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

Introduction

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. At the very least, it requires more bandwidth than is presently available (approximately 4 Gbps with CSI-2). Also, sophisticated devices within a mobile terminal need greater bandwidth to exchange control and configuration information, making 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

Purpose

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.

Scope

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 specification is to identify a standard interface between a camera and a host processor for mobile device applications.  

A host processor in this specification means the hardware and software that performs essential core functions for telecommunication or application tasks. The engine of a mobile terminal includes hardware and the functions, which enable the basic operation of the mobile terminal. These include, for example, the printed circuit boards, RF components, basic electronics, and basic software, such as the digital signal processing software.

Overview of CSI-2

The CSI-2 specification defines standard data transmission and control interfaces between transmitter and receiver. Data transmission interface (referred as CSI-2) is unidirectional differential serial interface with data and clock signals; the physical layer of this interface is the MIPI Alliance Standard for D-PHY [2]. 

Figure 1 illustrates connections between CSI-2 transmitter and receiver, which typically are a camera module and a receiver module, part of the mobile phone engine.

The control interface (referred as CCI) is a bi-directional control interface compatible with I2C standard.

 

Camera Parallel Interface

Introduction

The Camera Parallel Interface (CPI) is a parallel interface between a digital camera module and a mobile phone engine. This document specifies the requirements on the application processor engine (APE) and on the image sensor in order to allow the interfacing of the two components.

Overview

The Camera Parallel Interface (CPI) shall implement a 8-bit parallel link from image sensor to APE.

 

Camera Serial Interface

Introduction

The Camera Serial Interface (CSI) is a serial interface between digital camera module and mobile phone engine. The purpose of this document is to specify a standard interface between camera and phone engine. The mobile phone engine in this document means the hardware and software that performs essential core functions for telecommunication or application tasks. The engine of a mobile terminal includes hardware and the functions, which enable the basic operation of the mobile terminal. These include, for example, the printed circuit boards, RF components, basic electronics, and basic software, such as the digital signal processing software.

Overview

The interface between CSI transmitter and receiver consists of data transfer and control interface. The data transfer interface (referred as CSI in this document) is a unidirectional differential serial interface with data and clock signals. Figure 1 illustrates the connection between CSI transmitter and receiver, which typically are a camera module and a receiver module, part of the mobile phone engine.

The control interface (referred as CCI) is a bi-directional control interface compatible with I2C standard.

The physical layer of CSI is based on signaling scheme called SubLVDS, which is current mode differential low voltage signaling method modified from the IEEE 1596.3 LVDS standard for reduced power consumption. Electrical specifications for the SubLVDS I/O’s can be found from chapter 10. The use of SubLVDS enables the use of high data rates with low EMI with maximum transfer capacity of 208 Mbps. Thus transferring SVGA images (800 x 600 pixels) in YUV420 format at 30 fps is possible.

Transmission clock rate may vary between 1 MHz to 208 MHz. The maximum clock frequency of CSI should be chosen to be lowest possible for the application / transmitting device in question. It is recommended that CSI transmission clock is generated from host system clock using PLL or clock doubler. This simplifies greatly EMC design. Additional oscillators should be avoided in the camera module.

The CSI transmitter provides the data-qualifying clock to the CSI receiver. The transmission clock rate in practice can vary depending on transmitter implementation. The synchronization to receivers internal clock domain has to be handled inside the receiver. For example, a transmitter may use 13 MHz transmission clock for some type of data and 104 MHz clock for some other type of data. The principle of receiver operation is illustrated in Figure 2.