Many STM32 beginners struggle with UART communication, especially when using HAL functions for sending and receiving data. A single mistake in configuration or code can cause lost bytes, unwanted delays, or complete communication failure. This guide explains STM32 UART transmit and receive using HAL drivers in a clear, step-by-step way, with tested code examples for STM32F103, STM32F4, and STM32 Nucleo boards. By following these best practices, you’ll build fast, reliable, and maintainable UART applications.

Understanding UART in STM32

UART (Universal Asynchronous Receiver/Transmitter) is a simple serial protocol that uses:

  • TX (Transmit) pin – sends data out

  • RX (Receive) pin – receives data in

  • Configurable baud rate, parity, data bits, and stop bits

In STM32, UART hardware is often called USART because it can handle both asynchronous (UART) and synchronous modes.

Why Use HAL for UART?

STM32 HAL (Hardware Abstraction Layer) offers:

  • Pre-built API functions for transmit, receive, and interrupts

  • Portable code that works across STM32 families

  • Easy integration with CubeMX for pin and clock setup

HAL makes development faster, especially for beginners or teams working on multiple STM32 series.

HAL UART Functions You Should Know

The main HAL functions for UART are:

  • HAL_UART_Transmit() – Blocking transmit

  • HAL_UART_Receive() – Blocking receive

  • HAL_UART_Transmit_IT() – Non-blocking transmit using interrupts

  • HAL_UART_Receive_IT() – Non-blocking receive using interrupts

  • HAL_UART_Transmit_DMA() – Transmit using DMA

  • HAL_UART_Receive_DMA() – Receive using DMA

  • HAL_UARTEx_ReceiveToIdle_IT() – Receive until idle line detected

Basic STM32 UART Transmit Example

This example sends a string over USART1.

char msg[] = "Hello from STM32\r\n";

HAL_UART_Transmit(&huart1, (uint8_t*)msg, strlen(msg), HAL_MAX_DELAY);

Best practice:

  • Use HAL_MAX_DELAY for simple blocking sends.

  • Avoid blocking sends in time-critical tasks; use interrupt or DMA instead.

Basic STM32 UART Receive Example

This example reads one byte from USART1.

uint8_t rx_byte;

HAL_UART_Receive(&huart1, &rx_byte, 1, HAL_MAX_DELAY);

Best practice:

  • Blocking receive is fine for single-byte commands.

  • For streams of data, use interrupts or DMA.

Using UART Interrupts for Continuous Reception

For data that arrives at unpredictable times, use interrupts.

uint8_t rx_data;

void HAL_UART_RxCpltCallback(UART_HandleTypeDef *huart) {
    if (huart->Instance == USART1) {
        // Process received byte
        HAL_UART_Receive_IT(&huart1, &rx_data, 1); // Restart reception
    }
}

int main(void) {
    HAL_Init();
    SystemClock_Config();
    MX_USART1_UART_Init();
    HAL_UART_Receive_IT(&huart1, &rx_data, 1);
    while (1) {
        // Other tasks
    }
}

Best practice: Always restart reception inside the callback or you will only get the first byte.

Transmit and Receive with DMA

DMA is best for large data transfers. It offloads the CPU and can be combined with interrupts.

uint8_t tx_buffer[] = "STM32 DMA UART Example\r\n";
uint8_t rx_buffer[64];

HAL_UART_Transmit_DMA(&huart2, tx_buffer, sizeof(tx_buffer));
HAL_UART_Receive_DMA(&huart2, rx_buffer, sizeof(rx_buffer));

Best practice:

  • Use DMA for buffers longer than 16 bytes.

  • Avoid modifying buffers while DMA is active.

Using Idle Line Detection for Variable-Length Data

When you don’t know the exact number of bytes in advance, idle line detection helps.

uint8_t rx_buf[64];
uint16_t rx_len = 0;

void HAL_UARTEx_RxEventCallback(UART_HandleTypeDef *huart, uint16_t Size) {
    if (huart->Instance == USART3) {
        rx_len = Size;
        // Process data in rx_buf
        HAL_UARTEx_ReceiveToIdle_IT(&huart3, rx_buf, sizeof(rx_buf));
    }
}

int main(void) {
    HAL_Init();
    SystemClock_Config();
    MX_USART3_UART_Init();
    HAL_UARTEx_ReceiveToIdle_IT(&huart3, rx_buf, sizeof(rx_buf));
    while (1) {}
}

Best practice: Use idle detection for protocols like Modbus, GPS, or any packet-based system without fixed length.

Debugging UART Communication Problems

  • No data received: Check baud rate, wiring, and NVIC interrupt settings.

  • Garbled text: Ensure baud rates match exactly between devices.

  • Only one byte received: Forgot to restart receive in callback.

  • Data loss in DMA mode: Buffer too small or callback processing too slow.

Tips for Reliable UART Communication

  • Keep callbacks short; set a flag and handle processing in the main loop.

  • Use ring buffers for continuous streams.

  • Test with a USB-to-UART converter to confirm hardware is working.

  • For low-power applications, configure UART as a wake-up source.

Example: Full Duplex UART Echo

This code sends back whatever it receives.

uint8_t rx_char;

void HAL_UART_RxCpltCallback(UART_HandleTypeDef *huart) {
    if (huart->Instance == USART1) {
        HAL_UART_Transmit_IT(&huart1, &rx_char, 1);
        HAL_UART_Receive_IT(&huart1, &rx_char, 1);
    }
}

int main(void) {
    HAL_Init();
    SystemClock_Config();
    MX_USART1_UART_Init();
    HAL_UART_Receive_IT(&huart1, &rx_char, 1);
    while (1) {}
}

STM32 Families Covered in This Guide

  • STM32F103 – Popular in Blue Pill and many low-cost boards

  • STM32F4 – High performance with advanced UART options

  • STM32 Nucleo boards – Great for prototyping with built-in USB-UART

FAQ

1. How do I send a string with STM32 HAL UART?
Use HAL_UART_Transmit() with the string buffer and its length.

2. Which HAL function receives until idle line?
HAL_UARTEx_ReceiveToIdle_IT().

3. How to avoid blocking the CPU during transmit?
Use HAL_UART_Transmit_IT() or DMA.

4. Why does my UART only receive once?
Restart HAL_UART_Receive_IT() in the receive complete callback.

5. Is DMA faster than interrupt mode?
Yes, for large buffers.

6. Can STM32 UART wake the MCU from sleep?
Yes, if configured as a wake-up source.

7. How many UARTs do STM32F4 chips have?
Depends on the model; many have up to 8.

8. Can I use UART without HAL?
Yes, via register-level programming.

9. What is the maximum baud rate for STM32 UART?
Often up to 4.5 Mbps, depending on the series.

10. Does STM32 support single-wire UART?
Yes, using half-duplex mode.

Conclusion

STM32 HAL makes UART transmit and receive simple while still offering advanced modes like interrupts, DMA, and idle detection. For small data, blocking functions are fine. For continuous or high-speed transfers, use interrupts or DMA. Always restart reception when using interrupt mode, and use idle detection for variable-length packets.

If you want more tested STM32 code examples, check other UART tutorials on ControllersTech and share your project results in the comments.

Image Alt Text: STM32 HAL UART transmit and receive example with code

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  • All HAL functions verified with STM32CubeIDE 1.14 (2025)

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