Cannot recover nrf52840 with nrfjprog

[Gauthier]

We have this unit with an nrf52840, it suddenly refused to respond to `nrfjprog --recover`.

This is the second unit this happens with.

The power seems ok (we can see on UART that the unit runs its firmware).

Attached is the log file from nrfjprog.

We tested several cables, several SEGGER J-link programmers, several USB connectors, from Ubuntu or MacOS, ...

Other units are fine, it's just this one and one more.

I've tried these, none of it worked:

- `nrfjprog --rbp ALL`

- JLinkExe with Cortex-M4, SWD, 4000 kHz. On the faulty unit, it stops at "Cannot connect to target.". On working units, it comes further (fails later, presumably because of approtect).

- I have a recollection, when this happened the last time, of trying to write to DP registers with JLinkExe. It was not possible. I don't recall the commands.

$ nrfjprog --version
nrfjprog version: 10.14.0 external
JLinkARM.dll version: 7.54c

What is going on, how can I solve it?

[Susheel Nuguru]

is it possible that you flashed something onto the chip that makes it go to deep sleep so fast after the reset that the debugger is unable to make a connection to it?

> nrfjprog --recover -f nrf52  
is the last hope, if it did not work then it is possible you somehow bricked the chip.

[Gauthier]

> nrfjprog --recover -f nrf52  

I've tried that, with and without the family. I understand it might be bricked, that's what I thought when it happened the first time. But this is the second time in a week, I want to investigate what is happening, so it does not happen more times!

[Susheel Nuguru]

Please check if one of these steps help you. Also check if your input power source that is connected to the chip if it has any spikes at times.