Introduction of UART, RS232, RS485, I2C and SPI

Basic knowledge

• serial communication ： Multiple data passes through a data line , In turn
• Parallel communication ： Multiple data passes through multiple data lines , One time transmission
• Simplex communication ： Only receive data or send data （ The remote control 、 The radio ）
• Half duplex communication ： At the same time , Only receive data or send data （ Walkie-talkie ）
• Full duplex communication ： At the same time , It can receive data , You can also send data （ Telephone ）
• Baud rate ： Used to describe the communication speed during communication , Its unit is bps(bit per second), I.e. transmission every second bit The number of

UART

UART（Universal Asynchronous Receiver Transmitter）, General asynchronous transceiver .

• characteristic ：
  1、 There are two data cables （RXD、TXD）
  2、 Serial 、 asynchronous communication
  3、 Full duplex communication
  4、 Point to point communication ： Receiver and sender （ Two devices ）

• application ： In embedded systems , It is often used for communication between host and auxiliary equipment

• Sequence diagram
  
  A complete data transmission includes ： Start bit 、 Data bits 、 Check bit （ not essential ） And stop bits
  When transmitting data , High level （ Set up 1） I'm free
  Start bit ： Set low level （0）, Indicates the beginning of communication
  Data bits ： Start communication after the start bit , To transmit data , First transmit the low order data , Then transmit the data in high order （ Data bits can be sent 5-8 position , Generally send 8 Bit data （ A byte ））
  Check bit ：（ not essential ） Verify whether the data is sent correctly , Only errors can be checked , Can't fix , And after , Communication speed slows down
  Stop bit ： Set high level （ Set up 1）, Indicates the end of a communication （ Occupy 1 position 、1.5 Bits or 2 position ）

• UART Only one byte can be sent per communication , Cannot send cumulatively （ Avoid cumulative errors ）; To send multiple bytes of data , You need to send a byte first , Then end the communication , Start communication again , Send next byte

• Because it is asynchronous communication , There's no clock line , So pass Baud rate To distinguish between 01 still 0011

• Hardware connection diagram
  

• In general ,SOC Will integrate UART controller , In the use of UART When communicating, you only need to set its internal related registers , Communication can be completed

USART

USART It's universal synchronization / Asynchronous transceiver （ With synchronous clock line USART_CK）, It's just a way of agreement , It can be divided into RS232 The protocol and RS485 agreement .

UART Problems in serial communication ：
1、 The electrical interface is not unified
2、 Poor anti-interference ability ：UART Use it directly TTL The signal means 0 and 1, but TTL The anti-interference ability of the signal is very poor , It is easy to make mistakes during transmission
3、 Short communication distance ： because TTL The anti-interference ability of the signal is poor , So its communication distance is also very short . Generally, it is only used for the communication between two different chips on a circuit board （ Onboard communication ）

RS232 agreement ：（ Point to point communication ）
1、 Based on serial port , So sequence diagram and UART The serial port is consistent
2、 Some changes have been made on the electrical layer , Unified electrical interface （ A standard connector is defined , The function of each pin of the connector is specified in the standard , The level of the signal is also specified ）
3、 The anti-interference ability becomes stronger , The communication distance becomes longer （ Generally up to 15 rice ）
4、 The electrical interface is DB-9, In general use RXD、TXD、GND These three lines
5、 The logic is 1 The level of is -5V To -15V; The logic is 0 The level of is +5V To +15V

Hardware connection ：

RS232 The problem with the agreement ：
1、 The signal level of the interface is high , A chip that can easily damage the interface circuit
2、 And TTL Level incompatibility , You need a level conversion chip to work with TTL Circuit connection
3、 Low communication speed
4、 Prone to common mode interference , Weak noise immunity
5、 Communication distance 15 rice , Still short

RS485 agreement
characteristic ：（ Half duplex communication ）
1、 use Differential signal Data transfer （ Twisted pair ）, The voltage difference between the two lines is +2V To +6V To express logic 0; The voltage difference between the two lines is -2V To -6V To express logic 1, Thus realizing long-distance communication （1500 rice ）; And in the environment with large electronic noise , It can effectively transmit signals
2、 Allows multiple transceivers to be connected , Multi station capability , That is, a device network can be established
3、RS485 Level ratio of interface signal RS232 To reduce the , Therefore, it is not easy to damage the chip of the interface circuit

Hardware connection diagram （ Device network ,RS485 Multiple devices are mounted on the bus , Master slave mechanism reference I2C Bus ）
120Ω Resistors are used to eliminate common mode interference on signal lines

I2C

I2C Bus is a kind of serial 、 Sync 、 Multi host communication bus for half duplex communication , There are two data cables ：SDA（ cable ）、SCL（ Clock line ）; The hardware structure is simple , Lower cost
application ： Communication between different chips on the same circuit
characteristic ：
1、I2C Bus is a kind of multi host bus , Connected to the I2C The devices on the bus are divided into master and slave ;
2、 The host has the right to initiate and end a communication , The slave can only be called by the host ;
3、 When multiple hosts on the bus enable the bus at the same time ,I2C It also has the function of conflict detection and arbitration to prevent errors ;
4、 Each is connected to I2C Devices on the bus , Have a unique address （7 position ）, And each device can be used as a host （ There is only one host at a time ）, It can also be used as a slave ; The addition and deletion of devices on the bus will not affect the normal operation of other devices ;
5、I2C The bus is in communication , The device that sends data on the bus is called a transmitter , The device that receives data is called a receiver

Hardware connection diagram
Communication process ：
1、 Host send Start signal , Enable bus （ At this time, all devices on the bus can receive , And other devices will not enable the bus in this device ）
2、 The host sends a byte of data indicating Slave address And subsequent bytes Direction of transmission （ Host computer —> Slave , Or slave —> host ）
One byte of data ： front 7 Bit is the slave address , The last bit is the direction of transmission （ yes 0 Represents the host —> Slave ,1 Represents the slave —> host ）
3、 The addressed slave sends Answer signal Reply to the host , The check address of other devices is not their own address , And ignore
4、 The transmitter sends a byte of data
5、 The receiver sends a reply signal in response to the transmitter
The first 4、5 Step loop …
6、 When the communication is complete , Host send Stop signal Release the bus , One communication is completed

Start signal ： When SCL At high power level ,SDA From high to low
Stop signal ： When SCL At low power level ,SDA From low to high
Byte transfer and reply ： The transmitter sends a byte of data （ Transmit the high order first , Then transmit the low order ）; The receiver sends Response bit To respond to ,（1 Bit low level reply ）
Synchronous signal ：
During data transmission ,
When SCL by Low level period , The transmitter sends a bit of data to the data line , During this period, the signal on the data line is allowed to change
When SCL by High level period , The receiver reads a bit of data from the data line , During this period, the signal on the data line is not allowed to change （ because SCL by High level when , Signal changes on the data line will be considered Start signal perhaps Stop signal ）

SPI

SPI（Serial Peripheral Interface） Is the abbreviation of serial peripheral interface , It's a high speed 、 full duplex 、 Synchronous serial communication bus ; There are at least four wires ：MISO（ Master input 、 Output from the device ）,MOSI（ Main device output 、 Input from the device ）,SCLK（ Clock line ）,CS（ Film selection line ）
SPI Work in master-slave mode , There is usually a master device and one or more slave devices

Hardware connection diagram
Each additional slave is connected , It is necessary to connect more than one film selection line , and I2C Address is used in different ways
Communication process ：
1、SPI Bus transfers data ： Transmit the high order first , Then transmit the low order ; After a byte is transferred , No response required , You can start the transmission of the next byte
2、SPI The bus works synchronously , When the clock is on the rising or falling edge, the transmitter sends data to the data line , On the next falling or rising edge, the receiver reads data from the data line , So as to complete the transmission of one bit data ; Eight clock cycles can complete the transmission of one byte of data

Polarity and phase ： （ Judge according to the chip manual ）
SPI The bus has four different working modes , Depending on Polarity （CPOL） and phase （CPHA）

CPOL Express SCLK The state of being idle ：
When CPOL = 0,SCLK Low level indicates that the bus is idle
When CPOL = 1,SCLK A high level indicates that the bus is idle

CPHA Represents the sampling time ：
When CPHA = 0, The first clock of each cycle is sampled along , The phase is 0, Rising edge data , Falling edge data
When CPHA = 1, The second clock of each cycle is sampled along , The phase is 1, Falling edge data , Rising edge closing data

I2C and SPI Similarities and differences

The same thing ：
1、 Serial communication is adopted 、 How to synchronize
2、 All adopt TTL level , The transmission distance is similar to the application scenario （ Short range transmission , Onboard communication ）
3、 Both work in master-slave mode
Difference ：
1、I2C For half duplex communication ;SPI For full duplex communication
2、I2C There is a response mechanism ;SPI No response required
3、I2C Broadcast via bus Slave address To address ;SPI adopt ** Send the enable signal to the chip selection line ** To address
4、I2C The polarity and phase of the clock are fixed ;SPI The polarity and phase of the clock can be adjusted