Porting Zephyr RTOS to an STM32 microcontroller involves setting up the development environment, configuring the board support package (BSP), and building/flashing a sample application. Below is a step-by-step guide:

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1. Prerequisites
Hardware:

STM32 development board (e.g., STM32F4 Discovery, Nucleo-144).

Software:

  • Zephyr SDK (Toolchain for ARM Cortex-M).
  • Python 3.8+ (for west tool).
  • Git (to fetch Zephyr source).
  • STM32CubeProgrammer (for flashing).

2. Install Zephyr Development Environment
A. Install Dependencies (Linux/macOS/WSL)

bash

# Install Python and pip
sudo apt update && sudo apt install -y git cmake ninja-build gperf \
  python3 python3-pip

# Install west (Zephyr meta-tool)
pip3 install west

# Install Zephyr SDK (ARM toolchain)
wget https://github.com/zephyrproject-rtos/sdk-ng/releases/download/v0.16.0/zephyr-sdk-0.16.0_linux-x86_64.tar.gz
tar xvf zephyr-sdk-0.16.0_linux-x86_64.tar.gz
cd zephyr-sdk-0.16.0
./setup.sh

B. Fetch Zephyr Source

bash

west init ~/zephyrproject
cd ~/zephyrproject
west update

C. Set Environment Variables

bash

export ZEPHYR_BASE=~/zephyrproject/zephyr
source $ZEPHYR_BASE/zephyr-env.sh

3. Check STM32 Board Support
Zephyr supports many STM32 boards out-of-the-box. Check compatibility:

bash

ls $ZEPHYR_BASE/boards/arm | grep stm32

Example supported boards:

  • nucleo_f401re (STM32F4)
  • nucleo_h743zi (STM32H7)
  • stm32f4_disco (STM32F4 Discovery)

4. Build and Flash a Sample Application
A. Build "Hello World" for STM32

bash

west build -p auto -b nucleo_f401re $ZEPHYR_BASE/samples/hello_world
  • -p auto: Auto-detects available cores.
  • -b nucleo_f401re: Specifies the board.

B. Flash the Binary
Option 1: Using OpenOCD (via ST-Link)

bash

west flash

Requires ST-Link debugger (built into Nucleo boards).

Option 2: Manual Flashing (STM32CubeProgrammer)

  1. Locate the .hex or .bin file: 
bash

ls build/zephyr/zephyr.hex
  1. Flash using STM32CubeProgrammer GUI or CLI: 
bash

STM32_Programmer_CLI -c port=SWD -w build/zephyr/zephyr.hex 0x08000000

5. Customizing for a New STM32 Board
If your board isn’t officially supported, follow these steps:

A. Create a Custom Board Directory

bash

cd $ZEPHYR_BASE/boards/arm
mkdir my_stm32_board
cd my_stm32_board

B. Required Files

  1. board.cmake (Specifies CPU/flash settings): 
cmake

set(CPU_TYPE cortex-m4)  # e.g., M4 for STM32F4
set(FLASH_SIZE 512K)
set(RAM_SIZE 128K)
  1. Kconfig.board (Board-specific options): 
config BOARD_MY_STM32_BOARD
    bool "My Custom STM32 Board"
    depends on SOC_STM32F401XE
  1. .dts (Device Tree for peripherals): 
dts

/ {
    model = "My STM32 Board";
    compatible = "st,stm32f401";

    chosen {
        zephyr,console = &usart2;
    };
};
  1. _defconfig (Default Zephyr config): 
CONFIG_SERIAL=y
CONFIG_UART_INTERRUPT_DRIVEN=y

C. Build and Test

bash

west build -b my_stm32_board samples/hello_world
west flash

6. Debugging
A. Serial Output (UART)
Connect to the STM32’s UART (e.g., /dev/ttyACM0):

bash

picocom -b 115200 /dev/ttyACM0

Should show:

*** Booting Zephyr OS build v3.4.0 ***
Hello World! my_stm32_board

B. GDB Debugging

bash

west build -b nucleo_f401re -- -DCMAKE_EXPORT_COMPILE_COMMANDS=ON
arm-none-eabi-gdb build/zephyr/zephyr.elf
(gdb) target remote :3333  # Connect to OpenOCD
(gdb) monitor reset halt
(gdb) continue

7. Adding Peripheral Drivers
To enable I2C, SPI, or GPIO:

  1. Edit prj.conf: 
CONFIG_I2C=y
CONFIG_SPI=y
CONFIG_GPIO=y
  1. Use Zephyr’s Device Tree (DTS) to map pins: 
dts

&i2c1 {
    status = "okay";
    sda-pin = <&gpiob 7>;
    scl-pin = <&gpiob 6>;
};

8. Troubleshooting

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  1. Next Steps ✔ Explore Zephyr’s sensor drivers (e.g., BME280 over I2C). ✔ Implement multi-threading with Zephyr’s kernel API. ✔ Integrate RTOS features (message queues, timers).

For advanced use, refer to:

  • Zephyr STM32 Documentation
  • Zephyr Device Tree Guide