Watchdog Timer Issues and Pullup on nReset

[TylerS]

Hello,

We have been having strange issues with the watchdog on some of our development devices. The issue is that it seems the watchdog will sometimes not properly reset the device in the event of a lockup. We have determined that there is a bad issue that is causing a lockup in our system workqueue thread where the hardware watchdog is kicked. We have seen one device that is in this lockup state and the watchdog is not being kicked but the device is not being reset. We have none of the Zephyr watchdog options set. We are using NCS 1.7.0 with SICA B1-A hardware.

One hardware issue we have found is that our current revision has a 10k pullup resistor to 3V on the nReset pin. I have read that this is actually an issue and can potentially even cause damage to the SOC. Would this be a potential source of this watchdog issue? I also wanted to understand what the ramifications are of putting a pullup on this pin. The strange thing is that it is hard to reproduce the issue and almost every time we try to insert a lockup, the watchdog works properly. 

Thank you,

Tyler

[Simonr]

Hi again Tyler

No, I don't think I've seen/heard of behavior like this before, and I can't find anything either. You're right that the watchdog shouldn't need any "backup" as it generally just works. It could be related to the pull-up, as it will cause the side effects I described + that it might cause undefined behavior like this since we don't characterize how the SiP works in faulty states. 

Are you able to test your application on an nRF91 DK by the way? If you're not able to reproduce it there, I think it's safe to say there's a HW issue in your custom board. If you'd like we can do a HW review of your board before you do your next PCB revision to make sure everything looks good. You can make a new (private) ticket for that purpose.

Best regards,

Simon