35
Appendix A. Watch-Dog Timer
The WatchDog Timer is provided to ensure that standalone
systems can always recover from catastrophic conditions that
cause the CPU to crash. This condition may have occurred by
external EMI or a software bug. When the CPU stops working
correctly, hardware on the board will either perform a hardware
reset (cold boot) or a Non-Maskable Interrupt (NMI) to bring the
system back to a known state.
Two I/O ports control the WatchDog Timer :.
443
(hex)
Read
Enable to refresh the WatchDog
Timer.
043
(hex)
Read
Disable the WatchDog Timer.
To enable the WatchDog Timer, a read from I/O port 443H must
be performed. This will enable and activate the countdown timer
which will eventually time-out and either reset the CPU or cause
a NMI, depending on the setting of JP12. To ensure that this
reset condition does not 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 selected by
jumper group JP13.
A tolerance of at least 30% must be maintained to avoid
unknown routines within 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.
Note: when exiting a program, it is necessary to disable the
WatchDog Timer, otherwise the system will reset.
Summary of Contents for ROCKY-P258BX
Page 2: ......
