Universal Asynchronous Receiver Transmitter
The UART offers a very flexible, full-duplex, industrial standard, serial receiver / transmitter.
Including IrDA and modem operations (CTS/RTS).
Features
- Full-, half-duplex communication.
- Fractional baud rate generator.
- 8- and 9 bit word length.
- DMA and interrupt support.
Configuration Registers
USART_CR1 - Configuration register 1
| RE | 0 | Receiver disable (reset state) |
| 1 | Receiver enable | |
| TE | 0 | Transmitter disable (reset state) |
| 1 | Transmitter enable | |
| RXNEIE | 0 | Receiver not empty interrupt disable (reset state) |
| 1 | Receiver not empty interrupt enable | |
| TCIE | 0 | Transmission complete interrupt disable (reset state) |
| 1 | Transmission complete enable | |
| TXEIE | 0 | Transmitter empty interrupt disable (reset state) |
| 1 | Transmitter empty interrupt enable | |
| M | 0 | 8 data bits (reset state) |
| 1 | 9 data bits enable | |
| UE | 0 | USART disable (reset state) |
| 1 | USART enable |
USART_CR2 - Configuration register 2
| STOP | 00 | 1 stop bit (reset state) |
| 01 | 0,5 stop bit | |
| 10 | 2 stop bit | |
| 11 | 1,5 stop bit |
USART_BRR - Baud rate register
| Fraction | x | Defines fraction of PCLK divider |
| Mantissa | x | Defines Mantissa of the PCLK divider |
Status Register
USART_SR - Status register
| RXNE | 0 | Data is not received |
| 1 | Received data is ready to be read | |
| TC | 0 | Transmission not complete |
| 1 | Transmission complete | |
| TXE | 0 | Data is not transferred to the shift register |
| 1 | Data transferred to the shift register |
Data Register
USART_DR - Data register
- Reading from DR register reads from RXDR Buffer.
- Writing to DR register writes to TXDR Buffer.
Programming Example
Setup GPIO
#include "reg_stm32f4xx.h" RCC->AHBENR[0] |= (0x1 << 0u); /* Enable GPIOA clock */ /* Configure GPIO pin A.0 and A.1 in alternate function mode. */ GPIOA->MODER &= ~(0xf << 0u); /* Clear existing mode bits 0 to 3. */ GPIOA->MODER |= (0xa << 0u); /* Set pin 0..1 to alternate function mode. */ GPIOA->OSPEEDR &= ~(0xff << 0u); /* Clear existing output speed bits 0 to 3. */ GPIOA->OSPEEDR |= (0xff << 0u); /* Set pin 0..1 to high speed (100MHz) mode. */ GPIOA->AFR[0] &= ~(0xff << 0u); /* Clear existing af bits 0 to 7. */ GPIOA->AFR[0] |= (0x88 << 0u); /* Set pin 0..1 to AF8 (UART4). */
Transmitting Data
#include "reg_stm32f4xx.h" RCC->APBENR[0] |= (0x1 << 19u); /* Enable UART4 clock */ UART4->CR1 |= (0x1 << 13u); /* Enable UART */ UART4->BRR |= (42000000 / 115200); /* Set baudrate to 115'200 */ UART4->CR1 |= (0x1 << 3u); /* Enable transmission */ UART4->DR = data_send[0]; /* Send first data */ while(!(UART4->SR & (0x1 << 7u))); /* Wait til TXE flag is set */ UART4->DR = data_send[1]; /* Send first data */ while(!(UART4->SR & (0x1 << 7u))); /* Wait til TXE flag is set */ UART4->DR = data_send[2]; /* Send last data */ while(!(UART4->SR & (0x1 << 6u))); /* Wait til TC flag is set */
Receiving Data
#include "reg_stm32f4xx.h" RCC->APBENR[0] |= (0x1 << 19u); /* Enable UART4 clock */ UART4->CR1 |= (0x1 << 13u); /* Enable UART */ UART4->BRR |= (42000000 / 115200); /* Set baudrate to 115'200 */ UART4->CR1 |= (0x1 << 2u); /* Enable receiver */ while(!(UART4->SR & (0x1 << 5u))); /* Wait til RXNE flag is set */ data_received = UART4->DR /* Read data */