IEI Technology JUKI-C400 Series, Руководство пользователя

Страница: 53 / 61

53
Appendix A. Watchdog 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 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. For example, if the time-out period has been set to 10 seconds,
the I/O port 443H must be read within 7 seconds.