Cortex-M Debug Monitor

Monitor mode debugging is a Cortex-M feature, that provides a non-halting approach to debugging. With this it’s possible to continue the execution of high-priority interrupts, even when waiting on a breakpoint. This strategy makes it possible to debug time-sensitive software, that would otherwise crash when the core halts (e.g. applications that need to keep communication links alive).

Zephyr provides support for enabling and configuring the Debug Monitor exception. It also contains a ready implementation of the interrupt, which can be used with SEGGER J-Link debuggers.


Configure this module using the following options.

  • CONFIG_CORTEX_M_DEBUG_MONITOR_HOOK: enable the module. This option, by itself, requires an implementation of debug monitor interrupt that will be executed every time the program enters a breakpoint.

With a SEGGER debug probe, it’s possible to use a ready, SEGGER-provided implementation of the interrupt.

  • CONFIG_SEGGER_DEBUGMON: enables SEGGER debug monitor interrupt. Can be used with SEGGER JLinkGDBServer and a SEGGER debug probe.


When monitor mode debuging is enabled, entering a breakpoint will not halt the processor, but rather generate an interrupt with ISR implemented under z_arm_debug_monitor symbol. CONFIG_CORTEX_M_DEBUG_MONITOR_HOOK config configures this interrupt to be the lowest available priority, which will allow other interrupts to execute while processor spins on a breakpoint.

Using SEGGER-provided ISR

The ready implementation provided with CONFIG_SEGGER_DEBUGMON provides functionality required to debug in the monitor mode using regular GDB commands. Steps to configure SEGGER debug monitor:

  1. Build a sample with CONFIG_CORTEX_M_DEBUG_MONITOR_HOOK` and CONFIG_SEGGER_DEBUGMON configs enabled.

  2. Attach JLink GDB server to the target. Example linux command: JLinkGDBServerCLExe -device <device> -if swd.

  3. Connect to the server with your GDB installation. Example linux command: arm-none-eabi-gdb --ex="file build/zephyr.elf" --ex="target remote localhost:2331".

  4. Enable monitor mode debugging in GDB using command: monitor exec SetMonModeDebug=1.

After these steps use regular gdb commands to debug your program.

Using other custom ISR

In order to provide a custom debug monitor interrupt, override z_arm_debug_monitor symbol. Additionally, manual configuration of some registers is required (see debug monitor sample).