hiviah/nrf52840-monitor-mode-gcc-gdb-only

BLE monitor mode port for nRF52840 that requires only gcc, make and GDB (Ozone and SES not needed)

Example of monitor mode on nRF 52840 DK with only gcc and gdb (no Ozone or SES needed)

This is a port of BLE monitor mode example of nRF to pure make+gcc build, which requires only GDB to debug. This means that Segger Embedded Studio or Ozone is not required to debug. Makes possible to debug with other debuggers, such as CLion.

CMakeLists.txt is only just for CLion to resolve symbols, see build below.

Build

Copy this whole directory into nRF5_SDK_17.x.x_xxxx/examples/ble_peripheral/

make -C pca10056/s140/armgcc/ MMD=1
make flash
make flash_softdevice

To build with monitor mode, use make MMD=1, otherwise it's built without it.

By default, the build is unoptimized with -O0. You may want to change that to -Os or -Og (look at the Makefile).

Using in GDB or anything that is on top of GDB server

This requires JLink Plus/JTrace, the onboard JLink on the DK board won't suffice.

Run JLinkGDBServerExe, then connect to it from gdb via target extended-remote :2331 (or equivalent setting e.g. in CLion as "Remote debug")

To activate monitor mode on the JLink use this in GDB prompt:

    mon exec SetMonModeDebug = 1
    mon exec SetMonModeVTableAddr=0x27000
    monitor reset

The 0x27000 address is VTOR address, which can be found from linker script or looking into GDB at the VTOR address:

    >>> p __isr_vector
    $1 = {<text variable, no debug info>} 0x27000 <__isr_vector>

Now you can set breakpoints and step through code without softdevice crashing on breakpoint.

Seems that if you want to look at RTT, RTT viewer needs to be run before GDB server, then use in GDB.

How it works internally

If you look at the start of Makefile, there are few files added, that override DebugMon_Handler interrupt. You also need at the beginning of main set the priority of DebugMonitor_IRQn like

NVIC_SetPriority(DebugMonitor_IRQn, _PRIO_SD_LOW);

These things combined with the monitor commands from GDB make JLink/JTrace send periodic interrupts to keep the softdevice alive even when rest of CPU is stopped on breakpoint.

References

• https://wiki.segger.com/nRF52_Series_Devices#Monitor_Mode_Debugging_on_Nordic_nRF52
• https://www.segger.com/products/debug-probes/j-link/technology/monitor-mode-debugging/
• https://devzone.nordicsemi.com/nordic/nordic-blog/b/blog/posts/segger-embedded-studio-part-2-monitor-mode-debuggi
• https://devzone.nordicsemi.com/nordic/nordic-blog/b/blog/posts/monitor-mode-debugging-with-j-link-and-gdbeclipse