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:

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

Enabling non-halting debugging with Cortex-M Debug Monitor 

The debugging process can run in two modes. The halt-mode debugging stops the CPU when a debug request occurs. The monitor-mode debugging lets a CPU debug parts of an application while crucial functions continue. Unlike halt-mode, the monitor-mode is useful for scenarios like PWM motor control or Bluetooth, where halting the entire application is risky. The CPU takes debug interrupts, running a monitor code for J-Link communication and user-defined functions.

Use the following steps to enable monitor-mode debugging in the nRF Connect SDK:

In the application configuration file, set the Kconfig options CONFIG_CORTEX_M_DEBUG_MONITOR_HOOK and CONFIG_SEGGER_DEBUGMON.
Attach the debugger to the application.
Depending on debugger you are using, enable monitor-mode debugging:

For nRF Debug in nRF Connect for VS Code extension, enter -exec monitor exec SetMonModeDebug=1 in the debug console.

For debugging using Ozone, enter Exec.Command("SetMonModeDebug = 1"); in the console.

For more information about monitor-mode debugging, see Zephyr’s Cortex-M Debug Monitor documentation and SEGGER’s Monitor-mode Debugging documentation.

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:

You can use Debug libraries for any of your applications, for example to measure CPU load or trace hardware peripheral events on pins.
You can use nRF Profiler to measure performance and debug applications without introducing big performance overhead. This option requires introducing additional code changes: your application must register profiler events and log their occurrences.
When working with the SEGGER J-Link with the RTT feature, you can use the Ethernet over RTT driver, which is useful for handling data transfer.
The Thread protocol implementation offers pre-built libraries with debug symbols.
The Zigbee protocol implementation offers Zigbee ZBOSS OSIF and Zigbee shell with custom Kconfig options that you can set for debugging.

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.