| ??? 03/15/07 21:52 Read: times Msg Score: +1 +1 Informative |
#135066 - There are watchdogs and the there are WATCHDOGS Responding to: ???'s previous message |
There is a continuum of watchdogs from the very simple, where a single software instruction resets the watchdog. The next step is where it requires two actions, like setting an output and clearing an output. The next step is a windowed watchdog which requires the two actions to happen within a certain window. The most secure is one where there is a specific sequence of events before the watchdog is reset.
If at all possible, I believe you should avoid the first kind since it is a single action. In writing resilient code, several authors make the point that it would be nice to know if both the foreground and the interrupt are occurring, and if possible in the correct sequence. You can get some security by doing this in software and only clocking the watchdog in one place. On the other hand if you have the second type, you can set the pin in the normal course of events, and reset it in the interrupt routine. In that way, if either section of code stops running, the watchdog will not get kicked. Here are some artciles that pertain to the subject. Some are readily available, and some not. If you want tyo push this further, I could probably scan the stuff that is in print only. -Aubrey Born to Fail by Jack Ganssle, Embedded Systems Programming, December 2002: http://www.embedded.com/story/OEG20021211S0032 Li’l Bow Wow by Jack Ganssle, Embedded Systems Programming, January 2003: http://www.embedded.com/story/OEG20030115S0042 Watching the Watchdog by Jack Ganssle, Embedded Systems Programming, February 2003: http://www.embedded.com/story/OEG20030220S0037 Pedigree Protection- Watchdog Circuits by Bob Perrin, Circuit Cellar Online, July 1999: http://i.cmpnet.com/chipcente...9cdpdf.pdf Protection Techniques Ensure uC Reliabilty in Power Control Circuits by Richard J. Valentine, EDN October 1996 http://www.edn.com/archives/...+Valentine Designing Microcontroller Systems for Electrically Noisy Environments by Tim Williamson, Intel Application Note AP-125. Fault-tolerant software in real-time single-chip microcontroller systems by N. Q. Burnham and C.F. Cowling, Electronic Components and Applications, Vol 6: No. 1, 1984. Designing Reliable Software for Automotive Applications by Barry Yarkoni and John Wharton, Intel article reprint AR-102. Fault Tolerant Electronic Systems by George Novacek, Circuit Cellar, January 2004 4. Born to Fail by Jack Ganssle, Embedded Systems Programming, December 2002: http://www.embedded.com/story/OEG20021211S0032 Software fault tolerance staves off the errors that besiege uP systems by Dick Jarrett, Electronic Design August 9, 1984. |
