[embedded foundation] serial port communication

1. Preface

Serial communication is very commonly used in actual embedded development , It is usually used for data interaction with sensors .

2. Basic concepts

The concept of serial communication is very simple , Serial port by bit （bit） Send and receive bytes . Although by byte （byte） Parallel communication is slow , But the serial port can use one wire to send data and another wire to receive data at the same time .

Understand the principle of serial communication , You need to understand the basic concepts first ： Baud rate 、 Start bit 、 Data bits 、 Check bit 、 Stop bit 、 Free bit .

2.1 Baud rate

This is a parameter to measure the communication speed , Indicates the number of... Transmitted per second bit Number . such as 100 Indicates that the message is sent every second 100bit.

2.2 Start bit

The start bit is a logic that lasts one bit 0 level , Indicates the beginning of transmission of a character . The receiver can use the start bit to synchronize its receiving clock with the sender's data .

2.3 Data bits

The data bit follows the start bit , Is the really valid information in communication .

The number of data bits can be mutually agreed by both communication parties , It can generally be 5 position 、7 Bit or 8 position .

When transferring characters , Pass low first , Then pass to the high position . The standard ASCII Code is 0~127; Extended ASCII Code is 0 ~ 255(8 position ).

2.4 Check bit

The parity bit occupies only one bit , Used for odd check or even check , Parity bits are not required .

If it's odd , It is necessary to ensure that the transmitted data has an odd number of high levels in total ; If it's even parity , It is necessary to ensure that the transmitted data has an even number of high levels in total .

Suppose the transmitted data bits are 01010101, If it's odd , Then the odd check bit is 1（ Make sure there are an odd number 1）. If it's even parity , Then the parity bit is 0（ Make sure there are even numbers 1）.

2.5 Stop bit

The stop bit must be logical 1 level , Indicates the end of a character . The stop bit can be 1 position 、1.5 Bit or 2 position , It can be set by software .

2.6 Free bit

The free bit refers to the end of the stop bit of one character and the beginning of the start bit of the next character , Indicates that the line is idle , Must be filled by high level .

3. Working mode

Serial port communication is divided into... According to the data transmission direction 3 Working mode ： Simplex mode 、 Half duplex mode and full duplex mode .

3.1 Simplex mode

Simplex mode data transmission is unidirectional . Between the two sides of the communication , One party is fixed as the sender , One side is fixed as the receiver . Data can only be transmitted in one direction using one line .

3.2 Half duplex mode

Half duplex mode can send and receive data , But you cannot send and receive at the same time . Data transmission allows data to be transmitted in both directions , But only one of them can send data at any time , The other party accepts the data .

Half duplex mode can use one data line , You can also use two data cables .

3.3 Full duplex mode

Full duplex mode communication allows data to be transmitted in both directions at the same time , It is the combination of two simplex communication modes , It is required that both the sending equipment and the receiving equipment have independent receiving and sending capabilities .

In full duplex mode , Transmitter and receiver at each end , There are two transmission lines , Information transmission is higher .

4. Synchronous communication and asynchronous communication

The most obvious difference between synchronous communication and asynchronous communication is whether there is a clock signal .

4.1 Synchronous communication

Synchronous communication （SYNC:synchronous data communication） It means that at the agreed communication rate , The frequency and phase of the clock signal at the transmitter and receiver are always consistent （ Sync ）, This ensures that both sides of the communication have a completely consistent timing relationship when sending and receiving data .

Synchronous communication combines many characters into an information group （ Information frame ）, The beginning of each frame is indicated by synchronization characters , Only one frame of information is transmitted at a time . While transmitting data, it also needs to transmit clock signal , So that the receiver can use the clock signal to determine each information bit .

The advantage of synchronous communication is that the number of bits of information transmitted is almost unlimited , There are tens to thousands of bytes of data transmitted in a communication , High communication efficiency . The disadvantage of synchronous communication is that it is required to always maintain an accurate synchronous clock in communication , That is, the sending clock and receiving clock should be strictly synchronized （ The common practice is that two devices use the same clock source ）.

4.2 asynchronous communication

asynchronous communication （ASYNC:asynchronous data communication）, Also known as start stop asynchronous communication , Is transmitted in character units , There is no fixed time interval between characters , The bits in each character are transmitted at a fixed time .

In asynchronous communication , Synchronization is achieved by setting the start bit and stop bit in the character format . To be specific , Before a valid character is officially sent , The transmitter sends a start bit first , Then send valid character bits , Send another stop bit at the end of the character , The start bit to the stop bit form a frame . Between the stop bit and the next start bit is a free bit of variable length , And the start bit is specified as low level （ The logical value is 0）, The stop bit and the free bit are high （ The logical value is 1）, This ensures that there must be a jump edge at the beginning of the start bit , Thus, the beginning of a character transmission can be marked . According to the start bit and stop bit, it is easy to define and synchronize characters .

obviously , When asynchronous communication is adopted , The sender and receiver can control the sending and receiving of data by their own clocks , The two clock sources are independent of each other , Can be out of sync with each other .

5. Flow control

When the two devices communicate normally , Due to different processing speeds , There is such a problem , Some are fast , Some are slow , In some cases , It may lead to the loss of data .
Such as the communication between desktop computer and MCU , The receiving data buffer is full , At this time, the data that continues to be sent will be lost .

Flow control can solve this problem , When the receiving end can't process the data , Just send out “ No longer receive ” The signal of , The sender stops sending , Until receipt “ You can continue sending ” And then send the data . So flow control can control the process of data transmission , Realize the speed matching between the sender and the receiver , Prevent data loss .

PC Two kinds of flow control commonly used in computer are hardware flow control （ Include RTS/CTS、DTR/CTS etc. ） And software flow control XON/XOFF（ continue / stop it ）.

5.1 RTS

Require ToSend, Send a request , For the output signal , Used to indicate that the device is ready to receive data , Low level active .

Low level indicates that the device can receive data .

5.2 CTS

Clear ToSend, Send permission , Is the input signal , Used to determine whether data can be sent to the other party , Low level active .

Low level indicates that the device can send data to the other party .

5.3 Example of hardware flow control

RTS （Require ToSend, Send a request ） For the output signal , Used to indicate that the device is ready to receive ;CTS （Clear ToSend, Send clear ） Is the input signal , Used to indicate whether the peer device is ready to receive data .

such as A、B Two devices communicate , Two devices RTS、CTS Cross connect . Yes B Device send （A Equipment reception ） Come on , If A When the receiving buffer of the device is almost full , Put your own RTS pull up （ notice B The device stops sending ）, After a while A The device receive buffer has space , Put your own RTS Pull it down （ notice B The device can start sending ）.

5.4 Software flow control

Software flow control is the same as hardware flow control , It's just that there's a different way of doing it . In the process of communication , Software flow control by inserting Xoff（ Special characters ） and Xon（ Another special character ） Signal .A Once the device receives B From the device Xoff, Stop sending now ; conversely , If received B From the device Xon, Then resume sending data to B equipment . Empathy ,B The device is similar , So as to realize the speed matching between the sender and the receiver .

6. At the end

I have worked for many years , I don't think the serial communication has been thoroughly understood , be ashamed .

Even the simplest things , If there is no personal practice or no actual use of , Maybe I can't say I understand .

In practice , Debugging serial communication is generally to call the encapsulated interface of the platform , even so , Be familiar with the basic principles , When encountering an exception, you can check the cause more quickly .