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Flashing sample applications

Preparing environment

Prepare the development environment first as described in Environment setup document.

nRF52840 buttons and LEDs

Required hardware

You need following hardware to flash apps:

  • nRF52840 dongle - this is the target device on which Fobnail firmware runs.

  • nRF52840-DK - development board used for dongle flashing and debugging.

  • SWD Cable - 2x5 1.27 mm pitch IDC cable used for connecting dongle to debugger.

  • Adafruit 4048 header - nRF dongle comes without SWD header, so it must be soldered. This is the same header as used in nRF52840-DK board.

When soldered, 4048 header's notch must face direction opposite to USB plug.

Preparing debugger

We use cargo-embed for firmware flashing and debugging, which internally uses probe-rs, so you need a compatible debugger. We are using nRF52840-DK development board, which exposes a JLink-compatible interface. To check whether your debugger is supported, start Fobnail SDK and run probe-rs-cli list. You see similar output:

The following devices were found:
[0]: J-Link (J-Link) (VID: 1366, PID: 1015, Serial: 000683081460, JLink)

If probe-rs does not detect debugger, that could mean it is not supported. You can set RUST_LOG=trace environment variable to retry scanning with debug output enabled.

For debugger to work, you need to add an Udev rule granting you correct permissions:

SUBSYSTEM=="usb", ATTR{idVendor}=="1366", ATTR{idProduct}=="1015", OWNER="akowalski", MODE="0660"

Replace idVendor and idProduct and OWNER with correct VID, PID and your user name, save this to /etc/udev/rules.d/99-usb.rules, then run sudo systemctl reload systemd-udevd to apply these rules without rebooting.

Building and flashing blinky sample

In order to flash a sample application on the nRF52840 dongle we will use blinky sample application from our nrf-hal fork. Follow the steps below to build and flash blinky sample:

  1. Power on dongle and connect it to debugger.
  2. Go to nrf-hal directory
  3. Run the fobnail-sdk container with ./
  4. Go to examples/blinky-demo-nrf52840
  5. Execute cargo embed --target thumbv7em-none-eabihf. This command will build the sample app, flash it onto target device and spawn RTT console (sort of UART-over-JTAG).
  6. Both green and red LEDs should start blinking, you should see RTT console with similar message:
    10:31:50.887 Blinky demo starting


Cargo-embed provides great tools with very comfortable single-command solution for building and testing firmware.