Bluetooth: Peripheral UART

The Peripheral UART sample demonstrates how to use the Nordic UART Service (NUS). It uses the NUS service to send data back and forth between a UART connection and a Bluetooth® LE connection, emulating a serial port over Bluetooth LE.


When connected, the sample forwards any data received on the RX pin of the UART 0 peripheral to the Bluetooth LE unit. On Nordic Semiconductor’s development kits, the UART 0 peripheral is typically gated through the SEGGER chip to a USB CDC virtual serial port.

Any data sent from the Bluetooth LE unit is sent out of the UART 0 peripheral’s TX pin.


Thingy:53 uses second instance of USB CDC ACM class instead of UART 0, because it has no built-in SEGGER chip that could be used to gate UART 0.


In this sample, the UART console is used to send and read data over the NUS service. Debug messages are not displayed in this UART console. Instead, they are printed by the RTT logger.

If you want to view the debug messages, follow the procedure in Connecting using RTT.


On Thingy:53 debug logs are provided over the USB CDC ACM class serial port, instead of using RTT.

FEM support

You can add support for the nRF21540 front-end module to this sample by using one of the following options, depending on your hardware:

  • Build the sample for one board that contains the nRF21540 FEM, such as nrf21540dk_nrf52840.

  • Manually create a devicetree overlay file that describes how FEM is connected to the nRF5 SoC in your device. See Set devicetree overlays for different ways of adding the overlay file.

  • Provide nRF21540 FEM capabilities by using a shield, for example the nRF21540 EK shield that is available in the nRF Connect SDK. In this case, build the project for a board connected to the shield you are using with an appropriate variable included in the build command. This variable instructs the build system to append the appropriate devicetree overlay file. For example, to build the sample from the command line for an nRF52833 DK with the nRF21540 EK attached, use the following command within the sample directory:

    west build -b nrf52833dk_nrf52833 -- -DSHIELD=nrf21540_ek

    This command builds the application firmware. See Programming nRF21540 EK for information about how to program when you are using a board with a network core, for example nRF5340 DK.

Each of these options adds the description of the nRF21540 FEM to the devicetree. See Radio front-end module (FEM) support for more information about FEM in the nRF Connect SDK.

To add support for other front-end modules, add the respective devicetree file entries to the board devicetree file or the devicetree overlay file.

Minimal sample variant

You can build the sample with a minimum configuration as a demonstration of how to reduce code size and RAM usage. This variant is available for resource-constrained boards.

See Activating sample extensions for details.

USB CDC ACM extension

For the boards with the USB device peripheral, you can build the sample with support for the USB CDC ACM class serial port instead of the physical UART. This build uses the UART async adapter module that acts as a bridge between the USB CDC ACM that provides only the interrupt interface and the default sample configuration that uses the UART async API.

For more information about the adapter, see the uart_async_adapter source files available in the peripheral_uart/src directory. See Activating sample extensions for details about how to build the sample with this extension.


The sample supports the following development kits:

Hardware platforms


Board name

Build target

nRF5340 DK





nRF52840 DK




nRF52840 DK (emulating nRF52811)




nRF52833 DK




nRF52833 DK (emulating nRF52820)




nRF52 DK




nRF52 DK (emulating nRF52810)









nRF21540 DK





  • The boards nrf52dk_nrf52810, nrf52840dk_nrf52811, and nrf52833dk_nrf52820 only support the Minimal sample variant.

  • When used with Thingy:53, the sample supports the MCUboot bootloader with serial recovery and SMP DFU over Bluetooth. Thingy:53 has no built-in SEGGER chip, so the UART 0 peripheral is not gated to a USB CDC virtual serial port.

The sample also requires a smartphone or tablet running a compatible application. The Testing instructions refer to nRF Connect for Mobile, but you can also use other similar applications (for example, nRF Blinky or nRF Toolbox).

You can also test the application with the Bluetooth: Central UART sample. See the documentation for that sample for detailed instructions.


If you build this application for Thingy:53, it enables additional features. See Thingy:53 application guide for details.

The sample also enables an additional USB CDC ACM port that is used instead of UART 0. Because of that, it uses a separate USB Vendor and Product ID.

User interface

The user interface of the sample depends on the hardware platform you are using.

Development kits

LED 1:

Blinks with a period of 2 seconds, duty cycle 50%, when the main loop is running (device is advertising).

LED 2:

On when connected.

Button 1:

Confirm the passkey value that is printed in the debug logs to pair/bond with the other device.

Button 2:

Reject the passkey value that is printed in the debug logs to prevent pairing/bonding with the other device.



The RGB LED channels are used independently to display the following information:

  • Red channel blinks with a period of 2 seconds, duty cycle 50%, when the main loop is running (device is advertising).

  • Green channel displays if device is connected.


Confirm the passkey value that is printed in the debug logs to pair/bond with the other device. Thingy:53 has only one button, therefore the passkey value cannot be rejected by pressing a button.

Building and running

This sample can be found under samples/bluetooth/peripheral_uart in the nRF Connect SDK folder structure.

See Building and programming an application for information about how to build and program the application.

Activating sample extensions

To activate the optional extensions supported by this sample, modify OVERLAY_CONFIG in the following manner:

  • For the minimal build variant, set prj_minimal.conf.

  • For the USB CDC ACM extension, set prj_cdc.conf. Additionally, you need to set DTC_OVERLAY_FILE to usb.overlay.

See Providing CMake options for instructions on how to add this option. For more information about using configuration overlay files, see Important Build System Variables in the Zephyr documentation.


After programming the sample to your development kit, test it by performing the following steps:

  1. Connect the device to the computer to access UART 0. If you use a development kit, UART 0 is forwarded as a COM port (Windows) or ttyACM device (Linux) after you connect the development kit over USB. If you use Thingy:53, you must attach the debug board and connect an external USB to UART converter to it.

  2. Connect to the kit with a terminal emulator (for example, PuTTY). See How to connect with PuTTY for the required settings.

  3. Optionally, you can display debug messages. See Debugging for details.

  4. Reset the kit.

  5. Observe that LED 1 is blinking and that the device is advertising with the device name that is configured in CONFIG_BT_DEVICE_NAME.

  6. Observe that the text “Starting Nordic UART service example” is printed on the COM listener running on the computer.

  7. Connect to the device using nRF Connect for Mobile. Observe that LED 2 is on.

  8. Optionally, pair or bond with the device with MITM protection. This requires using the passkey value displayed in debug messages. See Debugging for details on how to access debug messages. To confirm pairing or bonding, press Button 1 on the device and accept the passkey value on the smartphone.

  9. In the application, observe that the services are shown in the connected device.

  10. Select the UART RX characteristic value in nRF Connect. You can write to the UART RX and get the text displayed on the COM listener.

  11. Type ‘0123456789’ and tap SEND. Verify that the text “0123456789” is displayed on the COM listener.

  12. To send data from the device to your phone or tablet, enter any text, for example, “Hello”, and press Enter to see it on the COM listener. Observe that a notification is sent to the peer.

  13. Disconnect the device in nRF Connect. Observe that LED 2 turns off.


This sample uses the following nRF Connect SDK libraries:

In addition, it uses the following Zephyr libraries:

  • include/zephyr/types.h

  • boards/arm/nrf*/board.h

  • Kernel Services:

    • include/kernel.h

  • Peripherals:

    • incude/gpio.h

    • include/uart.h

  • Bluetooth:

    • include/bluetooth/bluetooth.h

    • include/bluetooth/gatt.h

    • include/bluetooth/hci.h

    • include/bluetooth/uuid.h