Bluetooth: Central UART

The Central UART sample demonstrates how to use the Nordic UART Service (NUS) Client. It uses the NUS Client 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 1 peripheral to the Bluetooth LE unit. On Nordic Semiconductor’s development kits, the UART 1 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 1 peripheral’s TX pin.


In this sample, a UART console is used to send and read data over the NUS Client. Debug messages are not displayed in the UART console, but are printed by the RTT logger instead.

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


The sample supports the following development kits:

Hardware platforms


Board name

Build target

nRF5340 DK





nRF52840 DK




nRF52 DK




nRF52833 DK




nRF52833 DK (emulating nRF52820)




nRF21540 DK




The sample also requires another development kit running a compatible application (see Bluetooth: Peripheral UART).

Building and running

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

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


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

  1. Connect the kit to the computer using a USB cable. The kit is assigned a COM port (Windows) or ttyACM device (Linux), which is visible in the Device Manager.

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

  3. Optionally, connect the RTT console to display debug messages. See Debugging.

  4. Reset the kit.

  5. Observe that the text “Starting NUS Client example” is printed on the COM listener running on the computer and the device starts scanning for Peripherals with NUS.

  6. Program the Bluetooth: Peripheral UART sample to the second development kit. See the documentation for that sample for detailed instructions.

  7. Observe that the kits connect. When service discovery is completed, the event logs are printed on the Central’s terminal.

  8. Now you can send data between the two kits. To do so, type some characters in the terminal of one of the kits and hit Enter. Observe that the data is displayed on the UART on the other kit.

  9. Disconnect the devices by, for example, pressing the Reset button on the Central. Observe that the kits automatically reconnect and that it is again possible to send data between the two kits.


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:

    • include/uart.h

  • Bluetooth:

    • include/bluetooth/bluetooth.h

    • include/bluetooth/gatt.h

    • include/bluetooth/hci.h

    • include/bluetooth/uuid.h