Mipi interface, DVP interface and CSI interface of camera [easy to understand]

Hello everyone , I meet you again , I'm your friend, Quan Jun .

Our commonly used computer camera interface is USB Interface , The common camera on smart phones is MIPI Interface , There are also some cameras （ For example, some support DVP Interface hardware ） yes DVP Interface ; Generally speaking ,USB It's a universal serial bus （Universal Serial Bus） For short , and MIPI Is the mobile industry processor interface （Mobile Industry Processor Interface）,DVP It's a digital video port （digital video port） For short ,CSI Camera serial interface （CMOS Sensor Interface） For short .

Camera How it works

One 、 working process

The outside light passes through lens after , after color filter After filtering, it irradiates Sensor On the surface ,Sensor Will be taken from lens The transmitted light is converted into an electrical signal , And then through the inside AD Convert to digital signals . If Sensor No set become DSP, Through DVP Transfer to baseband, At this time, the data format is RAW DATA. If integration 了 DSP, RAW DATA Data after AWB、 be color matrix、 lens shading、 gamma、 sharpness、 AE and de-noise Handle , Post output YUV perhaps RGB Formatted data .

Eventually it will be CPU Deliver to framebuffer Intermediate display , So we see camera The scene captured 了 .

Two 、camera Main components

Generally speaking ,camera Mainly from lens and sensor IC Two parts , Some of them sensor IC Integrate 了 DSP, Some are not integrated , But it also requires external DSP Handle . In terms of segmentation ,camera The equipment consists of several parts below Component composition ：

• lens（ The lens ） commonly camera The lens structure is composed of several lenses , There are plastic lenses （Plastic) And glass mirror (Glass) , Usually the lens structure has ：1P,2P,1G1P,1G3P,2G2P,4G etc. .
• sensor（ image sensor ） Senor It's a semiconductor chip , There are two types of ：CCD（Charge Coupled Device） Abbreviation for charge coupled device and CMOS（Complementary Metal-Oxide Semiconductor） Complementary metal oxide semiconductor .

notes ：

• CCD sensor , The charge signal is transmitted first , Post amplification , Again A/D, High imaging quality and sensitivity 、 Good resolution 、 Small noise ; Slow processing speed ; High cost , Process complex .
• CMOS sensor , The charge signal is amplified first , after A/D, Retransmission ; Low imaging quality and sensitivity 、 Obvious noise ; Fast processing speed ; Low cost , Simple workmanship .
• sensor Will be taken from lens The transmitted light is converted into an electrical signal , And then through the inside AD Convert to digital signals . because sensor Each pixel Only light sensitive R Light or B Light or G light , So what each pixel stores at this time is monochromatic , We call it RAW DATA data .
• Want to put each pixel's RAW DATA Restore data to three primary colors , Need ISP To deal with it . ISP（ Image signal processing ） It mainly completes the processing of digital images , hold sensor Convert the collected raw data into display support The format of .
• CAMIF（camera controller ） On the chip camera Interface circuit , Control the equipment , receive sensor Give the collected data to CPU, And send it to LCD Display .

MIPI Interface

MIPI Cameras are common on mobile phones 、 In the plate , Support 500 High definition resolution of more than 10000 pixels . Its full name is “Mobile Industry Processor Interface”, It is divided into MIPI DSI and MIPI CSI, Corresponding to video display and video input standards respectively . at present ,MIPI Camera in embedded products , Like dash cam 、 Law enforcement instrument 、 HD micro camera 、 Network monitoring cameras are widely used .

One 、MIPI

1、 What is? MIPI？

MIPI union , Mobile industry processor interface （Mobile Industry Processor Interface abbreviation MIPI） union . MIPI（ Mobile industry processor interface ） yes MIPI An open standard and a specification for mobile application processors initiated by the alliance .

2、MIPI Characteristics

MIPI Differential serial transmission , Fast , anti-interference . Mainstream mobile phone modules now use MIPI transmission , Use... For transmission 4 Image data and a pair of differential clock signals are transmitted to the differential signal ; Originally to reduce LCD It is designed according to the number of wires between the screen and the main control chip , Later, it developed to high speed , Support high-resolution display , Now it's basically MIPI The interface .

MIPI The camera has three power supplies ：VDDIO（IO Power Supply ）,AVDD（ Analog power ）,DVDD（ Core digital power supply ）, Different sensor The camera power supply of the module is different ,AVDD Yes 2.8V or 3.3V Of ;DVDD In general use 1.5V Or higher , Different manufacturers have different designs ,1.5V Maybe by sensor Module supply or external supply , If external power supply can be used, it is recommended to use external power supply , The voltage must be greater than the internal voltage DVDD;VDDIO The voltage should be equal to MIPI The level of the signal line is consistent , If the signal line is 2.8V level , be VDDIO Also available for 2.8V, There are some sensor The module can also not be used for VDDIO, Provided internally .

Additional explanation ：MIPI Of camera The interface is called CSI,MIPI Of display The interface is called DSI.

Two 、DVP

DVP Bus PCLK The limit is about 96M about , And the length of the line should not be too long , all DVP The maximum speed should be controlled at 72M following ,PCB layout Easier to draw ;MIPI Bus rate lvds Interface coupling , The routing must be differential equal length , And need protection , So yes PCB Wiring and impedance control requirements are higher （ Generally speaking, the differential impedance is required at 85 ohm ~125 Between ohms ）.

DVP It's a parallel , need PCLK、VSYNC、HSYNC、D[0：11]—— It can be 8/10/12bit data , The specific situation depends on ISP or baseband Do you support ;MIPI yes LVDS Low voltage differential serial port , Just need to CLKP/N、DATAP/N—— The biggest support 4-lane, commonly 2-lane It can be done .MIPI Interface than DVP There are few interface signal lines , Because it is a low-voltage differential signal , The interference is small , The anti-interference ability is also strong . most important of all DVP The interface is limited in terms of signal integrity , The rate is also limited .500W You can barely use DVP,800W And above MIPI Interface .

3、 ... and 、 CSI Interface

1、CSI Basic concept of interface

CSI-2 The interface specification is made up of MIPI(Mobile Industry Processor Interface) The alliance was organized in 2005 Camera serial interface released in , As a new interface framework between camera equipment and processor , Here you are 、 Mobile camera and other related industries provide a flexible and high-speed device interface [28]. before , Traditional camera interfaces generally include data bus 、 Clock bus 、 Synchronization signal line, control line, etc , The physical interface block diagram is shown below ：

The physical interface of this camera occupies more data lines , The logic design is also complex , Horizontal synchronization signals are required , Vertical synchronization signal and clock signal , This puts forward higher requirements for the camera and the receiver , meanwhile , In the process of high-speed transmission , Directly using digital signals as data is easy to be disturbed by other external signals , Not as stable as the differential signal , This also greatly limits the transmission rate and the maximum image quality that the camera can transmit in real time .

be based on CSI-2 In the process of camera data transmission, the data differential signal is used to transmit the pixel value in the video , meanwhile CSI-2 The transmission interface can be simplified or expanded flexibly , For application scenarios with few interfaces ,CSI-2 The interface can only use a group of differential data signal lines and a group of differential clock lines to complete the data serial transmission process of the camera , This reduces the load , At the same time, it can also meet a certain transmission rate , For large arrays CCD The camera ,CSI-2 The interface can also extend its differential data line , So as to meet the high-speed requirements of parallel transmission of multiple groups of data lines .

meanwhile CSI-2 The control interface is also integrated in the interface CCI(Camera Control Interface),CCI Is one that can support 400KHz Communication control interface of full duplex master-slave equipment with transmission rate , It can be compatible with many existing processors IIC Standard interface , Therefore, it is very convenient to realize Soc On CCI Master Module To CSI-2 TX End CCI Slave Module The control of ,CSI-2 The physical interface block diagram is shown in the figure below .

2、CSI Physical protocol layer provisions

MIPI In addition to new regulations on the interface of the camera, the alliance also stipulates CSI-2 The software architecture of the interface is further developed ,CSI-2 The software framework is mainly divided into three layers , They are application layer 、 Protocol layer 、 The physical layer , The protocol layer can be subdivided into pixel byte packaging layer / Decontamination layer 、LLP(Low LevelProtocol) layer 、 Channel management (Lane Management), The main system software block diagram is shown below ：

CSI Protocol layer design ： application layer ： It mainly designs the encoding and decoding format of the upper data stream , It specifies the mapping relationship between pixels and bytes ; Protocol layer ： It mainly includes pixels / Byte packing / Byte unpacking layer ,LLP Layer provides the synchronization mechanism of serial transmission data , The channel management layer provides data bit width scalability , So as to flexibly adapt to different application scenarios ; The physical layer ： Defines the basic transmission medium specification , To determine the CSI-2 Input and output characteristic parameters of protocol physical layer , And determine its electrical characteristics and clock timing .

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Reference material

1. Xiaomiao blog ：CSI Basic introduction to camera interface
2. Camera MIPI Interface 、DVP Interface and CSI Interface
3. MIPI CSI-2 Interface protocol
4. CMOS Sensor Interface(CSI)

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