Overview

Bare-metal C firmware for the STM32F401RE (Nucleo board, ARM Cortex-M4). Everything between power-on and main() is written by hand: the startup code, the interrupt vector table and the linker script. The point of the project is to understand what the vendor libraries normally hide.

At a glance

Board

Nucleo STM32F401RE (Cortex-M4)

Language

C, GNU linker script

Button

B1 on PC13, input, rising-edge EXTI

LED

LD2 on PA5, output

Toolchain

arm-none-eabi-gcc, OpenOCD, ST-Link

The interrupt path

1. Button on PC13
2. EXTI line 13
3. NVIC IRQ 40
4. EXTI15_10_IRQHandler
5. event_count += 5
6. Main loop blinks LED

What it shows

• Startup and vector table. startup.c has a hand-written interrupt vector table and Reset_Handler. It copies .data from flash to SRAM and zeroes .bss before calling main().
• Custom linker script. linker.ld defines the FLASH (512K) and SRAM (96K) regions and places .text, .rodata, .data and .bss.
• Direct register access. RCC, GPIO, SYSCFG, EXTI and NVIC are configured through their registers directly, using the datasheet and reference manual.
• Safe sharing with the interrupt. The handler and the main loop share a volatile counter. The main loop reads and clears it inside a short interrupt-disabled section (cpsid i / cpsie i).
• Atomic LED writes. The LED is set and cleared through the GPIO BSRR register. It is a single write, so there is no read-modify-write that an interrupt could split.

How it works

1. main() enables the clocks for SYSCFG, GPIOA and GPIOC, then sets PA5 as output and PC13 as input.
2. SYSCFG routes PC13 to EXTI line 13. EXTI is set to rising edge and the line is unmasked. NVIC IRQ 40 is enabled.
3. On a button event, EXTI15_10_IRQHandler clears the pending bit and adds 5 to event_count.
4. The main loop copies and clears event_count with interrupts disabled, then blinks the LED that many times.

Build and flash

arm-none-eabi-gcc \
  -mcpu=cortex-m4 \
  -mthumb \
  -nostdlib \
  -T linker.ld \
  startup.c main.c \
  -o blink.elf

openocd -f interface/stlink.cfg -f target/stm32f4x.cfg \
  -c "program blink.elf verify reset exit"