ST-Link V2 programmers are seemingly USB flash drives that have an 8 pin debugging interface. They have a USB-A port on one side, and the debugging interface on the other. Sometimes, they may have an aluminum casing covering the ST-Link, while others may not have such protective coverings.
In flashing a bootloader to a factory unlocked nRF52840, I ran into a variety of issues, which I am documenting in hopes of saving someone else a couple of minutes, hours, or (in my case) days.
Please note that since my chips came unlocked, I do not have experience unlocking these chips. However, it appears the ST-Link V2 may require a CMSIS-DAP patch if using OpenOCD to unlock. In addition, I used Ubuntu 22.04 for my flashing, though the same process can be followed on any Linux-based distro or Windows. However, Windows users may need Zadig for proper USB drivers to allow the ST-Link V2 to properly interface with the nRF and the computer.
1. Plug the ST-Link into the computer
2. You now must choose one of two options: Blackmagic or OpenOCD. Personally, I only had luck with OpenOCD, though many have succeeded with Blackmagic. Using "Blackmagic" essentially turns the functionality of the ST-Link into a Blackmagic programmer, allowing the same actions to be done on a physical Blackmagic Probe. OpenOCD is much more flexible and can be done on a Raspberry Pi if you prefer (if you have one of those, that is, in today's day and age of the silicon shortage)
Using OpenOCD
The first download aims to update the firmware on the ST-Link V2 to a hopefully more reliable version that will be used for OpenOCD.
1. Take a look at and download https://github.com/blackmagic-debug/blackmagic/tree/master/src/platforms/stlink2. Download the link under "Reverting to original ST Firmware with running BMP firmware" on the second link. Follow the instructions to reflash the firmware on the ST-Link V2
3. Connect the debugging (SWD & SWC) pins on the nRF to the corresponding pins on the ST-Link V2 (SWDIO, SWCLK). In addition, bridge a GND connection between the chips, thus creating a common GND. I have found this creates a much better connection. In fact, it was necessary for the functionality of OpenOCD in my experience.
4. Use OpenOCD, running the base command of: openocd -f interface/stlink.cfg -f target/nrf52.cfg
5. For more information, take a look at this article by joric.