minimal-c-cortex-m

A minimal Arm Cortex-M example, including semihosting, for my own reference. not really recommended for anything beyond toy usage

# necessary tools
❯ sudo apt install openocd gcc-arm-none-eabi

# start openocd in one terminal
❯ make debug

# in another terminal, run this command to build with semihosting for default
# board (stm32f4discovery), and start gdb
❯ ENABLE_STDIO=1 ENABLE_SEMIHOSTING=1 make gdb

# 'continue' in gdb, you should see 'Hello there!' in openocd

For RTT, run this in another terminal (and set the ENABLE_RTT=1 variable at build):

# keep retrying until connection is made. also reopen on aborted connection
# on normal exit (eg ctrl+] then ctrl+d), will terminate the loop
❯ until telnet localhost 9090; do sleep 0.5; done

Build and test with Memfault

Assumes all prerequisites are installed (openocd, gcc-arm).

1. connect the STM32F4 Discovery board

2. start the debug server:

   ❯ make debug
   Open On-Chip Debugger 0.11.0-rc2
   Licensed under GNU GPL v2
   For bug reports, read
           http://openocd.org/doc/doxygen/bugs.html
   Info : The selected transport took over low-level target control. The results might differ compared to plain JTAG/SWD
   Info : clock speed 2000 kHz
   Info : STLINK V2J25M14 (API v2) VID:PID 0483:374B
   Info : Target voltage: 2.890490
   Info : stm32f4x.cpu: hardware has 6 breakpoints, 4 watchpoints
   Info : starting gdb server for stm32f4x.cpu on 3333
   Info : Listening on port 3333 for gdb connections
   semihosting is enabled
   
   Info : Listening on port 6666 for tcl connections
   Info : Listening on port 4444 for telnet connections

3. in another terminal, build and start gdb (this also flashes the binary):

   ❯ ENABLE_STDIO=1 ENABLE_RTT=1 ENABLE_MEMFAULT=1 ENABLE_MEMFAULT_DEMO=1 make gdb

4. in a 3rd terminal, start trying to connect telnet to the RTT server:

   ❯ until telnet localhost 9090; do sleep 0.5; done

5. in the gdb terminal (2nd terminal), run c or continue to start the board

6. you should see output in the RTT terminal. press enter a few times to start using the Memfault demo console

QEMU for stm32f407 discovery

You can run the application in QEMU, here's what I did:

1. download this forked version of QEMU and unpack:

   ❯ wget https://github.com/xpack-dev-tools/qemu-arm-xpack/releases/download/v2.8.0-12/xpack-qemu-arm-2.8.0-12-linux-x64.tar.gz

2. build the application with semihosting enabled:

   ❯ ENABLE_STDIO=1 ENABLE_SEMIHOSTING=1 make

3. start qemu in one window:

   # the -S halts the cpu at the first instruction
   ❯ ~/Downloads/xpack-qemu-arm-2.8.0-12/bin/qemu-system-gnuarmeclipse \
     -cpu cortex-m4 \
     -machine STM32F4-Discovery \
     -gdb tcp::3333 \
     -nographic \
     -S \
     -semihosting-config enable=on,target=gdb \
     -kernel build/main.elf

4. start gdb in another window:

   ❯ gdb-multiarch -q build/main.elf -ex 'target remote :3333'

5. continuing, you should see the startup print in the gdb window