[STM32] basic knowledge of serial communication

Basic knowledge of communication

• Serial and parallel
  According to the way of data transmission , Communication can be divided into serial communication and parallel communication

  serial communication ： Only one data bit can be transmitted at a time , Like a one lane highway

  Parallel communication ： Data that can transmit multiple data bits at a time , Like a multi lane highway

Feature comparison

• Transmission direction
  according to serial communication The direction of data transmission , The serial communication mode can be divided into ： Simplex 、 Half duplex 、 full duplex

  Simplex ： Data transmission can only go in one direction , Cannot reverse transmit

  Half duplex ： Data transmission can be in two directions , But not at the same time , That is, there is a sequence between sending and receiving

  full duplex ： Data transmission can be in two directions , And can be sent at the same time

  Half duplex both sides share a line to realize two-way communication , Full duplex uses two lines , Send data one by one 、 One receipt data .
  
  
  

• Synchronous and asynchronous
  Synchronous communication ： Both receiving and sending equipment will Use a signal line to represent the clock signal , Driven by the clock signal, the two sides coordinate 、 Synchronous data , In communication, both parties usually It is uniformly stipulated to sample the data line on the rising or falling edge of the clock signal

  asynchronous communication ： Do not use clock signals Data synchronization , Directly insert some signal bits for synchronization into the data signal , Or package the subject data , Transmit data in the format of data frames . Need both sides Agreed data transmission rate （ Baud rate ）, To better synchronize .

  Data synchronization means that both parties need to know when to start sending data 、 When data transmission ends , Otherwise, invalid data will be received .
  

Serial port communication

USART Universal Synchronous Asynchronous Receiver Transmitter Universal synchronous asynchronous transceiver , It can flexibly exchange full duplex data with external devices .UART And USART The difference is that it only has asynchronous communication , There is no synchronous communication .

USART yes STM32 Based on serial port protocol to achieve external communication with a peripheral , Because the serial communication protocol is simple 、 convenient , So most products support serial communication , It can be realized through serial port communication PC And STM32 Interaction , It plays a very important role in debugging and development .

Serial port communication is a common way of serial communication

Serial communication can be divided into physical layer and protocol layer . The physical layer specifies that the communication protocol has mechanical properties 、 Characteristics of electronic functions , Ensure the dissemination of raw data in physical media ; The protocol layer specifies the communication logic , Unified data packaging 、 Unpacking standard .

• The physical layer
  The main standard of serial communication physical layer is RS-232, It specifies the purpose of the signal 、 Communication interface and signal level standard , The communication structure is as follows
  
  DB9 Interface
  
  Levels of different standards
  
  For controller TTL level , If you want to use other standards for communication between controllers , Need to be connected to level conversion chip , take TTL The level is converted to other standard levels .

  TTL The level transmission distance is short , Generally, the communication distance is greater than 40cm When transmitting data, it will be interfered .

• Protocol layer
  The content of the data package is specified in the protocol layer , From start bit 、 Subject data 、 Check bit and stop bit , The data packet format of both communication parties should be agreed to be consistent in order to send and receive data normally

1. Baud rate
   Baud rate is the number of symbols transmitted per second . Because there is no clock signal in asynchronous communication , So both sides should agree on the baud rate . Common baud rates are 4800、9600、115200 etc.
2. Start and stop signals
   The beginning and end of the packet are the start bit and stop bit respectively , The starting signal consists of a logic 0 The data bits of the , Stop bit can be 0.5、1、1.5、2 A logic 1 The data bits of the , Both parties need to agree
3. Valid data
   Valid data specifies the length of the subject data , It's usually 8 or 9 position
4. data verification
   After the valid data , There is an optional data check bit . because Data communication is vulnerable to external interference, resulting in deviation of transmitted data , This problem can be solved by adding check bits in the transmission process . There are odd check methods (odd)、 Even check (even)、0 check (space)、1 check (mark) And none (noparity).

• Function pin
  TX： Send data output pin .

  RX： Receive data input pin .

  SW_RX： Data receiving pin , Only for single line and smart card modes , Belongs to the internal pin , No specific external pins .

  nRTS： Request to send (Request To Send),n Indicates that the low level is active . If enabled RTS flow control , When USART When the receiver is ready to receive new data, it will nRTS Turn to low level ; When the receive register is full , nRTS Will be set to high level . This pin is only suitable for hardware flow control .

  nCTS： Clear to send (Clear To Send),n Indicates that the low level is active . If enabled CTS flow control , The transmitter will detect before sending the next frame of data nCTS Pin , If it's low level , Indicates that data can be sent , If it is high, the transmission is stopped after the current data frame is transmitted . This pin is only suitable for hardware flow control .

  SCLK： Transmitter clock output pin . This pin is only available in synchronous mode .

  

  USART There are five pins ;UART Only asynchronous communication , Only two pins .

  Serial port pin connection ： TX Pick up RX,RX Pick up TX