Let's verify that all the tools were installed correctly.

Connect the micro:bit to your laptop using an USB cable.

The micro:bit should now appear as a USB device (file) in /dev/bus/usb. Let's find out how it got enumerated:

$ lsusb | grep -i NXP
Bus 002 Device 033: ID 0d28:0204 NXP ARM mbed
    ^^^        ^^^

In my case, the micro:bit got connected to the bus #2 and got enumerated as the device #33. This means the file /dev/bus/usb/002/033 is the Fmicro:bit3. Let's check its permissions:

$ ls -l /dev/bus/usb/002/033
crw-rw---- 1 root uucp 189, 160 Jul  8 14:06 /dev/bus/usb/002/033
                  ^^^^

The group should be uucp. If it's not ... then check your udev rules and try re-loading them with:

$ sudo udevadm control --reload-rules

First, connect the micro:bit to your computer using the micro-USB cable. The yellow LED next to the USB input should turn on right after connecting the USB cable to the board.

Next, run this command:

$ # *nix
$ openocd -f interface/cmsis-dap.cfg -f target/nrf51.cfg

$ # Windows
$ # NOTE cygwin users have reported problems with the -s flag. If you run into
$ # that you can call openocd from the `C:\OpenOCD\share\scripts` directory
$ openocd -s C:\OpenOCD\share\scripts -f interface/cmsis-dap.cfg -f target/nrf51.cfg

NOTE Windows users: C:\OpenOCD is the directory where you installed OpenOCD to.

You should see output like this:

Open On-Chip Debugger 0.10.0
Licensed under GNU GPL v2
For bug reports, read
    http://openocd.org/doc/doxygen/bugs.html
Info : auto-selecting first available session transport "swd". To override use 'transport select <transport>'.
cortex_m reset_config sysresetreq
adapter speed: 1000 kHz
Info : CMSIS-DAP: SWD  Supported
Info : CMSIS-DAP: Interface Initialised (SWD)
Info : CMSIS-DAP: FW Version = 1.0
Info : SWCLK/TCK = 1 SWDIO/TMS = 1 TDI = 0 TDO = 0 nTRST = 0 nRESET = 1
Info : CMSIS-DAP: Interface ready
Info : clock speed 1000 kHz
Info : SWD DPIDR 0x0bb11477
Info : nrf51.cpu: hardware has 4 breakpoints, 2 watchpoints

(If you don't ... then check the general troubleshooting instructions.)

openocd will block the terminal. That's fine.

Also, the yellow LED should start blinking very fast. It may seem concerning, but it is a good sign.

That's it! It works. You can now close/kill openocd.