nRF52840 Dongle

Next, we'll look into the radio API exposed by the dk HAL. But before that we'll need to set up the nRF52840 Dongle.

From this section on, we'll use the nRF52840 Dongle in addition to the nRF52840 DK. We'll run some pre-compiled programs on the Dongle and write programs for the DK that will interact with the Dongle over a radio link.

💬 How to find the buttons on the Dongle: Put the Dongle in front of you, so that the side with the parts mounted on faces up. Rotate it, so that the narrower part of the board, the surface USB connector, faces away from you. The Dongle has two buttons. They are next to each other in the lower left corner of the Dongle. The reset button (RESET) is mounted sideways, it's square shaped button faces you. Further away from you is the round-ish user button (SW1), which faces up.

The Dongle does not contain an on-board debugger, like the DK, so we cannot use probe-rs tools to write programs into it. Instead, the Dongle's stock firmware comes with a bootloader.

When put in bootloader mode the Dongle will run a bootloader program instead of the last application that was flashed into it. This bootloader program will make the Dongle show up as a USB CDC ACM device (AKA Serial over USB device) that accepts new application images over this interface. We'll use the nrfdfu tool to communicate with the bootloader-mode Dongle and flash new images into it.

✅ Connect the Dongle to your computer. Put the Dongle in bootloader mode by pressing its reset button.

When the Dongle is in bootloader mode its red LED will pulsate. The Dongle will also appear as a USB CDC ACM device with vendor ID 0x1915 and product ID 0x521f.

You can also use our cargo xtask usb-list tool, a minimal cross-platform version of the lsusb tool, to check out the status of the Dongle.

✅ Run cargo xtask usb-list to list all USB devices; the Dongle will be highlighted in the output, along with a note if in bootloader mode.

Output should look like this:

$ cargo xtask usb-list
Bus 001 Device 016: ID 1915:521f <- nRF52840 Dongle (in bootloader mode)

🔎 cargo xtask lets us extend cargo with custom commands which are installed as you run them for the first time. We've used it to add some helper tools to our workshop materials while keeping the preparation installations as minimal as possible.

Now that the device is in bootloader mode browse to the boards/dongle directory. You'll find some ELF files (without a file ending) there. These are pre-compiled Rust programs to be flashed onto your dongle.

For the next section you'll need to flash the loopback file onto the Dongle.

✅ Run the following command:

$ nrfdfu boards/dongle/loopback

Expected output:

[INFO  nrfdfu] Sending init packet...
[INFO  nrfdfu] Sending firmware image of size 37328...
[INFO  nrfdfu] Done.

After the device has been programmed it will automatically reset and start running the new application.

🔎 Alternatively, you can also use nordic's own nrfutil tool to convert a .hex file and flash it for you, among many other things nrfutil is a very powerful tool, but also unstable at times, which is why we replaced the parts we needed from it with nrfdfu.

🔎 The loopback application will make the Dongle enumerate itself as a CDC ACM device.

✅ Run cargo xtask usb-list tool to see the newly enumerated Dongle in the output:

$ cargo xtask usb-list
Bus 001 Device 020: ID 2020:0309 <- nRF52840 Dongle (loopback.hex)

The loopback app will log messages over the USB interface. To display these messages on the host we have provided a cross-platform tool: cargo xtask serial-term.

❗ Do not use serial terminal emulators like minicom or screen. They use the USB TTY ACM interface in a slightly different manner and may result in data loss.

✅ Run cargo xtask serial-term. It shows you the logging output the Dongle is sending on its serial interface to your computer. This helps you monitor what's going on at the Dongle and debug connection issues. You should see the following output:

$ cargo xtask serial-term
deviceid=588c06af0877c8f2 channel=20 TxPower=+8dBm app=loopback.hex

This line is printed by the loopback app on boot. It contains the device ID of the dongle, a 64-bit unique identifier (so everyone will see a different number); the radio channel that the device will use to communicate; and the transmission power of the radio in dBm.

If you don't get any output from cargo xtask serial-term check the USB dongle troubleshooting section.


At this point you should not get more output from cargo xtask serial-term.

❗If you get "received N bytes" lines in output like this:

$ cargo xtask serial-term
deviceid=588c06af0877c8f2 channel=20 TxPower=+8dBm
received 7 bytes (CRC=Ok(0x2459), LQI=0)
received 5 bytes (CRC=Ok(0xdad9), LQI=0)
received 6 bytes (CRC=Ok(0x72bb), LQI=0)

That means the device is observing interference traffic, likely from 2.4 GHz WiFi or Bluetooth. In this scenario you should switch the listening channel to one where you don't observe interference. Use the cargo xtask change-channel tool to do this. The tool takes a single argument: the new listening channel which must be in the range 11-26.

$ cargo xtask change-channel 11
requested channel change to channel 11

Then you should see new output from cargo xtask serial-term:

deviceid=588c06af0877c8f2 channel=20 TxPower=+8dBm
now listening on channel 11

Leave the Dongle connected and cargo xtask serial-term running. Now we'll switch back to the Development Kit.