Cortex-M Bare-Metal STM32 (No IDEs, No HALs)

A from-scratch bare-metal firmware project for the STM32F103RB (ARM Cortex-M3) Nucleo board.
This repo shows how to build embedded systems without vendor IDEs or auto-generated HAL code : using only Makefiles, linker scripts, and OpenOCD.

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Why This Project?

Most STM32 tutorials start with CubeIDE or HAL libraries. Here, everything is done manually:

• Custom startup code (vector table + Reset_Handler).
• Hand-written linker script.
• Minimal Makefile toolchain.
• Direct register access (no HAL, no CMSIS).
• Flash + debug using OpenOCD + GDB.

This repo proves end-to-end understanding of ARM Cortex-M architecture and the firmware build pipeline.

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Project Overview

• Target MCU: STM32F103RB (Cortex-M3, ARMv7-M)
• Board: Nucleo-F103RB
• Peripherals Used: GPIO (LED blink on PA5)
• Workflow: GCC ARM toolchain + Make + OpenOCD

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Features Implemented

• Vector table at 0x08000000
• Reset handler: .data copy, .bss zero, call main()
• Custom linker script with defined memory regions
• GPIOA clock enable + PA5 LED toggle
• Makefile build system
• Flash/debug with OpenOCD & GDB

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Repository Structure

• src/ – Application C sources (main.c)
• startup/ – Startup assembly (startup.s)
• include/ – Optional headers
• linker.ld – Custom linker script
• Makefile – Build system
• build/ – Generated build artifacts (.o, .elf, .bin)
• docs/ – In-depth technical explanations
  • startup.md
  • linker.md
  • gpio.md
  • toolchain.md
  • architecture.md
• README.md

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What You’ll Learn

• How Cortex-M boots after reset.
• Why startup code matters (stack pointer, Reset_Handler).
• How .text, .data, .bss, and stack are placed in memory.
• How to control peripherals via RCC and GPIO registers.
• How OpenOCD + GDB interact with the MCU via SWD.

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Toolchain

• arm-none-eabi-gcc → compile & link
• arm-none-eabi-objcopy → ELF → BIN/HEX
• OpenOCD → flash binary & debug bridge
• GDB → live debugging

Pipeline:
source.c ---> gcc ---> .o ---> ld ---> firmware.elf ---> objcopy ---> firmware.bin ---> OpenOCD ---> MCU

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Build & Flash

1. Build project

make

Generates build/firmware.elf → ELF executable build/firmware.bin → raw binary for flashing

2. Flash to board (via ST-Link)

make flash

3. Run + inspect binary size

make run

4. Debug with GDB over OpenOCD

bash

make gdb

Opens OpenOCD GDB server.

Launches arm-none-eabi-gdb connected to :3333.

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Further Reading

This repo doubles as a learning resource.

• Startup & Reset Flow
• Linker Script Deep Dive
• GPIO & Peripheral Registers
• Toolchain Explained
• ARM Cortex-M3 Architecture Notes

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Who Is This Repo For?

• 🔹 Beginners who want to learn Cortex-M bare metal.
• 🔹 Engineers tired of opaque IDEs/HALs.
• 🔹 Recruiters who want proof of deep embedded expertise.

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Future Work

• Add SysTick timer for delay (instead of busy loops).
• Implement UART driver (printf over USART2).
• Explore interrupts & NVIC.
• Extend docs into a mini Cortex-M bare-metal handbook.

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Author

Built with curiosity and first-principles thinking by Ronit.
Always learning, always building.