Testing and debugging an application

For simplicity, this guide will refer to both samples and applications as “applications”.

Follow the instructions in the testing section of the application’s documentation to make sure that the application runs as expected.

Information about the current state of the application is usually provided through the LEDs or through UART, or through both. See the user interface section of the application’s documentation for description of the LED states or available UART commands.

How to connect with nRF Connect for VS Code extension

The nRF Connect for VS Code extension includes an integrated serial port and RTT terminal, which you can use to connect to your board. For detailed instructions, see How to connect to the terminal on the nRF Connect for Visual Studio Code documentation site.

How to connect with PuTTY

To see the UART output, connect to the development kit (DK) with a terminal emulator, for example, PuTTY.

Connect with the following settings:

  • Baud rate: 115200

  • 8 data bits

  • 1 stop bit

  • No parity

  • HW flow control: None

If you want to send commands through UART, make sure to configure the required line endings and turn on local echo and local line editing:

PuTTY configuration for sending commands through UART

UART can also be used for logging purposes as one of the logging backends.

How to use RTT

To view the logging output using Real Time Transfer (RTT), modify the configuration settings of the application to override the default UART console:


SEGGER’s J-Link RTT can also be used for logging purposes as one of the logging backends.


SEGGER’s J-Link RTT is part of the J-Link Software and Documentation Pack. You must have this software installed on your platform to use RTT.

Connecting using RTT

To run RTT on your platform, complete the following steps:

  1. From the J-Link installation directory, open the J-Link RTT Viewer application:

    • On Windows, the executable is called JLinkRTTViewer.exe.

    • On Linux, the executable is called JLinkRTTViewerExe.

  2. Select the following options to configure your connection:

    • Connection to J-Link: USB

    • Target Device: Select your IC from the list

    • Target Interface and Speed: SWD, 4000 KHz

    • RTT Control Block: Auto Detection

    Example of RTT Viewer configuration
  3. Click OK to view the logging output from the device.

How to connect with Serial Terminal

You can also use the nRF Connect Serial Terminal app, which is part of nRF Connect for Desktop to send commands through UART.

The nRF Connect Serial Terminal app is also used to establish LTE communication with the cellular modem through AT commands, and it also displays the UART output. To connect to an nRF91 Series DK with Serial Terminal, perform the following steps:

  1. Launch the Serial Terminal app.

  2. Connect an nRF91 Series DK to the PC with a USB cable.

  3. Power on the DK.

  4. Click Select Device and select the particular kit entry from the drop-down list in the Serial Terminal.

  5. Observe that the Serial Terminal app starts AT communication with the modem of the nRF91 Series DK and shows the status of the communication in the display terminal. The app also displays any information that is logged on UART.


    In the case of the nRF9160 DK, the reset button must be pressed to restart the device and start the application.

Debugging an application

To debug an application, set up the debug session as described in the How to debug an application section in the nRF Connect for VS Code extension documentation. nRF Debug is the default debugger for nRF Connect for VS Code extension.

If you use a multi-core SoC, for example from the nRF53 Series, and you only wish to debug the application core firmware, a single debug session is sufficient. To debug the firmware running on the network core, you need to set up two separate debug sessions: one for the network core and one for the application core. When debugging the network core, the application core debug session runs in the background and you can debug both cores if needed.

Complete the following steps to start debugging the network core:

  1. Set up sessions for the application core and network core as mentioned in the How to debug applications for a multi-core System on Chip section in the nRF Connect for VS Code extension documentation.

  2. Select the appropriate CPU for debugging in each session, corresponding to the application core and the network core of your SoC, respectively.

  3. Once both sessions are established, execute the code on the application core.

    The startup code releases the NETWORK.FORCEOFF signal to start the network core and allocates the necessary GPIO pins for it.

  4. Start code execution on the network core in the other debug session.

If you want to reset the network core while debugging, make sure to first reset the application core and execute the code.

For more information about debugging in the nRF Connect for VS Code extension, for example testing and debugging with custom options, see the Debugging overview and other guides in the debugging section of the extension documentation.

Debugging secure and non-secure firmware

When using a build target with CMSE enabled (_ns), by default you can only debug firmware in the non-secure environment of the application core firmware.

To debug firmware running in the secure environment, you need to build Trusted Firmware-M with debug symbols enabled and load the symbols during the debugging session. To build Trusted Firmware-M with debug symbols, set the CONFIG_TFM_CMAKE_BUILD_TYPE_RELWITHDEBINFO Kconfig option.

nRF Debug in the nRF Connect for VS Code extension automatically loads the Trusted Firmware-M debug symbols.

Debug configuration

When you are following the How to debug an application process in the nRF Connect for VS Code extension and select the Enable debug options checkbox in the Add Build Configuration page, the following Kconfig options are set to y when you add the configuration:

  • CONFIG_DEBUG_OPTIMIZATIONS - This option limits the optimizations made by the compiler to only those that do not impact debugging.

  • CONFIG_DEBUG_THREAD_INFO - This option adds additional information to the thread object, so that the debugger can discover the threads. This will work for any debugger.

You can also set these options to y manually. There are many more Kconfig options for debugging that are specific to different modules. For details, see the respective documentation pages of the modules.

Debug build types

Some applications and samples provide a specific build type that enables additional debug functionalities. You can select build types when you are configuring the build settings.

Debug logging

You can use logging system to get more information about the state of your application. Logs are integrated to many various modules and subsystems in the nRF Connect SDK and Zephyr. These logs are visible once you configure logger for your application.

You can also configure log level per logger module, for example to get more information about a given subsystem. See Logging in nRF Connect SDK for details on how to enable and configure logs.

Debug libraries

The nRF Connect SDK also provides several libraries and drivers for debugging different components. For example:

Debugging stack overflows

One of the potential root causes of fatal errors in an application are stack overflows. Read the Stack Overflows section on the Fatal Errors page in the Zephyr documentation to learn about stack overflows and how to debug them.

You can also use a separate module to make sure that the stack sizes used by your application are big enough to avoid stack overflows. One of such modules is for example Zephyr’s Thread analyzer.

Debug tools

The main recommended tool for debugging in the nRF Connect SDK is nRF Debug of the nRF Connect for VS Code extension. The tool uses Microsoft’s debug adaptor and integrates custom debugging features specific for the nRF Connect SDK.

  • When working with the nRF Connect for VS Code extension, use nRF Debug after adding the required Kconfig options to the prj.conf file.

  • If you are working from the command line, you can use west with nRF Debug. For details, read the Debugging with west debug section on the Building, Flashing and Debugging page in the Zephyr documentation.

A useful tool for debugging the communication over Bluetooth and mesh networking protocols, such as Thread or Zigbee, is the nRF Sniffer for 802.15.4. The nRF Sniffer allows you to look into data exchanged over-the-air between devices.