The use of cameras and displays is increasing in automotive, IoT and multimedia applications, and designers need a variety of image and display interface solutions to meet increasingly stringent power and performance requirements. Traditionally, designers have used MIPI Camera Serial Interface (CSI-2) and Display Serial Interface (DSI) to connect image sensors or displays to application processors or mobile applications such as smart phones. The SOC is connected. However, due to the reliable advantages and successful implementation of MIPI interfaces, they are now implemented in many new applications, such as Advanced Driver Assistance Systems (ADAS), infotainment, wearables, and enhanced/virtual reality headsets. In this article, we introduce designers to implement MIPI DSI and CSI-2 in automotive, IoT, and multimedia applications to enable multiple uses of camera and display input and output while meeting their bandwidth and power requirements.
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MIPI CSI-2 and DSI: used in mobile applications
In the past 10 years, the mobile market, especially smart phones, has grown tremendously, choosing MIPI CSI-2 and DSI interfaces to use multiple cameras and some displays in mobile devices. Using these interfaces, low-power, low-latency, and low-cost chip-to-chip connections between the host and the device enable designers to connect low-resolution and high-resolution cameras and displays. Both interfaces use the same physical layer, MIPI D-PHY, which is used to transfer data to the application processor or SOC (Figure 1).
Figure 1: MIPI DSI and CSI-2 implementation in mobile applications
MIPI interface in mobile applications
In Figure 2, a MIPI camera and display interface implemented in an ADAS and entertainment information application is shown. In today's cars, multiple cameras, front, rear and side, are installed to be able to view 360 degrees around the driver. In such an embodiment, the MIPI CSI-2 image sensor is coupled to an image signal processor, which is then coupled to the bridge device such that the entire module is coupled to the vehicle host system. In some cases, the in-vehicle entertainment information system employs DSI to implement a display interface that uses the same implementation.
Figure 2: Example of an ADAS application using the MIPI DSI and CSI-2 specifications
MIPI provides a complete set of specifications for automotive applications (Figure 3):
DSI: for driver information, no anti-display and entertainment information.
• CSI: for ADAS applications, backup cameras, collision avoidance, mirrorless vehicles, and in-vehicle passenger information acquisition.
Other interfaces: MIPI I3C for sensor connectivity, JEDEC UFS for embedded and removable card storage, and SoundWire and RFFE.
Figure 3: Using the MIPI Alliance Specification (image of the MIPI Alliance) in automotive applications
MIPI interface in IoT applications
There are many forms of IoT SOCs, and we're here to introduce a superset of components or interfaces, including MIPIs typically used in IoT SOCs. CSI-2, possible DSI, and processor with SOC vision processing components. Storage components include LPDDR for low power DRAM and embedded multimedia card (EMMC) for embedded flash. For wired and wireless communications, leverage adjustments are made in many specifications, such as Bluetooth low energy, Secure Data Input and Output (SDIO), and USB, depending on the target application. Security is an essential part of ensuring data transmission to the cloud and stored in the device. It is primarily composed of multiple engines, such as true random number generators and encryption algorithm accelerators.
Dozens or more sensors are connected in one component, which is a sensor and control subsystem with I2C or I3C and Serial Peripheral Interface (SPI). I3C is a new MIPI specification that takes the key features of I2C and SPI together and unifies them while retaining the two-wire serial interface. System designers can connect a large number of sensors to the device while minimizing power consumption and reducing component and implementation costs. At the same time, by using a single I3C bus, manufacturers can combine multiple sensors from different manufacturers to achieve new concepts while supporting longer battery life and cost-effective systems.
MIPI interface in multimedia applications
New use cases for MIPI cameras and display interfaces have emerged in multimedia applications such as virtual/enhanced display devices with high resolution cameras and displays. In such devices, the interface is responsible for transmitting and receiving multiple graphics from multiple sources, which are then processed and sent to the user with extreme quality. Three examples of multimedia application implementations are given below:
High-end multimedia processor: In this implementation, multiple display and camera inputs enter the image signal processor through CSI-2 and DSI, which typically comes from another application processor that has processed and received the image. The image signal processor then transmits the image to the camera or display via CSI-2 or DSI.
- Multimedia Processor: This implementation is mainly used for gesture or motion recognition, or human-machine interface. The two image sensors cooperate with the processor via the CSI-2 protocol to identify and process motion or pose data at the processor for further analysis and manipulation. The processed work or pose data is then transmitted to the application processing portion via the CSI-2 protocol.
- Bridge IC: Due to the presence of multiple image inputs and outputs, as described in the Automotive section, there is a need for bridge ICs. With a bridge IC, the output of the application processor can be split into two display streams.
Advantages of the MIPI interface
MIPI CSI-2 leverages the MIPI D-PHY physical layer to communicate with the application processor or SOC. The image sensor or CSI-2 device takes the image and transmits it to the CSI-2 host where the SOC is located. Before the image is transmitted, it is located in memory in a separate frame. Each frame is then transmitted over the virtual channel through the CSI-2 interface. For multiple image sensors that support different pixel streams and sometimes multiple exposures, a virtual channel is employed and a virtual channel identification is assigned to each frame. Considering the transmission of a complete image from the same image sensor but with multiple pixel streams, each virtual channel is divided into multiple lines, one line at a time.
MIPI CSI-2 communicates using data packets, which contain data formats and Error Correction Code (ECC) functions to protect the header and perform Cyclic Redundancy Check (CRC) on the payload. This type of implementation applies to all transmitted data packets from the image sensor to the SOC. A single packet is transmitted through the D-PHY, through the CSI-2 device controller, and then separated into multiple data channels. The D-PHY distributes the data to several data channels operating in high speed mode and transmits the data packets to the receiver over the channel. For CSI-2 receivers using their D-PHY physical layer, it is responsible for packet extraction and decoding, and finally transmits the packets to the CSI-2 host controller. This process is then repeated frame by frame between the CSI-2 device and the host in an efficient, low power, and low cost implementation.
In a typical system with multiple cameras and displays, the CSI-2 and DSI protocols will use the same physical layer (D-PHY). Depending on the specific target application, there are many things to consider during the discovery phase, such as the required bandwidth and device type. Understanding such considerations helps designers determine the D-PHY version, as well as the number of data channels and data channels required, and then determine the number of pins that need to be implemented in the system. Finally, the designer can determine the interface required and the memory of its target application. For example, there are various implementations in which CSI-2, built on top of the D-PHY, operates at 1.5 Gbps per data channel, and in other embodiments, operates at speeds up to 2.5 Gbps per data channel. Working at lower speeds means more power and area optimization, but most importantly, newer image sensors and displays that support faster speeds are not suitable for all applications.
to sum up
Multiple cameras and displays are now used in applications other than mobile, such as cars, the Internet of Things, and multimedia applications that include augmented/virtual reality. For all of these applications, high-speed, low-power camera and display interface solutions that meet today's high-resolution image processing requirements are needed. MIPI CSI-2 and DSI are reliable interfaces in the mobile market, mainly smartphones, and because of their successful implementation, they are now being tested in many new applications. Synopsys has a number of MIPI IP solutions, including controllers, PHYs, verification IP, and IP prototyping suites that are compatible with the latest MIPI specifications, enabling designers to incorporate the required functionality into their mobile, automotive and IoT SOCs. At the same time, it can meet the requirements of power consumption, performance and timely market introduction.
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