NRF52840 dongle USB bootloader with external power supply

[scttnlsn]

In section 6.3.2 of the NRF52840 dongle user guide it describes how to modify the board for use with an external, regulated power supply.  Once those modifications are made to the board, is it still programmable using the USB bootloader (with or without the external supply connected)?

[ketiljo]

Yes, you can still use the USB interface, but you must power the dongle from the external power supply.

[scttnlsn]

I have 3v (from 2 AA batteries in series) across VDD OUT and GND.  I press the bootloader reset button (and get a red pulsing LED) and then try to flash my firmware with nrfutil but I get the following error:

pc_ble_driver_py.exceptions.NordicSemiException: Serial port could not be opened on /dev/ttyACM0. Reason: could not open port /dev/ttyACM0: [Errno 2] No such file or directory: '/dev/ttyACM0'

I do not get this error when flashing an unmodified dongle powered via USB 5v.

[ketiljo]

If you break SB2 and connect SB1, you will just disable the regulator so that you can power the chip externally. VBUS is still connected to the VBUS pin of the nRF52840 to power the USB-PHY. Make sure SB2 is completely disconnected, else you may damage the chip when VBUS is applied to the USB connector. I can't see why it shouldn't work, unless something is broken.

[scttnlsn]

I got it working but the current consumption is very high.  ~600uA whereas the same program running on the NRF52840 DK only consumes around 75uA.  Are there other components that could be sinking some current?

[ketiljo]

No, there are no other components on the dongle. If the current draw is higher on the dongle, it's possibly damaged somehow.