Bluetooth: Central SMP Client

The Central Simple Management Protocol (SMP) Client sample demonstrates how to use the GATT DFU SMP Service Client to connect to an SMP Server and send a simple echo command. The response, which is received as CBOR-encoded data, is decoded and printed.


This sample does not provide the means to program a device using DFU. It demonstrates the communication between SMP Client and SMP Server.


The sample supports the following development kits:

Hardware platforms


Board name

Build target

nRF5340 DK



nrf5340dk_nrf5340_cpuapp_ns nrf5340dk_nrf5340_cpuapp

nRF52 DK




nRF52840 DK




When built for an _ns build target, the sample is configured to compile and run as a non-secure application with Cortex-M Security Extensions enabled. Therefore, it automatically includes Trusted Firmware-M that prepares the required peripherals and secure services to be available for the application.

The sample also requires a device running mcumgr with transport protocol over Bluetooth® Low Energy, for example, another development kit running the SMP Server Sample.


This sample does not program the device using DFU.


After connecting, the sample starts MTU size negotiation, discovers the GATT database of the server, and configures the DFU SMP Client. When the configuration is complete, the sample is ready to send SMP commands.

To send an echo command, press Button 1 on the development kit. The sent string contains a number that is automatically incremented. This way, you can verify if the correct response is received. The response is decoded using the zcbor library and displayed after that.

User interface

Button 1:

Send an echo command.

Building and running

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

When built as firmware image for the _ns build target, the sample has Cortex-M Security Extensions (CMSE) enabled and separates the firmware between Non-Secure Processing Environment (NSPE) and Secure Processing Environment (SPE). Because of this, it automatically includes the Trusted Firmware-M (TF-M). To read more about CMSE, see Processing environments.

To build the sample with Visual Studio Code, follow the steps listed on the How to build an application page in the nRF Connect for VS Code extension documentation. See Building and programming an application for other building and programming scenarios and Testing and debugging an application for general information about testing and debugging in the nRF Connect SDK.


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

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

  3. Reset the kit.

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

  5. Program the SMP Server Sample to another development kit. See the documentation for that sample only in the section “Building the sample application”. When you finish building the SMP Server Sample, you can program it to the kit by calling:

    west flash
  6. Observe that the kits connect. When service discovery is completed, the event logs are printed on the Central’s terminal. If you connect to the Server with a terminal emulator, you can observe that it prints “connected”.

  7. Press Button 1 on the Client. Observe messages similar to the following:

    Echo test: 1
    Echo response part received, size: 28.
    Total response received - decoding
    {_"r": "Echo message: 1"}
  8. 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:

It uses the following Zephyr libraries:

  • include/zephyr/types.h

  • boards/arm/nrf*/board.h

  • Kernel Services:

    • include/kernel.h

  • Bluetooth APIs:

    • include/bluetooth/bluetooth.h

    • include/bluetooth/gatt.h

    • include/bluetooth/hci.h

    • include/bluetooth/uuid.h

In addition, it uses the following external library that is distributed with Zephyr:

The sample also uses the following secure firmware component: