4.1.5.5. USART on STM32F411

4.1.5.5.1. Overview of USART Functionality

USART (Universal Synchronous/Asynchronous Receiver/Transmitter) is a key feature in STM32F411 for serial communication. It enables bidirectional data exchange between the microcontroller and external devices like computers, sensors, or other microcontrollers.

4.1.5.5.2. Common Use Cases

  • Debugging via a serial monitor

  • Communication with peripheral devices

  • Wireless data transmission modules (e.g., Bluetooth or Wi-Fi)

4.1.5.5.3. Key Configurations

  1. Baud rate: The speed of data transmission.

  2. Data format: Frame format (number of data bits, parity, and stop bits).

  3. Control registers: Settings for enabling transmission, reception, and interrupts.

4.1.5.5.4. USART Modes: Synchronous vs Asynchronous

Asynchronous Mode

  • Uses only two lines: TX (transmit) and RX (receive).

  • The transmitter and receiver operate independently, with communication timing controlled by the baud rate.

  • Commonly used in standard UART communication (e.g., serial terminals).

Synchronous Mode

  • Uses an additional clock line (SCLK) for synchronized communication between the transmitter and receiver.

  • The transmitter generates the clock, ensuring precise timing for data exchange.

  • Suitable for high-speed communication or scenarios where the clock signal needs to be shared.

Enabling Synchronous Mode

  • Set the USART_CR2.CLKEN bit to enable the clock signal.

  • Configure the clock polarity (CPOL) and phase (CPHA) in USART_CR2.

4.1.5.5.5. Registers and Settings for USART Communication

  1. Baud Rate Configuration

    The baud rate is set in the USART_BRR (Baud Rate Register). The exact formula to calculate the baud rate includes oversampling and is defined as:

    \[Baud\ Rate = \frac{f_{PCLK}}{8 \times (2 - OVER8) \times USARTDIV}\]

    Where:

    • f_PCLK: Peripheral clock frequency (APB1 or APB2 clock, depending on the USART instance).

    • OVER8: Over-sampling factor (0 for 16x, 1 for 8x).

    • USARTDIV: Value set in the USART_BRR register, determined by the desired baud rate.

    If OVER8 = 0, the formula simplifies to:

    \[Baud\ Rate = \frac{f_{PCLK}}{16 \times USARTDIV}\]

  2. Control Registers

    • USART_CR1 (Control Register 1): Enables USART, selects word length, parity control, and interrupts.

    • USART_CR2 (Control Register 2): Configures stop bits, synchronous mode settings, and clock options.

    • USART_CR3 (Control Register 3): Configures flow control and DMA settings.

  3. Data Register

    • USART_DR: Used for reading received data or writing data to be transmitted.

4.1.5.5.6. Practical Example: Setting Up USART for Asynchronous Communication

  1. Enable USART Clock

    RCC->APB2ENR |= RCC_APB2ENR_USART1EN;  // Enable USART1 clock

  2. Configure GPIO Pins for USART

    GPIOA->MODER |= (2 << (9 * 2)) | (2 << (10 * 2));  // Set PA9 and PA10 to alternate function mode
GPIOA->AFR[1] |= (7 << (1 * 4)) | (7 << (2 * 4));  // Set PA9 and PA10 to AF7 (USART)

  3. Set Baud Rate

    USART1->BRR = 16000000 / 9600;  // Assuming 16 MHz clock and 9600 baud rate

  4. Enable USART

    USART1->CR1 |= USART_CR1_UE | USART_CR1_TE | USART_CR1_RE;  // Enable USART, TX, and RX

  5. Transmit Data

    while (!(USART1->SR & USART_SR_TXE));
USART1->DR = 'A';  // Send character 'A'

  6. Receive Data

    while (!(USART1->SR & USART_SR_RXNE));
USART1->DR;  // Read received data

4.1.5.5.7. Summary

USART in STM32F411 supports two modes:

  • Asynchronous Mode: Simple, widely used for serial communication without a shared clock.

  • Synchronous Mode: Adds a clock line for precise timing, ideal for high-speed or clock-sensitive applications.

The baud rate is configured using a precise formula to ensure accurate communication timing. This flexibility makes USART highly adaptable for various communication needs.