Serial communication interface 8250

1、 The basic concept of serial communication

1. serial communication ： Decompose data into binary bits , Transmit bit by bit with a signal line .
(1) advantage ： There are few lines for communication , The cost of long distance communication can be reduced .
(2) shortcoming ： The communication speed is slow .
(3) purpose ： It is often used for data transmission between systems with low speed requirements .
2. The two sides of serial communication must solve 6 A question ：
(1) Both parties agree on the rate of data transmission and reception ( Baud rate );
(2) What data format is agreed upon ( The frame format );
(3) How does the receiver know the beginning and end of a batch of data ( Frame synchronization );
(4) How does the receiver sample the data in place correctly from the bit stream ( Bit synchronization );
(5) How does the receiver judge the correctness of the received data ( data verification );
(6) How to deal with errors in sending and receiving ( Error handling ).
3. The classification of serial communication （ According to the synchronization mode ）
(1) asynchronous communication ： Think of a character as an independent unit of information , And the relative time that characters appear in the data stream is arbitrary , And the bits in each character are transmitted at a fixed time .
namely ： In character synchronization , Asynchronous between characters .
The communication protocol of serial asynchronous communication is called ： Start stop asynchronous communication protocol .
The character format of the transmission ：

① Start bit ： Using logic 0 The level serves as a symbol of the beginning of transmission of each character ;
② Data bits ： The data bit is transmitted immediately after the start bit . from 5～8 Binary bits make up , Low first ;
③ Check bit ： Used to verify whether the transmission is correct ; The odd test is optional 、 Even check or not transfer check bit ;
④ Stop bit ： Indicates the end of the character transmission . The stop bit uses logic 1 level , Can choose 1、1.5 or 2 position ;
⑤ Free bit ： Transfer logic between characters 1 level , Indicates that there is no transmission .
(2) Synchronous communication ： In a block of data ( frame ) Is the transmission unit , Each data block is attached with 1 Or 2 Sync characters , Finally, it ends with a check character .
The most common synchronous communication protocol is ： Advanced data link control protocol HDLC
Transmission format ( One frame )：

4. Transmission mode ：

1、 modems

1. modulation ： Converting digital signals into analog signals transmitted over telephone lines .
2. demodulation ： Convert analog signal of telephone line to digital signal .
3. modems ： Devices with modulation and demodulation functions are combined in one device .
   2,8250 Universal asynchronous receiver transmitter
   8250 It's a typical UART chip