USB to serial communication in stm32f4USART

Beginner learning STM32, first contact with USART serial communication.

A personal summary of the USART communication hardware principle. The TTL level from the MCU (Micro Controller Unit, also known as the ST chip) is converted to the USB level through a level conversion chip, thereby achieving communication with the PC.

USART (Universal Synchronous/Asynchronous Receiver/Transmitter) Universal synchronous/asynchronous serial receiver/transmitter.

Here we only talk about asynchronous communication. The main difference between asynchronous communication and synchronous communication is the clock, so SLCK is turned off here. USART has a total of 5 external pins: TX, RX, nRTS, nCTS, and SCLK (SW_RX in the block diagram is internal, and n means low level is valid). When learning to communicate with the PC host computer, the hardware flow control mode is turned off, so asynchronous communication only involves the two pins TX (Transmit) and RX (Receive).

The general idea is: data is transferred to the transmit data register USART_DR (TRD) by software, and TC (Transmit complete) is judged to be set to 1. If it is set to 1, it means that the transmit shift register has completed the transmission, and the data in the transmit data register is sent to the transmit shift register to start transmission. TXE (Transmit data register empty) is judged, and the next data is read to TDR; if TC is 0, wait. TC is set to 0 by the software sequence (reading the USART_SR register and then writing to the USART_DR register), or it can be achieved by writing "0" to this bit. When the hardware is reset, TC is set to 1 by hardware.

    When receiving data, the data is transferred to the receive shift register and then to the read data register. At this time, RXNE (Read data register not empty) is set to 1, and the interrupt flag RXNEIE can be generated.

Data format: 1 start bit, 8 data bits (USART_CR1, bits 12-M in control register 1 are set to 0), no check bit (USART_SR, bits 1-PE in status register are set to 0 - no parity error), 1 stop bit (USART_CR2, bits 13:12-STOP in control register 2 are assigned to 00). Because the communication is between the development board and the PC, the distance is short and the communication environment is good, so no parity check is set.

USART1 is used, PA9 is selected for TX and PA10 is selected for RX.

Code flow: 1. Initialization a. Turn on the peripheral clock (if you want to use any peripheral, you must enable the clock first), turn on the clock of GPIOA mounted on AHB1 and USART1 mounted on APB2, and call the firmware library functions RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOA, ENABLE) and RCC_APB2PeriphClockCmd(RCC_APB2Periph_USART1, ENABLE)

b. Initialize the GPIO port related to the serial port, assign values ​​to the GPIO_InitTypeDef GPIO_InitStruct structure, and then map it to the register through GPIO_Init(GPIOA,&GPIO_InitStruct)

c. Configure and select the multiplexing function, call the firmware library function GPIO_PinAFConfig (form parameter, form parameter, form parameter) to determine the multiplexing function of the pin

d. Initialize the serial port, initialize the USART structure, set the baud rate, data bit length, stop bit, check bit selection, hardware flow control mode selection, USART mode control (enable receiving/transmitting), and finally complete the configuration through USART_Init(...)

e. If interrupt execution service is required, initialize NVIC (Nested Vectored Interrupt Controller). If not, skip

f. Enable the serial port and call USART_Cmd (.....) to enable USART1

    2. Write a data transmission function. Since USART_DR has only 8 bits valid, it is divided into byte transmission, double-byte transmission, string transmission, and array transmission. These four transmissions are based on the firmware library function USART_SendData (...). When sending a byte, determine whether TXE is 1. If it is 1, prepare to send the next data to the transmission data register; if it is 0, wait. The firmware library function for obtaining the interrupt status bit is ITStatus USART_GetITStatus (...). Finally, it can be determined whether TC is 1, that is, whether the transmission is completed.

    3. Write the data receiving function and call the firmware library function USART_ReceiveData (...) to obtain the latest USARTx data. Judge RXNE, 1 means the reception is complete, 0 means no data is received, so the content of the data receiving function should be written under the condition that RXNE is judged to be 1.

   4. Just call the main() function.

According to the Wildfire tutorial, reference manual, and novice understanding, I just reviewed it for my own reference. If there is anything wrong, please point it out. If it is not well written, please forgive me.

As a novice, there are still many questions I don't understand. 1. If parity check is set, should the receiver handle it? How to handle it? If this is the case for MCU to PC communication, what about MCU to MCU?

2. To send 8-bit data, you need to write a function, and to send 16-bit data, you need to write another function. When sending an array, you can only send an array of 8 bytes. Maybe I don’t know enough and I don’t understand it.