Difference and application of SPI, UART and I2C communication

Communication between electronic devices is like communication between humans , Both sides need to speak the same language . In Electronics , These languages are called communication protocols .

     I've shared it alone before SPI、UART、I2C Communication articles , This article makes some comparisons between them .

Serial VS parallel

     Electronic devices talk to each other by sending data bits . Bits are binary , Can only be 1 or 0. Through the rapid change of voltage , Bits are transferred from one device to another . In order to 5V In the working system ,“0” adopt 0V Short pulses of communication , and “1” adopt 5V Short pulses of communication . 

     Data bits can be transmitted in parallel or serial form . You can also learn about... Through this video : Video Explanation UART、I2C、SPI Serial port communication . In parallel communication , Data bits are transmitted simultaneously on the wire . The figure below shows binary （01000011） Middle letter “C” Parallel transmission of ：

     In serial communication , Bits are sent one by one over a single wire . The figure below shows binary （01000011） Middle letter “C” Serial transmission of ：

SPI signal communication

    SPI It is a common device general communication protocol . It has a unique advantage that it can transmit data without interruption , Any number of bits can be continuously transmitted or received . And in the I2C and UART in , Data is sent in packets , With a limited number of digits .

     stay SPI In the device , Equipment is divided into master and slave systems . The host is a control device （ It's usually a microcontroller ）, And the slave （ It's usually a sensor , Display or memory chip ） Get instructions from the host .

     A set of SPI The communication consists of four signal lines ：MOSI (Master Output/Slave Input) – The signal line , Host output , Slave input .MISO (Master Input/Slave Output) – The signal line , Host input , Slave output .SCLK (Clock) – Clock signal .SS/CS (Slave Select/Chip Select) – Piece of optional signal .

SPI Characteristics of the agreement

     actually , The number of slaves is limited by the system load capacitance , It will reduce the ability of the host to accurately switch between voltage levels .

Clock signal

     One bit of data per clock cycle , So the speed of data transmission depends on the frequency of the clock signal .  The clock signal is generated by the host configuration , therefore SPI Communication is always initiated by the host . 

     Any communication protocol in which devices share clock signals is called synchronization .SPI It's a synchronous communication protocol , There are also asynchronous communications that do not use clock signals .  For example, in UART In communication , Both sides are set to the pre configured baud rate , The baud rate determines the speed and timing of data transmission .

Piece of optional signal

     The master pulls down the slave CS/SS Enable communication .  At leisure / In the non transmission state , The chip selection line remains high . There can be multiple on the host CS/SS Pin , Allow the host to communicate with multiple different slaves .

Uploading … Re upload cancel ​

     If the host has only one chip select pin available , These slave devices can be connected in the following ways ：

MOSI and MISO

     The host by MOSI Send data to the slave in serial mode , The slave can also pass through MISO Send data to the host , Both can be done at the same time . So theoretically ,SPI It is a full duplex communication protocol .

Transmission steps

1.  Host output clock signal

2. The main engine is pulled down SS / CS Pin , Activate slave

3. The host by MOSI Send data to the slave

4. If you need a response , Then the slave passes MISO Return the data to the host

     Use SPI There are some advantages and disadvantages , If you choose between different communication protocols , Then it shall be fully considered according to the project requirements .

SPI advantage

    SPI Communication has no start and stop bits , So the data can flow continuously without interruption ; Didn't like I2C Such a complex slave addressing system , The data transmission rate ratio is I2C Higher （ Almost twice as fast ）. independent MISO and MOSI line , Can send and receive data at the same time .

SPI shortcoming

    SPI Use four wires （I2C and UART Use two wires ）, No confirmation of successful signal reception （I2C With this feature ）, There's no form of error checking （ Such as UART Parity bits in ）.

    UART Stands for universal asynchronous receiver / The transmitter is also called serial communication , It is not like SPI and I2C Such a communication protocol , It's a physical circuit in the microcontroller or an independent IC.

    UART The main purpose of is to send and receive serial data , Its best advantage is that it uses only two lines to transmit data between devices .UART The principle of is easy to understand , But if you haven't read SPI Communication protocol , That may be a good starting point .

UART signal communication

     stay UART In communication , Two UART Communicate directly with each other .  send out UART Control the equipment （ Such as CPU） The parallel data is converted to serial form , It is sent serially to the receiver UART. Only two lines are needed to work in two UART Transfer data between , Data is sent from UART Of Tx The pin flows to the receiver UART Of Rx Pin ：

    UART It belongs to asynchronous communication , This means that there is no clock signal , Instead, add start and stop bits to the packet . These bits define the beginning and end of the packet , So receive UART Know when to read this data . 

     When receiving UART When the start bit is detected , It will read at a specific baud rate . Baud rate is a measure of data transmission speed , In bits per second （bps） Express . Two UART Must operate at approximately the same baud rate , Send and receive UART The baud rate difference between can only be about 10％.

UART working principle

     send out UART After obtaining parallel data from the data bus , It adds a start bit , A parity bit and a stop bit are used to form a packet and start from Tx Bit by bit serial output on the pin , receive UART In its Rx Read the packet bit by bit on the pin .

    UART The packet contains 1 Starting bits ,5 to 9 Data bits （ Depending on UART）, An optional parity bit and 1 Or 2 Stop bits ：

Start bit ：

    UART The data transmission line is usually maintained at a high voltage level when data is not transmitted . Send when transmission starts UART Pull the transmission line from high level to low level in one clock cycle , When receiving UART When a high to low voltage transition is detected , It starts reading bits in the data frame at baud rate .

Data frame ：

     The data frame contains the actual data being transmitted . If parity bits are used , It can be 5 position , most 8 position . If you don't use parity bits , Then the length of the data frame can be 9 position . 

Check bit ：

     The parity bit is received UART How to determine whether there is any data change during transmission . receive UART After reading the data frame , It will have a pair value of 1 Count the number of bits , And check whether the total number is even or odd , Whether it matches the data .

Stop bit ：

     To signal the end of a packet , send out UART Drive the data transmission line from low voltage to high voltage for at least two bits .

Transmission steps

1. send out UART Receive data in parallel from the data bus ： 

2. send out UART Set start bit , Parity bits and stop bits are added to the data frame ：

3. The whole packet is sent from UART Serial send to receive UART. receive UART The data line is sampled at a pre configured baud rate ：

4. receive UART Discard the start bit in the data frame , Parity bit and stop bit ：

5. receive UART Converting serial data back to parallel data , And transmit it to the data bus at the receiving end ：

     No communication protocol is perfect , however UART Very good at their work . Here are some pros and cons , Can help you determine if they are suitable for your project needs ：

advantage

• Use only two wires

• No clock signal needed

• Have parity bits to allow error checking

• As long as both sides set up the packet structure

• Well documented and widely used methods

shortcoming

• The maximum size of the data frame is 9 position

• Multiple slave systems or multiple master systems are not supported

• Every UART The baud rate must be within each other's 10％ within

I2C signal communication

    I2C The bus is made up of Philips The company developed a simple 、 Two way two wire synchronous serial bus . It only needs two wires to transmit information . It is a combination of SPI and UART The advantages of , You can connect multiple slaves to a single host （ Such as SPI like that ）, Multiple hosts can also be used to control one or more slaves . When you want multiple microcontrollers to record data to a single memory card or display text to a single memory card LCD when , This will be very useful .

    SDA (Serial Data) – cable .

    SCL (Serial Clock) – Clock line .

    I2C It's a serial communication protocol , So the data goes along SDA Bit by bit . And SPI equally ,I2C A clock synchronization signal is also required and the clock is always controlled by the host .

working principle

    I2C The data transmission is based on multiple msg In the form of , Every msg Contains the binary address frame of the slave , And one or more data frames , It also includes start conditions and stop conditions , read / Between write bits and data frames ACK / NACK position ：

Starting conditions ： When SCL It's high level ,SDA Switch from high level to low level .

Stop conditions ： When SCL It's high level ,SDA Switch from low level to high level .

Address frame ： Unique to each slave 7 Bit or 10 Bit sequence , Used for address identification between master and slave devices .

read / Write in ： a , If the master sends data to the slave, it is low , The request data is high .

ACK/NACK： Each frame in the message is followed by a ACK/NACK position . If the address frame or data frame is successfully received , The receiving device will return a ACK Bits are used to indicate confirmation .

Addressing

     because I2C Didn't like SPI That kind of film selection line , Therefore, it needs to use another way to confirm a slave device , And this way is ——  Addressing  .

     The host sends the slave address to be communicated to each slave , Then each slave compares it with its own address . If the address matches , It will send a low level to the host ACK position . If it doesn't match , Do nothing ,SDA The line remains high .

read / Write in  

     The end of the address frame contains a read / Write in . If the master wants to send data to the slave , Low level . If the master requests data from the slave , High level .

Data frame

     When the host detects the of the slave ACK Behind you , You can send the first data frame . The data frame is always 8 position , Each data frame is followed by ACK / NACK position , To verify the reception status . When all data frames are sent , The master can send a stop condition to the slave to terminate the communication .

Transmission steps

1. stay SCL The line is at high level , The host will SDA The line switches from high level to low level to start bus communication .

2. The master sends to the bus the information of the slave to communicate with 7 Bit or 10 Bit address , And reading / Write in ：

3. Each slave compares the address sent by the host with its own address . If the address matches , Then the slave will pass SDA Pull the line down one bit and return one ACK position . If the address of the master does not match the address of the slave , Then the slave will SDA The line goes up .

4. The host sends or receives data frames ：

5. After transmitting each data frame , The receiving device will have another ACK Bits are returned to the sender , To confirm that the frame has been successfully received ：

6. Then the host will SCL Switch to high level , And then SDA Switch to high level , So send stop conditions to the slave .

Single host VS Multiple slaves

     because I2C Use the addressing function , Multiple slaves can be controlled by one master . Use 7 Bit address , You can use at most 128(27) A unique address . Use 10 Bit addresses are not common , But it can provide 1,024(210) A unique address . If you want to connect multiple slaves to a single host , Please use 4.7K The pull-up resistor of Ohm connects them , For example SDA and SCL The wire is connected to Vcc：

Multiple hosts VS Multiple slaves

    I2C Support multiple hosts to connect with multiple slaves at the same time , When two hosts try to pass SDA When the line sends or receives data at the same time , There will be problems . Therefore, each host needs to detect before sending messages SDA Is the line low or high . If SDA The line is low level , It means that another host is controlling the bus . If SDA Line height , You can safely send data . If you want to connect multiple hosts to multiple slaves , Please use 4.7K The pull-up resistance of Ohm will SDA and SCL The wire is connected to Vcc：

     Compared with other agreements ,I2C It may sound complicated . Here are some pros and cons , Can help you determine if they are suitable for your project needs ：

I2C advantage

• Use only two wires

• Support multiple hosts and multiple slaves

• Every UART The baud rate must be within each other's 10％ within

• Hardware ratio UART It's simpler

• A well-known and widely used protocol

I2C shortcoming

• The data transmission rate ratio is SPI slow

• The size of the data frame is limited to 8 position