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™.
Note!
This article is based on an ongoing R&D work, now live @ TechNode
PROTOLABZ. Although it is a commercial project,the project will be
solely published (sometime later) in
electroschematics.com
(R&D @ TechNode PROTOLABZ)
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
