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 Tyler

Seems strange that this pullup would affect the watchdog, but I guess that could be the case... 

The external pull-up is indeed not recommended, as the nRESET is internally pulled up to the SiP internal VDD rail. During sleep, if the external rail is higher (I.E. 3V), the internal regulator is in a state where it doesn't leak the current fed by the external pull-up, causing the internal supply to rise from 2.2V to the external rail voltage. When the nRF91 wakes up, the internal supply will be too high, and might cause issues with calibrations done at startup and causing excess power consumption (that I know of). The recommendation is therefore to not use external pull resistors, keep any controllers etc. in high-Z except when pulling low to reset nRF9160. Reset switches/buttons can use the internal pullup.

Best regards,

Simon