B&B Electronics PC Watchdog Timer Card ATRWDT Bedienungsanleitung PDF herunterladen (Seite 19)

P104-WDG-CSM User Manual

Background Watchdog Mode

An alternative method to refresh the counters that provides some flexibility but potentially

reduces dependability is to use an interrupt service routine (ISR). The card will generate an

interrupt one Counter 0 load value before timeout occurs. If that interrupt is used by a user-

defined ISR, it's possible for that ISR to refresh the counters and, thus, avoid need for the

main program loop to refresh the counters.

There is the possibility of reduced dependability because this method will not detect certain

types of program lock-ups. For example, if the application program were to lock up in a loop

such as "JMP $", the interrupt service routine would never know it and would therefore

continue refreshes despite the lockup. It is possible to modify the host program to enable the

ISR to detect such loops (perhaps by checking the return stack location for the ISR each time,

and making sure it changes...) However, if the code is going to be modified, the next, hybrid,

method might be more effective.

This method does, however, allow "off-the-shelf" existing programs to be used with the card

without modification. Also, this method could cause spurious resets if your program disables

interrupts for an extended duration. The IRQ that would have caused the ISR might be missed

and allow the counter to time out. If the counter counts-down, the reset signals will be

generated and the counters will restart.

Combination Reset Mode

By combining the two methods ("Foreground" and "Background") it is possible to create a very

elaborate scheme for monitoring the status of the computer. This combined method eliminates

the negatives associated with either method used individually, but does require extensive

modifications to existing code, or even rewriting code entirely. In this scheme, both foreground

and background routines can prompt the watchdog.

The foreground routine would prompt the watchdog under normal conditions, and the ISR

running in the background would only prompt the watchdog if the foreground routine failed to

do so. The ISR, noting that the foreground routine missed its prompt, could take steps to

determine why, and even try to correct it. For example, the foreground program can post its

current status to a shared-memory variable, indicating that it is about to enter a long dedicated

process (calculations for a print job, perhaps) and that it might miss several of its prompts.

The ISR, when executed by the warning IRQ, notes that the foreground program indicated it

might miss its turn, and starts counting misses. After prompting the watchdog, it returns

control to the foreground process. If the count of missed prompts gets too high, it could write

status to a disk file or serial port, then skip the prompt, allowing the computer to reset.

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B&B Electronics PC Watchdog Timer Card ATRWDT Bedienungsanleitung Seite 19