Camera Module Interface Introduction

Introduction

The common computer camera interface is USB, while the common camera on smartphones is MIPI, and some of the cameras (such as certain hardware that supports DVP interface) are DVP; in layman’s terms, USB is short for Serial Universal Serial Bus, while MIPI is short for Mobile Industry Processor Interface ( In layman’s terms, USB stands for Universal Serial Bus, MIPI stands for Mobile Industry Processor Interface, DVP stands for digital video port, and CSI stands for CMOS Sensor Interface.

Camera Module Introduction

I. Working process

After the external light passes through the lens, it is filtered by the color filter and then shines on the sensor surface. The sensor converts the light transmitted from the lens into an electrical signal, which is then converted into a digital signal by the internal AD. If the sensor is not integrated with DSP, the data is transmitted to the baseband by DVP, and the data format is RAW DATA. If the DSP is integrated, the RAW DATA data is processed by AWB, color matrix, lens shading, gamma, sharpness, AE and de-noise, and then output in YUV or RGB format.

Finally, it will be sent to the frame buffer by the CPU for display, so that we can see the scene captured by the camera.

Camera Module Interfaces

I. MIPI Camera Module

1. What is MIPI?

MIPI Alliance, that is, the Mobile Industry Processor Interface (MIPI) Alliance.
MIPI (Mobile Industry Processor Interface) is an open standard and a specification for mobile application processors initiated by the MIPI Consortium.

2、Characteristics of MIPI inteface

MIPI is a differential serial port transmission, fast speed, anti-interference. Mainstream cell phone modules are now using MIPI transmission, transmission using four pairs of differential signals to transmit image data and a pair of differential clock signals; initially designed to reduce the number of wires between the LCD screen and the main control chip, and later developed to high speed, to support high-resolution display, now basically MIPI interface.

MIPI camera has three power supplies: VDDIO (IO power), AVDD (analog power), DVDD (kernel digital power), different sensor module camera power supply is different, AVDD has 2.8V or 3.3V; DVDD generally use 1.5V or higher, different manufacturers’ design is different, 1.5V may be provided by the sensor module or external 1.5V may be provided by the sensor module or externally, external power supply is recommended, the voltage should be greater than the internal DVDD; VDDIO voltage should be consistent with the level of the MIPI signal line, if the signal line is 2.8V level, then VDDIO should also provide 2.8V, some sensor modules can also not provide VDDIO, provided by the internal.

Additional note: MIPI camera interface is called CSI, and MIPI display interface is called DSI.

II. DVP interface camera

DVP bus PCLK limit of about 96M, and the length of the line can not be too long, the maximum rate of all DVP is best controlled below 72M, PCB layout easier to draw; MIPI bus rate lvds interface coupling, the line must be differential equal length, and the need for protection, so the PCB alignment and impedance control requirements are a little higher (in general, differential impedance requirements in 85 Ohm ~ 125 Ohm between).

DVP is a parallel port and requires PCLK, VSYNC, HSYNC, D[0:11] – can be 8/10/12bit data, depending on ISP or baseband support; MIPI is an LVDS low-voltage differential serial port and only requires CLKP/N, DATAP /The MIPI interface has fewer signal lines than the DVP interface, and because it is a low-voltage differential signal, it generates less interference and is more resistant to interference. The most important thing is that the DVP interface is limited in signal integrity, the rate is also limited. 500W can barely use DVP, 800W and above are using MIPI interface.

III. The CSI interface

1. CSI interface basic concepts

CSI-2 interface specification is released by the MIPI (Mobile Industry Processor Interface) Alliance organization in 2005 on the camera serial interface, which serves as a new interface framework between the camera device and the processor, to portable, cell phone cameras and other related industries to provide a flexible and high-speed device interface [28]. Previously, the traditional camera interface generally included a data bus, clock bus, synchronization signal line control lines, etc., and the physical interface block diagram is shown below.

This camera physical interface occupies more data lines, the logic design is also more complex and requires strict synchronization including horizontal synchronization signal, vertical synchronization signal as well as clock signal, which puts high requirements on this end of the camera as well as on this end of the receiver, at the same time, in the process of high-speed transmission, the direct use of digital signals as data is easily disturbed by other external signals, which is not as stable as differential signals. This also greatly limits the rate of transmission and the maximum image quality that the camera can transmit in real time.

Based on the CSI-2 camera data transmission process uses a differential data signal for the transmission of pixel values in the video, while the CSI-2 transmission interface can be streamlined or expanded very flexibly, for applications with fewer interfaces, the CSI-2 interface can use only one set of differential data signal lines and one set of differential clock lines to complete the camera data serial transmission process, which reduces the For large arrays of CCD cameras, the CSI-2 interface can also be extended with differential data lines to meet the high-speed requirements of parallel transmission of multiple data lines.

CSI-2 interface also integrates the control interface CCI (Camera Control Interface), CCI is a full-duplex master-slave device communication control interface capable of supporting 400KHz transmission rate, it is compatible with many existing processor IIC standard interface, so it can be very convenient to implement the Soc CCI Master Module to the CSI-2 TX side of the CCI Slave Module control, CSI-2 physical interface block diagram is shown below.

2. CSI physical protocol layer provisions

MIPI Alliance in addition to the new provisions in the camera interface also on the CSI-2 interface software architecture for further development, CSI-2 software framework is mainly divided into three layers, respectively, the application layer, protocol layer, physical layer, and for the protocol layer can be subdivided into pixel byte packing layer / unpacking 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 as well as decoding formats of the upper layer data streams, and specifies the mapping relationships of pixel to byte conversion.
Protocol layer: mainly includes the pixel/byte packing/byte unpacking layer, the LLP layer provides a synchronization mechanism for serial transmission data, and the channel management layer provides a data bit-width scalable function so as to flexibly adapt to different application scenarios.
Physical layer: defines the basic transmission medium specification, determines the input and output characteristics parameters of the physical layer of the CSI-2 protocol, and determines its electrical characteristics as well as clock timing.II