Chapter 9: USART

📚 CMPE1250: USART Lesson — Building a UART Library

🎯 Learning Objectives

  • Understand the UART protocol and frame structure
  • Configure the STM32 USART peripheral using register-level access
  • Build a minimal UART library using a provided header file
  • Transmit and receive data using polling

1️⃣ UART Protocol Fundamentals

What is UART?

UART (Universal Asynchronous Receiver/Transmitter) is a serial communication protocol that transmits data one bit at a time over a single wire. It’s asynchronous, meaning no shared clock — both devices must agree on timing.

UART

UART Frame Structure

Component Description
Start Bit Always 0 (low). Signals the beginning of a frame
Data Bits Usually 8 bits (can be 5–9)
Parity Bit Optional error-checking bit
Stop Bit(s) One or two bits set to 1 (high)

Example: A frame with 8 data bits, no parity, and 1 stop bit:

START D0 D1 D2 D3 D4 D5 D6 D7 STOP

Baud Rate

  • Baud rate = bits per second (bps)
  • Common values: 9600, 115200

  • STM32 formula:

    BRR = (uint32_t)((float)SystemCoreClock / BaudRate + 0.5f)

2️⃣STM32 USART Registers (Chapter 33 RM)

Register Purpose
USART_CR1 Enable USART, TX, RX, interrupts
USART_CR3 Overrun disable
USART_BRR Baud rate configuration
USART_ISR Status flags (TXE, RXNE, etc.)
USART_TDR Transmit data register
USART_RDR Receive data register
USART_RQR Request register (clear RX flags)

3️⃣ Minimum steps to Initialize the USART/UART

  • Before configuring the peripheral, the proper alternate function must me anabled to have the Tx and Rx PINs in the GPIO port.

  • Set Baud Rate in the BRR register

    BRR = SystemCoreClock / BaudRate (33.8.5)

  • Disable overrun in the CR3 register: OVRDIS (33.8.4)

  • Enable peripheral, receiver , and transmitter (33.8.1)

    UE, RE, TE in the CR1 register

  • If interrupt is needed SET RXNEIE in the CR1 register (33.8.1)

  • CLEAR receiving flag, just in case: RXFRQ in the RQR register. This enables to discard the received data without reading them, and avoid an overrun condition. (33.8.8)

4️⃣ Transmitting a Byte

  • Wait for the TXFNF flag in the ISR register to be SET to 1 (33.8.9)

  • Write data into the transmit data register: TDR (33.8.13)

5️⃣ Receiving a Byte

  • When a byte has been received and is ready to be read, the RXFNE flag in the ISR register is SET to 1 (33.8.9)

  • Read the byte from the receive data register RDR (33.8.12)

6️⃣Library Construction

Provided Header: uart.h

void UART_Init(USART_TypeDef*, uint32_t, char);
void UART_TxByte(USART_TypeDef*, uint8_t);
void UART_TxStr(USART_TypeDef*, const char*);
void UART_TxBuffer(USART_TypeDef*, uint8_t*, uint16_t);
uint8_t UART_RxByte(USART_TypeDef*, uint8_t*);