Electronic – Start firmware with blackmagic probe after flashing

I've got a blackmagic probe and a board with an STM32F412KB microcontroller. Flashing the board works great (and fast!) – but I can't find a way to start the firmware after the flash (aside from manually pushing the reset button on the board).

Context:
I'm trying to support the blackmagic probe in our free microcontroller IDE (https://embeetle.com). Embeetle can already flash firmware using this probe, but the user might forget to push the reset button on his microcontroller board. Displaying a warning message is our last-resort solution. For now, I try to get the blackmagic probe to work in the same way as the other probes: you click "flash" and you see your firmware running instantly.

1. Hardware Setup

1.1 probe

I've got a blackmagic probe (see https://1bitsquared.com/products/black-magic-probe and https://github.com/blacksphere/blackmagic/wiki) for flashing and debugging purposes. The probe version is:

Black Magic Probe (Firmware v1.6.1-1-g74af1f5) (Hardware Version 3)

1.2 microcontroller board

The microcontroller board is self-designed:

Here are the relevant parts of the schematic. First of all, the microcontroller:

Here is the physical reset button:

And this is the flash/debug-connector (to be connected to the blackmagic probe):

As you can see, the flash/debug-connector follows the 10-pin Cortex-debug standard, and can be used for both JTAG and SWD flashing/debugging.

2. Software setup

2.1 toolchain

I've got the following toolchain installed on my Windows 10 PC:

gcc-arm-none-eabi-9-2020-q2-update-win32

Which I recently downloaded from:

https://developer.arm.com/tools-and-software/open-source-software/developer-tools/gnu-toolchain/gnu-rm/downloads

With this toolchain, I built the microcontroller project. I also use the arm-none-eabi-gdb.exe tool to interact with the blackmagic probe (but more on that later).

2.2 Microcontroller project

I've got a microcontroller project at the following location on my Windows 10 PC:

C:/Users/Kristof/beetle_projects/beetle_l412kb

After compilation, the build output ends up in:

C:/Users/Kristof/beetle_projects/beetle_l412kb/build

For example, my .elf file is at:

C:/Users/Kristof/beetle_projects/beetle_l412kb/build/application.elf

Now I open a console and navigate to the toplevel project folder. Then I invoke the makefile its flash target (we'll get to the makefile in a minute):

$ cd C:/Users/Kristof/beetle_projects/beetle_l412kb

$ make flash -C build -f ../config/makefile "COM=\\.\COM16"

I basically invoke the flash target from the makefile and pass it the $(COM) variable to inform it where the blackmagic probe is connected.

The relevant part of the makefile is:

ELF_FILE = application.elf
GDB_FLASHFILE = ../config/.gdbinit

GDB_FLASHFLAGS = \
    -n \
    -batch \
    -x $(GDB_FLASHFILE) \
    -ex "flash-remote $(ELF_FILE) $(COM)" \

.PHONY: flash
flash: $(ELF_FILE)
    arm-none-eabi-gdb $(GDB_FLASHFLAGS)

The flash target launches arm-none-eabi-gdb and passes it the $(GDB_FLASHFLAGS). These flags point to the following .gdbinit file where the actual flash instructions are listed inside the flash-remote() user-defined function:

# This user-defined function flashes the given
# .elf file to the microcontroller and should then
# exit.
define flash-remote
  echo \n
  echo \n--------------------------------------------------------------------------------
  echo \n  Run flash-remote function:
  echo \n  ==========================
  echo \n  flash-remote(
  echo \n      elf_file = $arg0,
  echo \n      com_port = $arg1,
  echo \n  )
  echo \n--------------------------------------------------------------------------------
  echo \n
  target extended-remote $arg1
  monitor version
  monitor swdp_scan
  attach 1
  file $arg0
  load
  start
  detach
  quit
end

3. The problem explained

The flashing works great (it's super fast!) and the firmware runs when done. I can see the firmware running because the LED on the board is blinking – that's basically what the firmware does. Unfortunately, the flash-remote() function hangs at the start command, probably because GDB is waiting for a breakpoint that never comes.

The goal here is not to debug, but simply to flash the firmware. So I want to exit after the flash. This is what I tried so far:

1. I simply delete the start command. Now the function flash-remote() exits cleanly. However, to start the firmware, I need to push the reset button on the board. I'd like to start the firmware without manual intervention.

2. Replacing start with monitor hard_srst seems to work at first glance (function continues, doesn't hang). However, it causes something to malfunction. Even after pushing the reset button on the board, the firmware doesn't start (LED doesn't blink).

I can't find a way to start the firmware after the flash (aside from manually pushing the reset button on the board).