why rst pin voltage drops in STM32L462CE?

RMart.1 · ‎2020-10-28

Hi guys,

we have a several hundred of devices deployed on the field with a STM32L462 mcu. We are noticing that some devices are stopping operation and returning to operation in a very random way.

We have analyzed some of the devices that eventually fail and we are noticing that the pin rst is dropping below the brown out reset level. rst is connected to just to a 100nF capacitor. Connecting the stlink gives under reset error. Eventually and randomly some of them come back to live after a time ranging from few hour to up to 20 days without intervention.

Also by applying a small voltage to the rst pin with the multimeter in diode mode we can recover some of the devices from the under reset state but eventually they fail to this state again after some time.

Some of the devices that have entered into this state do not seem to work correctly anymore, somo other they do even though they enter to this under reset state again.

I have read in the manual that in case of internal reset the internal pull up is deactivated and that could explain the decrease in the voltage in the rst pin, but all reset situation I have read on the manual come back to a reset giving voltage to rst pin...

I just wanted to share this with you in case you have any advice or recommendation... any thoughts or suggestion will be more than welcome..

Thanks a lot to all in advance!

Have you experience something like this?

Uwe Bonnes · ‎2020-10-28

I suspect your circuit somehow damaged the devices. Are external voltages beside supply applied to the board. I suspect this voltage near or at the damage level so that some of the boards fail.

Also have a look with the scope on the NRST pin and look it the voltage pumps. Maybe something early in the startup causes reset, and so the part gets in a loop.

RMart.1 · ‎2020-11-03

Hi Uwe,

Thanks a lot for your advice, it is much appreciated.

As you very well said we are suspecting that there is something external that is causing this effect. Just to complete with more information we are feeding the system with battery and we are stepping up the voltage to read a sensor y a cycle fashion every 15 minutes, this process only takes a few hundreds of ms. By the fuel of the system comes from a battery a there is no other voltage supply on the system.

Another issue would be if at some locations there are power mains around that might be affecting... but again the system has passed all CE certification at the lab....

Thans again for your feedback!!!