STM32F401CCU6-Blink-Bare-Metal

RTC-wakeup blink demo for STM32F401CCU6. On each RTC wakeup event it flashes the PC13 LED three times rapidly and prints the current date and time via USART6 at 115200 baud. Register-level, no HAL.

Features

• Register-level, bare-metal firmware (no HAL, no CMSIS-DSP)
• System clock 100 MHz from HSE 25 MHz via PLL (25 / 15 x 120 / 2)
• RTC with LSE oscillator, wakeup timer period ~1 s
• Three fast LED flashes (30 ms each toggle) on each RTC wakeup
• USART6 output: date and time in DD.MM.YYYY HH:MM:SS format at 115200 baud
• Instruction & data cache enabled, prefetch active, 2 wait states

Hardware

Component	Detail
MCU	STM32F401CCU6 (Cortex-M4, 256 KB Flash, 64 KB RAM)
Clock	HSE 25 MHz -> PLL -> 100 MHz SYSCLK
Cache	2 WS, ICache + DCache + prefetch
LED	PC13 (on-board), push-pull output
RTC	LSE 32768 Hz, wakeup timer
UART	USART6 on PA11, 115200 baud, 8N1
Debug	SWD on PA13/PA14

Build (Makefile)

Prerequisites

• ARM GCC toolchain (arm-none-eabi-gcc 13.x or later)
• GNU Make
• curl, unzip

Build

make

All third-party files (CMSIS headers, SVD, IDE startup files) are downloaded automatically at build time.

Output in build/: project.elf, project.hex, project.bin.

Targets

Target	Description
make	Download deps + build
make download_cmsis	Download CMSIS core + STM32F4xx headers
make download_svd	Download STM32F401.svd
make download_iar_startup	Download IAR startup file
make download_mdk_startup	Download MDK-ARM startup file
make download_licenses	Download third-party license files
make download	Download all of the above
make program	Flash via ST-LINK
make jprogram	Flash via J-Link
make clean	Remove build artifacts
make clean_all	Remove build artifacts + all downloaded files

Build (IDE)

Important: All third-party dependencies (CMSIS headers, startup files, SVD) are downloaded at build time via make. Run make download from the project root before opening the project in any IDE. Without this step the IDE project will not compile.

Each IDE folder has a README with setup and CLI build instructions.

IDE	Project	CLI Tool
SEGGER Embedded Studio	ide/SES/Project.emProject	emBuild.exe
Keil MDK-ARM	ide/MDK-ARM/Project.uvprojx	UV4.exe
IAR EWARM	ide/EWARM/Project.eww	IarBuild.exe

Project Structure

.
+-- main.c                        Application + init
+-- main.h                        Macros, clock tree, GPIO helpers
+-- crt0.c                        __libc_init_array stub (SES bundled GCC)
+-- Makefile                      GCC build + download targets
+-- STM32F401CCUX_FLASH.ld        GNU linker script
+-- LICENSE                       MIT License + third-party notice
+-- stm32f401cc.jflash            J-Flash project
+-- project.jdebug                J-Link debugger project
+-- CMSIS/                        Downloaded at build time (gitignored)
+   +-- core/                     ARM CMSIS headers + LICENSE.txt
+   +-- device/                   STM32F4xx headers + startup + LICENSE.md
+-- ide/
    +-- EWARM/                    IAR Embedded Workbench project
    +-- MDK-ARM/                  Keil MDK-ARM project
    +-- SES/                      SEGGER Embedded Studio project

Clock Tree

HSE 25 MHz
  |
  v
PLLM = 15   25 / 15 = 1.667 MHz (VCO input)
PLLN = 120  1.667 x 120 = 200 MHz (VCO output)
PLLP = /2   200 / 2 = 100 MHz
  |
  v
SYSCLK = 100 MHz
  |
  +-- AHB1 (HPRE = /1) -> HCLK = 100 MHz
  |     +-- APB1 (PPRE1 = /2) -> PCLK1 = 50 MHz
  |     +-- APB2 (PPRE2 = /1) -> PCLK2 = 100 MHz
  |     +-- SysTick timer (AHB/8 = 12.5 MHz)
  |
  +-- FLASH: 2 WS, ICache + DCache + Prefetch ON

RTC Configuration

Parameter	Value
Clock source	LSE (32768 Hz)
Asynchronous prescaler (PREDIV_A)	127
Synchronous prescaler (PREDIV_S)	255
Wakeup timer clock	RTCCLK/16 = 2048 Hz (WUCKSEL = 000)
Wakeup timer reload (WUTR)	2047
Wakeup interval	~1 second

Firmware Flow

Reset -> main()
  |
  init_sys()         RCC: HSE -> PLL -> 100 MHz
  |                  FLASH: 2 WS, cache + prefetch
  |                  PWR: enable backup domain
  |                  RCC: LSE ON, RTC clock = LSE
  |                  RTC: init time/date, wakeup timer
  |                  SysTick: start (for DELAY_MS)
  |
  init_gpio()        GPIOA: USART6 TX (AF8)
  |                  GPIOC: PC13 output (LED)
  |
  init_usart(115200) USART6: 115200 baud, TX only
  |
  loop:
    wait RTC wakeup flag (WUTF)
    capture time/date (TR/DR)
    6 toggles of PC13 (3 full flashes, 30 ms each)
      u_put_bcd(field) + separator per toggle
      -> USART6: "DD.MM.YYYY HH:MM:SS\n"

License

Source code is MIT (see LICENSE). Third-party components downloaded at build time are covered by their own licenses (see LICENSE for details).