Unable to connect debugger after nrfjprog --recover due to default REGOUT0

[Benjamin]

I have a custom board with an nRF52840 that uses high-voltage mode (5V on VDDH) and the chips internal voltage regulators. This requires setting REGOUT0 to 3.3V. My firmware sets this correctly (along with both DC/DC enables) and the board works great.

However, after wiping the chip (usually via --recover), REGOUT0 returns to its default of 1.8V. This sometimes causes the debugger to fail to access the chip. It happens when using an nRF52840-DK as well as a stand-alone JTAG debugger. I suspect a brown-out, because when I inject 1.8V externally, the debugger functions just fine.

nrfjprog doesn't allow using the --memwr flag along with the --recover flag. Executing a separate nrfjprog --memwr command immediately following a --recover command sometimes succeeds, but sometimes fails.

I would like a better solution than hooking up an external power supply to my debugger every time I need to bring up a new board, or want to wipe a board to build from scratch.

[Dmitry]

Hi,

you can try to erase chip and then write UICR without device reset directly with jlink script, as explained here.

[Vidar Berg]

As Dimitry said, the default REGOUT0 setting should not get applied if you can avoid resetting the device before the --memwr command.

Could you try using --erase instead of --recover, or is --recover needed to disable APPROTECT in your case?

e.g.

nrfjprog --erase

nrfjprog --memwr UICR.REGOUT0 --val <3V3 setting>

nrfjprog --reset

Also, I'm not sure why programming becomes more stable with the external supply. Standalone debuggers usually come with built-in level shifters so the IO voltage should not have been a problem. Is the VREF pin connected to Vdd on your target board?