What is the mipi interface? How does it work?

In embedded vision systems, efficient transmission of image data is of utmost importance. The MIPI interface plays a central role in this process. You may have heard of it in smartphones, drones, car cameras, or industrial edge devices. So, what is a MIPI interface? How does a MIPI camera work? This article will provide a detailed analysis of MIPI technology, particularly the MIPI CSI-2 standard, and delve into the entire process of MIPI cameras from image capture to data transmission.

MIPI stands for Mobile Industry Processor Interface. It is not a single interface but a set of specifications developed by the MIPI Alliance to provide a unified interface standard for processors, peripherals, and sensors in mobile and embedded devices. The MIPI Alliance is a global collaborative organization comprising hundreds of technology companies. Founded in 2003, its mission is to define and promote hardware and software interface specifications to simplify component integration in mobile devices and devices influenced by mobile technology. This includes interfaces for cameras, displays, CPUs/GPUs, sensors, and memory.

what is mipi interface

Achieve interconnect standardization: Provide a common method for interfacing components from different vendors.
Accelerate time to market: Simplify design and integration processes.
Reduce costs: Optimize component integration and system complexity.
Improve performance: Ensure high bandwidth, low power consumption, and low electromagnetic interference (EMI) communications.

For cameras, the most relevant and widely adopted MIPI specification is MIPI CSI-2 (Camera Serial Interface-2).

CSI-1 is the original Mipi standard interface architecture, which defines the connection interface between camera modules and host processors.

CSI-2 was first released in 2005 and includes multiple protocol layers, such as:

Physical Layer
Lane Merger Layer
Low-Level Protocol Layer
Pixel to Byte Conversion Layer
Application Layer

In the second half of 2017, the second version of MIPI CSI-2 was released, supporting RAW-16 and RAW-20 color depths. Additionally, it supports increasing the number of virtual channels from 4 to 32, effectively reducing latency and improving transmission efficiency (LRTE).

The third version of MIPI CSI-2 was released in 2019 and supports RAW-24 color depth.

MIPI CSI-3 was first released in 2012, followed by the next version in 2014. It provides a high-speed bidirectional protocol for image and video transmission between cameras and hosts.

Of these three types, MIPI CSI-2 is the most commonly used interface in mobile and remote applications such as autonomous driving, drones, and computer vision.

High-speed serial data transmission: CSI-2 uses high-speed differential signals (D-PHY or C-PHY) to transmit image data at extremely high bandwidths. This makes it ideal for transmitting high-resolution and high-frame-rate images.
Low power consumption: The MIPI interface design prioritizes low power consumption, which is critical for battery-powered mobile and embedded devices.
Low pin count: Compared to traditional parallel interfaces, MIPI CSI-2 uses fewer pins, helping to reduce PCB size and lower costs.
Packet-based communication: MIPI CSI-2 uses packets for communication, with each packet containing data format and error correction code (ECC) functionality to ensure reliable data transmission.
Flexible channel configuration: Supports 1, 2, 4, or more data channels (Lanes), allowing flexible configuration based on bandwidth requirements.

In summary, MIPI CSI-2 is a camera interface connecting image sensors to embedded motherboards, designed for efficient and reliable control and processing of image data.

mipi camera module

Theoretically, the maximum bandwidth of a USB interface is 5 gigabits per second. However, the actual achievable bandwidth is approximately 3.6 gigabits per second.

This limits the ability of visual systems in most applications to quickly transmit image data for various processing and analysis tasks. CSI-2 addresses this issue. It offers a maximum bandwidth of 6 Gb/s, with an actual achievable bandwidth of approximately 5 Gb/s. Compared to a standard USB interface, CSI-2 enables more efficient and faster processing.

MIPI CSI-2 supports high-performance applications and high-resolution imaging. It can seamlessly transmit images and videos in 1080p, 4K, and 8K formats. It is suitable for both single-camera and multi-camera deployments. MIPI CSI-2 also provides enhanced machine perception capabilities across multiple applications.

Generally speaking, development boards in visual systems support MIPI CSI-2 and can be used in conjunction with other smart sensor solutions. In addition, they are compatible with different CPU boards.

MIPI CSI-2 supports communication between the MIPI D-PHY physical layer and the application processor or system-on-chip (SoC). It can be implemented on either the MIPI C-PHY℠ v2.0 or MIPI D-PHY℠ v2.5 physical layer. As a result, its performance enables channel expansion.

In a MIPI camera, the camera sensor captures images and transmits them to the CSI-2 host. During image transmission, the images are stored in memory as individual frames. Each frame is transmitted via a virtual channel. Then, each channel is split into multiple lines, with one line transmitted at a time. This allows for complete image transmission from the same image sensor, but with multiple pixel streams.

MIPI CSI-2 communicates using packets that include data format and error correction code (ECC) functionality. A single packet traverses the D-PHY layer and is then split into the required number of data channels. The D-PHY operates in high-speed mode and transmits the packet to the receiver via the channels.

The CSI-2 receiver is equipped with a D-PHY physical layer to extract and decode the data packets. This process is repeated frame by frame from the CSI-2 device to the host through an efficient and cost-effective implementation.

The MIPI interface, particularly MIPI CSI-2, serves as the critical bridge connecting image sensors to processors. Through efficient, low-power serial data transmission, it enables modern embedded vision systems to capture, process, and interpret complex visual information. Understanding what the MIPI interface is and how MIPI cameras function forms the foundation for exploring mobile and embedded imaging technologies, providing robust visual capabilities for a wide range of applications from smartphones to autonomous vehicles.

With over 10 years of experience and expertise in the embedded vision field, Muchvision has developed more than 40 MIPI camera modules compatible with all mainstream processing platforms. Please check out our embedded vision camera product portfolio for more information.