Microcontroller failure modes?

I am using a SAM3N00B microcontroller in an emergency light PCB.

We have to make provision for when the micro fails.

But when it fails, will the pins be..

1...High impedance
2...'stuck' logic high
3...'stuck' logic low
4...a combination of the above
?


SAM3N00B DATASHEET:
http://www.atmel.com/Images/doc11011.pdf

What you say to use Watch Dog ?

Best regards,
Peter

A watchdog doesn't help with microcontroller failure - only with a working microcontroller which has managed to get stuck with a software bug or where the function has otherwise been corrupted temporarily.

My information is probably out of date, but failure analysis used to consider any pin could be shorted to any pin and any component could be short or open. I seems to remember it being acceptable to only consider resistors going open not short circuit. This was back in the days when the standards were 'British Standards'. The European ones may be different.

Keith

Perhaps this is an over-simplified answer but as a micro can fail in almost any condition and even randomly produce varying signals, the most obvious solution would be a manual switch that electronically overrides the micro's output.

Brian.

Yes I mean on that when uC stuck with working and need reset.

Regarding hardware failure, I agree with Keith that case 4 should be considered.

But that's only part of the problem. There's also a possibility that the µP seems to service the outputs as expected (e.g. sending a watchdog trigger sequence) but is no longer acting on input conditions due to software faults or corrupted binary image. So software safety must be included in a failure analysis. There are different standards for software safety and respective compliance requirements in application specific standards.

For moderate requirements as probably applicable for emergency lights, repetive RAM and ROM integrity check (CRC) together with an external watchdog forcing a safe state (e.g. "light on") might be sufficient, or software safety is simply ignored in the regulations.