STM32F401 ↔ ESP32 UART Button-to-LED

Overview

UART communication between an STM32F401CCU6 microcontroller and an ESP32 development board(bare-metal, no HAL).

• A push button on the STM32 (PA0) is used as input.
• When pressed, the STM32 sends a text message (BTN: PA0 pressed) over USART2 (115200 baud).
• The ESP32 receives this message and toggles its onboard LED (GPIO2).

STM32 - ESP32

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🔌 What is UART?

UART stands for Universal Asynchronous Receiver/Transmitter.
It is a very common way for two devices to talk to each other using just two wires:

• TX (Transmit): sends data out
• RX (Receive): receives data in

Key facts:

• UART is asynchronous → no clock line is shared. Both devices must agree on the baud rate (bits per second).
• Common baud rates: 9600, 115200, etc.
• Data is sent frame by frame:
  • 1 start bit (0)
  • 8 data bits (like a character 'A')
  • 1 stop bit (1)
  • Optional parity bit (error check)

For example, at 115200 baud each bit is ~8.68 µs long.
So the STM32 can send "BTN: PA0 pressed\r\n" and the ESP32 reconstructs it on the other side.

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Hardware Required

• STM32F401CCU6 board (Black Pill)
• ESP32 DevKit (ESP32-WROOM or similar)
• Push button + jumper wires
• USB cables for both boards
• Common ground between STM32 and ESP32

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Wiring

STM32 Pin	Function	ESP32 Pin	Notes
PA2	USART2_TX	GPIO16	STM32 → ESP32 data
PA3	USART2_RX	GPIO17	ESP32 → STM32 data (optional)
GND	Ground	GND	Must be common
PA0	Button input	—	Button between PA0 and GND

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STM32 Code (Bare-metal C)

Steps inside the code

1. Clock Configuration

   • Enable clock for GPIOA (RCC_AHB1ENR)
   • Enable clock for USART2 (RCC_APB1ENR)

2. GPIO Setup (PA2, PA3 for USART2)

   • GPIOA_MODER: set PA2 and PA3 to Alternate Function mode (10)
   • GPIOA_AFRL: select AF7 (USART2) for PA2 and PA3
   • GPIOA_OSPEEDR: set PA2, PA3 to high speed
   • GPIOA_PUPDR: enable pull-up on PA3 (RX)

3. GPIO Setup (PA0 as Button)

   • Configure PA0 as input
   • Enable internal pull-up resistor
   • Button connects PA0 → GND → pressed = logic 0

4. USART2 Setup

   • Disable USART (CR1=0)
   • Configure baud rate via helper function usart_brr_from()
     • Formula ensures correct baud at 16 MHz system clock
   • Enable Transmitter (TE) and Receiver (RE)
   • Enable USART (UE)

5. Main Loop

   • Poll PA0 for press (active low).
   • On press (with debounce):
     • Send string "BTN: PA0 pressed\r\n" via usart2_write_str()
   • Wait for button release before next toggle.

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ESP32 Code (Arduino IDE)

Steps inside the code

1. Setup

   • Initialize USB Serial (Serial) at 115200 baud (debug output to PC).
   • Initialize Serial2 at 115200 baud, pins RX=16, TX=17 (UART connection to STM32).
   • Configure LED pin (GPIO2) as output, start LOW.

2. Loop

   • handleFromSTM32()
     • Reads incoming characters from STM32.
     • Builds a line until \n.
     • If line is "BTN: PA0 pressed", toggles LED state.
     • Prints status to Serial Monitor.
   • handleFromPC()
     • Forwards any text typed in Serial Monitor to STM32.

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How to Build & Run

STM32 (Bare-metal C)

1. Copy the main.c into your STM32CubeIDE or bare-metal GCC project.
2. Make sure system clock is left at 16 MHz HSI (default).
3. Compile and flash to STM32F401CCU6.
4. Connect UART pins to ESP32 as described.

ESP32 (Arduino IDE / PlatformIO)

1. Open Arduino IDE.
2. Select ESP32 Dev Module.
3. Copy the ESP32 sketch (ESP32-Code/UART-LED-control.ino.ino).
4. Upload to ESP32.
5. Open Serial Monitor @ 115200 baud.

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Serial Monitor Example

When pressing STM32 button:

[STM32] BTN: PA0 pressed
[ESP32] LED is now ON

[STM32] BTN: PA0 pressed
[ESP32] LED is now OFF

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License

MIT License