Contec SPI-8451-LLVA, SPI-8451-LVA, SPI-8452-LLVA user manual }

7. Watch-Dog-Timer (WDT) Setting
SPI-8451-LLVA, SPI-8452-LLVA, SPI-8451-LVA 71
7. Watch-Dog-Timer (WDT) Setting
The watchdog timer serves as a safeguard against possible system lock-up in your industrial computer
system. In most industrial environments, there are heavy equipment, generators, high-voltage power
lines, or power drops that have adverse effects on your computer system. For instance, when a power
drop occurs, it could cause the CPU to come to a halt state or enter into an infinite loop, resulting in a
system lock-up.
The application software created by user with the watchdog timer enabled, a RESET automatically
generated unless the software periodically triggers the timer within the setting time-out interval. That is,
while the system gets hung up, the running program can’t trigger the timer periodically. The timer will
generate a reset signal to reboot the system. This feature allows a running program to restart in an
orderly way when a power glitch or any abnormal condition occurs.
The watchdog timer comes with 255-level time-out interval, 1 - 255 seconds per interval, which can be
adjusted by software setting. There is a tolerance of 2 second for this time-out interval. For example,
if the time-out interval has been set to 32 seconds, your program should trigger the watchdog timer before
28 seconds are escaped. Otherwise, after 28-32 seconds are escaped, the system will automatically
reboot. To keep the system running normally, your program should trigger the watchdog timer every 28
seconds.
The I/O port is defined at address 2e/2fH. You can trigger/enable disable the timer by writing address
2e/2fH.
Here is an example for flow chart and programming how to use the watch-dog-timer.
(1) Example flow chart
WDT
Restart?
END
START
No
WDT Stop
WDT Initial
WDT Start
Yes
If [WDT Stop] is not performed within the limit time set up
by [WDT Start], system reset occurs.
}
When performing the re-start of WDT, it repeats [WDT Start] and [WDT Stop].
* It is also possible not to perform [WDT Stop] instead of performing [WDT Stop] to [WDT Start], but to perform [WDT
Start] continuously at the time of a re-start.