Beyond Logic



CMOS Digital Image Sensors

    Adding vision to your projects needs not be a difficult task. Whether its machine vision for robot control or the sampling and storage of images for security, CMOS images sensors can offer many advantages over traditional CCD sensors. Just some of the technical advantages of CMOS sensors are,

    • No Blooming
    • Low power consumption. Ideal for battery operated devices
    • Direct digital output (Incorporates ADC and associated circuitry)
    • Small size and little support circuitry. Often just a crystal and some decoupling is all that is needed.
    • Simple to design with.

    There are many manufacturers making CMOS Image Sensors. Just some of the more notable ones are Micron who acquired Photobit, OmniVision, ST who acquired VLSI Vision, Mitsubishi and Kodak.

    There are two different categories of CMOS Sensors based on their output. One type will have a analog signal out encoded in a video format such as PAL, NTSC, S-Video etc which are designed for camera on a chip applications. With these devices you simply supply power and feed the output straight into you AV Equipment. Others will have a digital out, typically a 4/8 or 16 bit data bus. These 'digital' sensors simplify designs, where once a traditional 'analog' camera was feed into a video capture card for conversion to digital. Today, digital data can be pulled straight from the sensor.

    The main components to a Digital Video Camera design are

    • CMOS Image Sensor. The heart of the camera. It produces a digital/analog output representing each pixel. It's support circuitry will normally include a Crystal Oscillator and power supply decoupling. Some sensors may need a resistive bias network of some type. All of these components are normally surface mounted on the back of the PCB and occupies very little real estate.
    • The lens Holder. This will be either a plastic or metal mount which attaches to your PCB and allows a standard size lens to be screwed in. The screw thread facilitates focusing for fixed lens systems. The base of the lens mount may also have a IR (Infra Red) filter.
    • The Lens. This will determine your Field of view among other things. Lenses range from fish-eye to telescopic and need to be purchased to fit the parameters of your sensor and lens holder.

    Once you have completed the above, you have yourself a imaging system which constantly spits out a pixel data stream synchronised to a pixel, frame and/or line clocks. Connecting this directly to a microcontroller/processor system will cause headaches. Trying to clock this raw data in will use up great amounts of CPU time, if your uC could do it in the first place. If you drop a pixel because an ISR is doing some thing more privileged, then you have no ability to sample that location again, and thus no method of error correction.

    While the frame rate on many devices can be slowed down by using internal divisors, it still doesn't reach an acceptable speed nor allow random access to pixels. Reducing the master clock rate of the device will effect exposure times and other time dependent settings, thus is not an option. Clearly some additional circuitry will need to be designed.

    By using a CPLD/FPGA and RAM, you can program the CPLD to dump the data straight into RAM. Your micro could then read this RAM through the PLD which could be memory mapped. If you really want performance (And budget is not a problem), you could use Dual Port RAM. If however you only want to capture one frame, then the PLD could copy one frame into memory and ignore subsequent pixel data until an event such as when your device has read all the data out of RAM. Other options are to use a LVDS (Low Voltage Differential Signalling) serial bus, to relay your data over a few metres or more. At a high enough clock rate, you won't wait all day for a frame.

    The other thing you must not forget is how to control the sensor. Most of it's internal parameters are controlled by a serial bus, typically I2C for the majority of sensors. This can either be controlled through a memory mapped Register programmed into your PLD or via an I2C port straight from your uC. All up this makes quite a cheap way to capture video. Ideal for your Embedded Linux Systems.

    OmniVision Technologies

      OmniVision not only develops CMOS Image Sensors, but also support device ICs such as the OV-511 & OV-518 Advanced Camera to USB Bridge. OmniVision is one of the more popular manufacturers with devices such as the OV7910 NTSC/PAL Camera on a Chip being used in many small analog camera modules around the world. This would be the recommended starting point if you are starting out designing with CMOS Image Sensors.

        OV9620

        • SXGA 1280 x 1024 Colour (OV9620) or Monochrome (OV9121) (1.3 mega-pixel)
        • 1/2" Optical Format
        • 15 fps @ SXGA
        • 10 bit Raw RGB Data Out
        • Requires 3.3V and 2.5V supplies
        • CLCC-48 Package

        OV8610

        • SVGA 800 x 600 Colour
        • 1/3" Optical Format
        • 15 fps @ SVGA
        • 10 bit Raw RGB Data Out
        • Requires single 3.3V supply < 30mA
        • CLCC-48 Package

        OV7640

        • 640 x 480 Colour (OV7640) or Monochrome (OV7141)
        • 1/4" Optical Format
        • 30fps @ VGA, 60fps @ QVGA
        • YUV/YCbCr 4:2:2, RGB 4:2:2 or Raw 8 bit RGB Outputs
        • 2.5V Core & Analog Supply / 2.5 or 3.3V I/O supplies. Power Consumption under 40mW
        • PLCC-28 Package

        OV7620

        • 664 x 492 Colour (OV7620) or Monochrome (OV7110)
        • 1/3" Optical Format
        • 0.5 to 30 fps
        • YUV/YCbCr 4:2:2, RGB 4:2:2 or Raw 8 bit RGB Outputs
        • 5V Supply <120mW
        • 48-pin LCC

        OV6630

        • 352 x 288 Colour (OV6630) or Monochrome (OV6130)
        • 1/4" Optical Format
        • up to 60 fps
        • YUV/YCbCr 4:2:2, RGB 4:2:2 or Raw 8 bit RGB Outputs
        • 3.3V Core < 20mA / 3.3V or 5V I/O
        • LCC-48 Package

      OmniVision and some third party vendors (e.g. COMedia) have evaluation modules for the OmniVision sensors. This allows you to get up to speed with the sensor, incorporating a PCB with de-coupling, a Lens and Lens Holder. The majority of the sensor's signals are broken out to a header which you can use to interface to your own designs. The evaluation modules in small quantities are normally much easier to obtain than the sensors themselves, and are typically cheaper as a result.


      A picture of the M3188 Evaluation Module with
      the lens holder removed. The signals can be obtained
      from the 32 pin header on the top of the module

      DIY Electronics (http://www.kitsrus.com) are just one outlet which sells the third party evaluation boards.


    Kodak

      In August 2001 Kodak launched it's first two CMOS Images to its Kodak Digital Science range of image sensors. Kodak has been in the game of CCD Sensors for twenty plus years with a wealth of imaging expertise and research.

        KAC-0311

        • 640 x 480 resolution
        • 1/3" Optical Format
        • 0 - 60 frames per second
        • Single 3.3V Supply <200mW
        • 48 pin ceramic LCC package

        KAC-1310

        • 1.3 megapixel CMOS sensor, 1280 x 1024 resolution
        • 1/2" Optical Format
        • 15 frames a second at 20MHz Clock (Full SXGA)
        • Single 3.3V Supply, <250mW
        • 48-pin ceramic LCC package

    Mitsubishi Chips

      Mitsubishi have broken the pack, to produce smaller resolution sensors. These sensors can typically be used for a range of applications such as finger print sensing, motor detection, gaming, tracing of moving parts etc. Just one application is the new optical mice flooding the market place. They use a low resolution Image Sensor to track movement on a wide variety of surfaces.

      Also unique to these sensors is in-built image processing. Both sensors can output edge enhanced or extracted data, making them ideal for tracking on small robots, industrial control etc. The sensors can also process 2D images into 1D. The output of each pixel is by the means of a analog potential, thus this must be fed into an ADC to return digital image data.

        M64285FP CMOS Image Sensor

        • 32 x 32 Pixel Black & White, 1/6" Optical Format
        • 5V Supply < 15mW
        • In Built Edge Extraction
        • Max 5000 frames per second
        • Analog Output to uC ADC
        • 10pin SO Package

        M64282FP Artificial Retina LSI

        • 128 x 128 Pixel Black & White, 1/4" Optical Format
        • 5V Supply < 15mW
        • Positive and negative image output, Edge enhancement / extraction
        • 10 to 30 frames per second
        • 16pin TSOP Package


    Micron

      Micron Imaging has aquired Photobit Corporation and inherited its IP and Image Sensors. CMOS APS (CMOS active pixel sensor) was first created by a team of JPL engineers lead by Dr Eric Fossum. Dr Fossum is now Fellow at Micron Tecnology Inc. Micron's Product range can be sought from Micron's Product Matrix

        MI-0111

        • CIF Resolution - 352 x 288 Colour
        • 1/5 Inch Optical Format
        • 0-30 Frames a Second
        • 3.3V Supply, < 55mW
        • 28-pin CLCC

        MI-0330

        • VGA Resolution - 640 x 480 Colour
        • 1/4 Inch Optical Format
        • 0-30 Frames a Second
        • 3.3V Supply, <100mW
        • 48-pin CLCC

    ST Microelectronics Imaging Division

      Spectronix have used the ST Sensors in their RoboCam Series. ST also offer a couple of CoProcessors, a STV0657 Digital CoProcessor, a STV0672 USB CoProcessor and a STV0680 DSC (Digital Still Camera) CoProcessor. The DSC CoProcessor offers an RS-232 / USB Interface and on board SDRAM Storage.

        VV5301/VV6301

        • VV5500 Monochrome / VV6500 Colour 648 x 484 VGA Sensor
        • 10bit ADC Output RAW
        • 3.3V-6.0V (Built In Regulator) <25mA
        • 48 LCC Package





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