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Forum Index : Microcontroller and PC projects : WATCHDOG command vs real one...
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Grogster Admin Group  Joined: 31/12/2012 Location: New ZealandPosts: 9072 |
I see now that the latest MMBASIC has a watchdog timer you can set in the software, which is a nice feature - just after we perfected a 555 design in another thread a month or so ago. 
With that in mind, and now that we HAVE a watchdog timer in the BASIC, what is the opinion on using an external WDT circuit? Is it still preferable to use an external one, or is the MM software one good enough that an external hardware WDT can be omitted? Opinions? Thoughts? Smoke makes things work. When the smoke gets out, it stops! |
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| JohnS Guru  Joined: 18/11/2011 Location: United KingdomPosts: 3674 |
I'd use simple external if it's important. The chip or software can hang up and the internal WDT won't necessarily happen. John |
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Geoffg Guru Joined: 06/06/2011 Location: AustraliaPosts: 3165 |
The WATCHDOG command will catch the more obvious failures such as a program being stuck in a loop, an error being generated by your program or your program ending up at the command prompt for some reason. An internal error in the processor (an exception) caused by a bug in MMBasic also causes a restart (and runs RESTART.BAS) the same as a WATCHDOG timeout so it will be caught also. But there is also an outside chance that a cosmic ray strike (or whatever) could cause the PIC32's processor to hang and only an external circuit could catch that. The chances of this are very very rare but it could happen. But an external circuit does not catch everything. For example a close lightning strike could fry the PIC32's processor and resetting it will not bring it back to life. For that you would need a number MM's running in parallel with some sort of supervisor circuit to disconnect one if it died. It is really a matter of risk. I would say that the WATCHDOG command would be fine for most uses (such as Gizmo's windmill power system) but if there was the possible loss of many thousands of dollars I would use an external circuit. If there was the risk of injury or loss of life then the MM should not be used in the first place. Don't forget that the external watchdog circuit is also a risk factor. If, for example, it fails and holds the MM in reset the effect could be just as bad as a bug in your BASIC program. Geoff Geoff Graham - http://geoffg.net |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9072 |
So you're saying that nothing is infallible then, right?(rhetorical) 
So noted - I think I will still use the external WDT, only because we spent time here on the forums developing that idea, so I would like to use it rather then just chuck it in the bin.  Smoke makes things work. When the smoke gets out, it stops! |
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