Pull-up circuit on nReset (P0.21) malfunctioning?

[Konrad Hammel]

We have a product that uses an nrf52832 module (vendor listed on the 3rd part modules list) and have shipped ~30,000 units using the current circuit design.  We are using SDK 14.2 and SD v5.1.0.

The product uses the P0.21 reset pin;  the pin is connected to a 10k pull-up resistor connected to Vdd and a 100nF capacitor to ground.

In our last production run we have found a number of samples (~1% at this time) that appear to be stuck in the P0.21 reset state.  These devices were working normally, for months in some cases, but are now stuck in reset across power cycles.

Measuring the voltage on P0.21 shows different values per sample but the samples that are stuck show a voltage below 2.3V (3.4V is our VDD).  We have also isolated a number of samples that show a P0.21 voltage between 2.3V and VDD that are functioning but showing an elevated current draw.

I used hot-air gun to remove the pull-up resistor on one sample that was stuck.  After measuring the resistance and  soldering the pull-up resistor again (using hot-air) the device is no longer stuck in reset and P0.21 shows a voltage of ~VDD

On another sample that was stuck in reset, metal tweezers were used to short the pull-up resistor to VDD.  This sample now works normally across power cycles as well.

Any suggestion on what the cause of this problem may be?  Any hardware or SDK erratas that might apply?

[Konrad Hammel]

Thanks for the replies!

We have removed power from the failing samples for over 1 minute (several days to weeks for some samples) and the issue persists.

Vdd waveform from a good sample (bad sample is the same):

nreset waveform for a good sample:

nreset waveform for a bad sample:

At this time we consider the issue to be caused by a faulty capacitor.

In future builds we will remove the circuit and disable the reset pin in firmware.

[Kenneth]

Thanks for sharing measurement, VDD rise time is good, but it's indeed something suspicious about the voltage on nreset, not something I have seen earlier. Based on your current description and fix I don't think it's anything faulty with the nRF52832, so I don't see a need to transfer this case to QA and ask for an RMA to return samples here for ATE retest. If you later find samples that are permantely stuck in reset or other symptoms please let us know (and possible with a link to this case) so we may follow up.

Kenneth