Lock-ups and freezes w/ an STM32F4Discovery board with ST-Link v2, OpenOCD and GDB

This post is especially an outing of my joy now that I have finally resolved my issue debugging the STM32F4xx using OpenOcd.

There exist many blogpost, tutorials, manuals about setting up an STM32F4Discovery + OpenOcd + gdb combo.
I won't explain how to do it here, but it is an awesome, powerful and very cheap set-up.

I chose the STM32F4Discovery board as the demo board for my course on embedded C, because it's cheap, full of features and has on-board debugging hardware.

Writing a custom crt0, I ran into problems however. Stepping through the early init code, assembly instruction per assembly instruction, gdb would just lock-up, freeze, at random moments. Not always on the same instruction, not always at the same moment. I could also set a breakpoint on a certain line, then type 'c' to continue, and the debugger would never break, or even respond to a Ctrl+C.

Furthermore, using st-util, from the texane github (https://github.com/texane/stlink) would work.It's awefully slow (especially when using the split layout in gdb-tui, that disassbles on-the-fly)

Digging up a mailinglist message from about a year ago (http://www.mail-archive.com/openocd-devel@lists.sourceforge.net/msg01605.html) I finally found the solution to the problem.

The STM32F4 features a "DBGMCU" register,
which by default will leave the debug function of the mcu in an unpowered state when in standby, stop or sleep mode.

Changed my openocd.cfg file to:

 #Include configs from openocd  
 source [find board/stm32f4discovery.cfg]  
 source [find mem_helper.tcl]  
 # Disable power-saving for debug circuitry  
 $_TARGETNAME configure -event reset-init {  
   # allow debugging during sleep/stop/standby modes:  
   # set DBG_SLEEP, DBG_STOP and DBG_STANDBY bits in DBGMCU_CR  
   mmw 0xe0042004 0x7 0x0  
 }  

Then, additionally, it is very important to reset the board in a clean way.
For this, issue a "monitor reset halt" command. I automated it by adding it to my .gdbinit file:

 tar ext :3333
 monitor halt
 file out/main.elf
 load out/main.elf
 monitor reset halt
 stepi

This config will make gdb(tui) automatically connect to openocd, then stop the cpu, flash the ELF file, load symbols from the elf file, reset the mcu in a clean way and then step to the first instruction to be executed.
This leaved the cpu halted, at the first assembly instruction. From there, you can set-up breakpoints, or just hit 'c' to start running. 

Once I had taken care of both these topics, gone was my trouble!

Happy debugging!