Monitor mode debugging does not work

[boz]

I am evaluating the monitor mode debugging support in nRF Connect SDK with nRF52840-DK and an arbitrary BLE sample (like peripheral_uart). My SDK/toolchain version is 3.1.1 and J-Link version v8.60 on WSL.

I added the Kconfig options as described in https://docs.nordicsemi.com/bundle/ncs-3.1.1/page/nrf/test_and_optimize/debugging.html#enabling_non-halting_debugging_with_cortex-m_debug_monitor to enable monitor mode, but it did not have any effect. After I start debugging with the VS Code extension, I run the example and view the BLE advertisement from the nRF Connect app. When I pause the execution, the advertisements stop and they restart after resuming. Same with the connection: it breaks when I pause the execution. This is the same behavior without monitor mode. Is this supposed to work as I expect? If not, how to verify that the monitor mode is working? There are tutorials to enable the monitor mode but no guides to verify it.

[Vidar Berg]

Hello,

Please try to cherry-pick this commit: https://github.com/zephyrproject-rtos/segger/commit/50892fdbcf2f570e67baa72b8894a66b16946f72. This fix is needed, and was not included in this SDK version.

Best regards,

Vidar

[boz]

Thank you, it solved the issue. FYI I just pulled the latest version as the cherry pick caused merge conflicts.

I have another related issue: It works with nRF52840-DK but fails with nRF54L15-DK. I know that the M33 core was previously not supported by monitor mode, but it was added in a later J-Link release in late 2025. However the J-Link OB firmware of the nRF54L15-DK seems to be outdated (July 2025). I checked the firmware update with J-Link tools but it shows up to date. Do you know if there will be an update for the nRF54 OB firmware to support monitor mode?

[Vidar Berg]

Thanks for the update. Regarding the error message, this is a limitation imposed bt the on-board debugger FW. I can't comment on whether support for this will be added future, but I have reported this internally to indicate that there is demand for it.