. When the system stops working, 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 WatchDog Timer.
443 (hex)
Write
Set WatchDog Time period
443 (hex)
Read
Enable the refresh the WatchDog Timer.
043/843 (hex)
Read
Disable the WatchDog Timer.
Prior to enable the WatchDog 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. You need to send the time-out value to
the I/O port – 443H, and then enable 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 WatchDog Timer must be periodically refreshed by reading
the same I/O port 443H. This must be done within the time-out period
that is set by the software, please refer to the example program. Finally,
we have to disable the WatchDog 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
.
Summary of Contents for ROCKY-3732EVS
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