Mobile Phone Camera Interface Primer – 1

Today almost every mobile phone contains a camera. In principle, mobile phone camera is a sensor/camera module designed for use across a range of mobile phone handsets and accessories. It embeds high quality still camera functions and also supports rich video. For these camera modules designed to work with any host with a standardized camera interface, separate hardware accelerator device (coprocessor) can be integrated in the mobile phone system to run the associated image processing algorithms in hardware where the baseband cannot support this processing load.
Or these camera modules can be directly connected to a baseband or multimedia processor. No dedicated coprocessor is required in the second configuration because the image processing is done in software (or hardware) within the baseband processor. Ofcourse, you can take these cameras from mobile phones and inteface them yourself with your advanced hobby electronics projects just as with any other standard add-on modules. However, good knowledge in popular camera interface techniques is a prerequisite to proceed with your succeeding dream project.

Behind The Camera Interface
Because the companies that make mobile phone cameras and the companies that make the application processors are usually different, there is a need for standardization of the camera/application processor interface. MIPI (mobile industry processor interface) Alliance has been on top of this, and the main connection is a fast serial interface known as CSI (camera serial interface).
The mobile phone handset industry had a need for a standard interface to attach camera subsystems to a host device, such as an application processor. In response, MIPI developed CSI2 several years ago. The Camera Working Group – develops and maintains camera serial interface and supporting documents – released the CSI-2 v1.0 specification in 2005. The group produced CSI-3, a next generation interface specification based on the MIPI foundation of UniPortM, in 2012.
CSI-2 consists of a DPHY and a CSI-2 transmitter at the camera and receiver on the application processor. The DPHY provides the physical interface, and the transmitter and receiver cover encoding, packing, error handling, lane distribution, assembly of image data stream, etc. However, the increasing pixel count and frame-rate is driving the need for even higher bandwidth, hence CSI-3. CSI-3 has a new MPHY, and each MPHY has a bandwidth of up to 6Gb/s per lane, with up to 4 lanes. The next level up is the Unified Protocol layer (UniPro). This defines a unified protocol for connecting devices and components designed to have high speed, low power, low pin count, small silicon area high reliability and so on.
CSI-2: The “Camera Serial Interface2 Specification” defines an interface between a peripheral device (camera) and a host processor. The host processor (baseband, application processor) here denotes the hardware and software that performs essential core functions for telecommunication or application tasks. Two high-speed serial data transmission interface options are defined. The first option – referred to in this specification as the “DPHY 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 DPHY. The second high-speed data transmission interface option, -referred to in this specification as the “CPHY 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 CPHY. The Camera Control Interface (CCI) for both physical layer options is a bidirectional (SDL-SDA) control interface compatible with the I2C standard.

CSI-3: This interface technology is much easier to implement in both hardware and software than the existing technologies. CSI-3 is a new standardized data and control interface between the camera subsystem and the host device. Note that, within a camera subsystem, various components such as a RAW camera sensor, an SoC (system – on a – chip) camera, or a multi-chip camera module can be connected to each other using a proprietary interconnect, or CSI-3.

The VX6953CB Camera Module
The VX6953CB 5.1 megapixel EDOF (Extended depth of field) camera module (from ST) is designed for use across a range of mobile phone handsets and accessories. It embeds high quality still camera functions and also supports HD video. VX6953CB produces raw Bayer 5 Mpixel images at 15 fps in RAW10, and supports the CCI control as well as CCP 2.0 and CSI-2 (D-PHY v1.0 compliant) data interfaces. As stated, the VX6953CB has both CCP2.0 and MIPI CSI-2 video data interfaces selectable over the camera control interface (CCI).
The image data is digitized using an internal 10-bit column ADC. The resulting pixel data is output as 8-bit, 10-bit or 10-8 bit compressed data and includes checksums and embedded codes for synchronization. The interface conforms to both the CCP 2.0 and MIPI CSI-2 interface standards. The sensor is fully configurable through a CCI serial interface. The module is available in a SMOP (small optical package) type package measuring 6.5 x 6.5 x 4.6 mm. It is designed to be used with a board-mounted SMIA65 (standard mobile imaging architecture) socket or flex cable.

Pinout and pin description of VX6953CB camera module, as viewed from the bottom of the module, is shown below. In the pinout table, note that pads T1-T8 are ST Test Points.

Since only a minimal list of external components is required, the VS6953CB features allow straight forward integration into custom-designs. VS6590 is another near-similar camera module from ST, but with only 0.5 Megapixel resolution (800Hx600V)and CCP 1.0 serial video interface.

• CCP → CCP stands for Compact Camera Port, the interface standard for portable cameras, developed by SMIA (standard mobile imaging architecture) -an organization promoting the standardization of mobile phone (cellphone) interfaces.
• CCI → This is usually a two or three-wire interface used to control the sensor module. Though named differently by different vendors (e,g. Serial Camera Control Bus, SCCB by Omni Vision), it usually confirms to the I2C standards (defined by Philips).
• SMIA → SMIA (Standard Mobile Imaging Architecture) is an imaging architecture especially suitable for mobile application use. The scope of SMIA covers a raw bayer output image sensor head: It specifies housing, mechanical interconnection, functionality, register set and electrical interface

In the next figure, you can see the camera wiring in a Nokia 2700C (Nokia 2700c2 RM-561) mobile phone circuitry. In the schematic diagram, the 12-pin camera connector is labelled as X3300. The camera module can be safely removed from this connector/socket using a special “Nokia Camera Remover Tool”, available as a service accessory. For more details, refer the official service documentation/service schematics published by NOKIA™.

Referenced Documents (including but not limited to):

• MIPI Alliance Standard for Camera Serial Interface 2 (CSI-2) v1.0
• MIPI Alliance D-PHY Specification (v00-90-00)
• High-Speed interface Technology for Image Data Transmission (FIND Vol.26)
• Camera Sensor Driver Development and Integration (PATH PARTNER)
• SMIA 1.0 Introduction and Overview (NOKIA & ST)
• Arasan’s White Papers & Articles

Part 2 → Mobile Phone Camera and Arduino/Raspberry Pi