Camera Interface Specifications

The Camera Working Group develops and maintains Camera Serial Interface and supporting documents. The group released produced the CSI-2 v1.0 specification in 2005. The group produced CSI-3, a next generation interface specification based on the MIPI foundation of UniPort-M, in 2012

Specifications are available to MIPI members only. For more information on joining MIPI, please go to Join MIPI.

MIPI Alliance Standard for Camera Serial Interface CSI-3

The mobile handset industry has long had a need for a standard interface to attach camera subsystems to a host device, such as an application processor. In response, MIPI developed CSI-2 (see below) several years ago. Many companies adopted the technology to support a range of devices in millions of mobile products. As end-user requirements continue to evolve, MIPI meets current and future market needs with a new generation of camera serial interface, complementary to CSI-2, which remains useful for many camera applications.
 
Camera subsystems in mobile phones tend to closely follow similar subsystems in still and video cameras. High resolution sensors potentially allow digital zoom, and deep color pixels increase dynamic range, for still images, as well as for moving images at high frame rates. New applications such as stereo image capture introduce new challenges to handset and mobile terminal designers.

A greater feature set, with more options, has a direct impact on the interface between camera subsystem and host. The need for bandwidth increases every year, while sophisticated devices within a mobile terminal need greater bandwidth to exchange control and configuration information, making also a high-speed reverse link essential. Pin count reduction as well as lower power consumption are additional requirements.

On the manufacturing and engineering side, in order to keep assembly lines running at capacity in a typical manufacturing environment, it might be necessary to substitute components from a list of alternative devices. For complex devices, such as a camera subsystem, substituting one device for another typically requires loading different software drivers to fully support the new device. Therefore, the ability for a device to identify its type and operational parameters can simplify the manufacturing process.

In addition, to reduce design-time effort and to focus the know-how of camera experts on their application rather than on interface issues, a flexible, reusable and scalable protocol stack is required from the networking layer down to physical layer, Features of the protocol stack might include data integrity, scalable bandwidth, burst mode to maximize power efficiency and support for different data formats. Also, EMI concerns on a PCB (trace location, length) dictate that the physical layer optionally supports optical interconnects. Finally, any new interface technology must be easier to implement in both hardware and software than existing technologies.

To fulfill these requirements, this document defines a standardized data and control interface between a camera subsystem and a host device called CSI-3. Within a camera subsystem, various components such as a RAW camera sensor, an SOC camera, a multichip camera module or an ISP can be connected to each other using a proprietary interconnect, or CSI-3.
 

 

MIPI Alliance Standard for Camera Serial Interface CSI-2

Demand for increasingly higher image resolutions is pushing the bandwidth capacity of existing host processor-to-camera sensor interfaces. Common parallel interfaces are difficult to expand, require many interconnects and consume relatively large amounts of power. Emerging serial interfaces address many of the shortcomings of parallel interfaces while introducing their own problems. Incompatible, proprietary interfaces prevent devices from different manufacturers from working together. This can raise system costs and reduce system reliability by requiring “hacks” to force the devices to interoperate. The lack of a clear industry standard can slow innovation and inhibit new product market entry. CSI-2 provides the mobile industry a standard, robust, scalable, low-power, high-speed, cost-effective interface that supports a wide range of imaging solutions for mobile devices.
 
The Camera Serial Interface 2 Specification defines an interface between a peripheral device (camera) and a host processor (baseband, application engine). The purpose of this document is to specify a standard  interface between a camera and a host processor for mobile applications.
 
This Revision of the Camera Serial Interface 2 Specification leverages D-PHY 1.2 and introduces C-PHY 1.0, both with improved skew tolerance and higher data rates. These enhancements enable higher interface bandwidth and more flexibility in channel layout. The CSI-2 1.3 Specification was designed to ensure interoperability with CSI-2 1.2 when the former uses the D-PHY physical layer. If the C-PHY physical layer only is used, backwards compatibility cannot be maintained.
 
A host processor in this document refers to the hardware and software that performs essential core functions for telecommunication or application tasks. The engine of a mobile terminal includes hardware 10 and the functions, which enable the basic operation of the mobile terminal. These include, for example, the 11 printed circuit boards, RF components, basic electronics, and basic software, such as the digital signal processing software.
 
The CSI-2 Specification defines standard data transmission and control interfaces between transmitter and receiver. Two high-speed serial data transmission interface options are defined. The first option, referred to in this specification as the “D-PHY physical layer option”, is a unidirectional differential interface with one 2-wire clock Lane and one or more 2-wire data Lanes. The physical layer of this interface is defined by the MIPI Alliance Specification for D-PHY. Figure 1 illustrates the connections for this option between a CSI-2 transmitter and receiver, which typically are a camera module and a receiver module, part of the mobile phone engine.
 
The second high-speed data transmission interface option, referred to in this specification as the “C-PHY physical layer option”, consists of one or more unidirectional 3-wire serial data Lanes, each of which has its own embedded clock. The physical layer of this interface is defined by the MIPI Alliance Specification for C-PHY. Figure 2 illustrates the CSI transmitter and receiver connections for this option.
 
The Camera Control Interface (CCI) for both physical layer options is a bi-directional control interface compatible with the I2C standard.