Specifications

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The Watch-Dog Timer is provided to ensure that standalone
systems can always recover from catastrophic conditions that
caused 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.
The Watch-Dog Timer is controlled by two I/O ports.
443
(hex)
Read Enable the refresh the Watch-Dog Timer.
043
(hex)
Read Disable the Watch-Dog Timer.
To enable the Watch-Dog 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 an
NMI depending on the setting of JP19. To ensure that this reset
condition does not occur, the Watch-Dog Timer must be
periodically refreshed by reading the same I/O port 433H. This
must be done within the time out period that is selected by jumper
group JP17.
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 Watch-
Dog Timer, otherwise the system will reset.