c-examples

Minimal bare-metal C examples for OnMCU. Most count upwards and stream the count over RTT on real hardware in the cloud; one logs over a serial port instead. Each signals success to the host via ARM semihosting so the run exits 0. The structure mirrors the [rust-examples](../rust-examples) repository.

cd nucleo-h743zi
make run        # build + flash + run on a real NUCLEO-H743ZI, returns 0 on success

make run builds the ELF and invokes onmcu run --board <BOARD> --file <elf>, which uploads the firmware, runs it on real hardware, streams the RTT log back to your terminal, and exits 0 on success.

Prerequisites

1. Arm GNU toolchain: arm-none-eabi-gcc (and objcopy, size) on PATH.

2. GNU make.

3. OnMCU CLI: install the onmcu binary, then authenticate once:

   onmcu login            # stores your API key in the OS keyring
   onmcu list-boards      # confirm which boards are available

   Create your API key under app.onmcu.com/settings. See github.com/onmcu/onmcu-rs for details.

Layout

lib/                     reusable, board-independent libraries
  rtt/                   minimal SEGGER-RTT compatible up channel (target -> host)
  semihosting/           ARM semihosting exit, for the run's return code
common.mk                shared build rules (toolchain, lib build, run target)
nucleo-h743zi/           one example crate per board
  Makefile               sets NAME, BOARD, linker script, MCU flags, sources
  stm32h743.ld           memory map + sections
  startup_stm32h743.c    vector table + reset handler
  main.c                 the counter

Semihosting is compiled into every example, and RTT into all but the serial-only one (which sets USE_RTT = 0), so adding a new board only means adding a board directory with its Makefile, linker script, startup and main.c.

How it works

• RTT (lib/rtt): a statically initialised control block placed at the start of RAM. The debugger OnMCU attaches scans RAM for the signature, then drains channel 0 and prints it as plain text. Only a single blocking up channel is implemented, which is all the examples need.
• Semihosting (lib/semihosting): sys_exit(code) issues SYS_EXIT_EXTENDED via bkpt 0xAB. The debugger services the trap and ends the session with that code (0 = success, non-zero = failure).
• Serial (nucleo-f401re-serial): logs over USART2 (PA2/PA3, the ST-LINK virtual COM port) at 115200 8N1 instead of RTT. It sets USE_RTT = 0 and runs with --logging serial, so OnMCU captures the board's serial output.

Boards

Directory	Board	MCU	Core	Logging
nucleo-h743zi	NUCLEO-H743ZI	STM32H743ZIT6	Cortex-M7	RTT
nucleo-f401re	NUCLEO-F401RE	STM32F401RET6	Cortex-M4F	RTT
nucleo-f401re-serial	NUCLEO-F401RE	STM32F401RET6	Cortex-M4F	serial (USART2)
nucleo-f439zi	NUCLEO-F439ZI	STM32F439ZIT6	Cortex-M4F	RTT
nucleo-wb55rg	NUCLEO-WB55RG	STM32WB55RGV6	Cortex-M4F	RTT
nucleo-wl55jc	NUCLEO-WL55JC	STM32WL55JC	Cortex-M4	RTT