USB Camera module have been using in various difference device in our life. As the developing of technology, camera module is not untouchable in civil usage, even customized OEM Camera module is available in many maufacturer. Today we will go thought the basic knowledge of USB camera module.
Signal from DSP to Sensor
I2C: SCL, SDA
The signal interface used by the DSP for the illegal command of the sensor, there will be no signal at ordinary times, only when entering/leaving preview mode (when AP is turned on/off) and when doing AE, AWB . All I2C interfaces have an open-drain structure, so a pull high resistor must be added externally.
XVCLK: 24M Hz
pixel clock input. All output digital signals use this clock as the reference point.
The voltage level depends on the different sensors. The pin name with “/” at the beginning or B or # at the end is a low-level honor; otherwise, it is a high-level dynamic margin. DSP does not necessarily control this pin.
Turn on the sensor when entering preview mode; turn off at other times.
Signal from Sensor to DSP
There will be a reply signal only when the DSP has a command.
24M Hz pixel clock output
Vertical sync signal
Horizontal sync signal
D0-D11 (12bits), D0~D9 (10bits), D0~D7 (8bits): expensive image
The image is composed of many small dots arranged and combined to form the entire graphic. For example 20X20 image means that the image is composed of 20 equal parts wide and 20 equal parts high, with a total of 400 small dots (pixels). 48X48 is composed of 2304 small dots to form a complete picture.
In the same area, the more pixels there are, the higher the resolution and the clearer the image. Generally so-called The 300,000 pixels refers to the resolution of 640X480; 1.3 million pixels is 1280X960; rain million pixels is 1600X1200.
Lens is an important original used to capture the image and present it on the Sensor. The quality of the Lens has a considerable relationship with the presentation of the image.
IR Filter: It is used to filter light other than red, because infrared will affect the sensor’s sensing, and human eyes cannot see light other than red, so IR filter is used to filter infrared.
Cover Glass: The protective glass layer is used to isolate the IR filter and the Sensor.
Image Plane: The area where the image is presented.
In the picture, the subject appears on the photosensitive original through Lens, and the image will appear completely reversed. Generally speaking, Lens is made of glass or plastic, and can also be used in combination, such as two layers of plastic and one layer of glass. Glass material has better image quality than plastic material, but relatively expensive glass. When the resolution is CIF (352 X 288), only one layer of plastic is enough, while VGA (640 X480) requires two layers of material, and 1.3M (1280 X 1024) requires three layers. At present, there are already photovoltaic power plants that can develop a 1.3M Lens, and only two layers of materials are required.
The full name is Erasable Programmable ROM, which is a non-volatile memory (NV RAM) used to store the Firmware code of the Camera module. The transmission interface used is the most common I2C on the market.
Similar to EEPROM, it is also a device used to store firmware code, which has more advantages than EEPROM. The transmission interface is SPI, but because the SPI interface is not yet common, EEPROM is mostly used on the Camera module, even though FLASH has more advantages than EEPROM.
Sensor is a device used to sense light and convert it into electronic signals. The Sensor can be divided into two different types: CMOS and CCD. Regardless of whether it is CMOS or CCD, silicon photodiodes are used to convert light to electricity, so the stronger the light, the stronger the signal. Because the working principles of CMOS and CCD are quite different, relative CMOS and CCD also have different advantages and disadvantages.
In the early days of the development of imaging technology, most high-end imaging devices (such as digital cameras, digital cameras, etc.) mostly used CCD as the main photosensitive element. In contrast, CMOS products are mostly regarded as secondary products of imaging devices. At present, a considerable number of high-end imaging devices have changed to adopt CMOS as the main photosensitive element.
Definition: Charge-coupled device, which is a relatively mature imaging device at present, and is a current signal in units of rows. The surface of the photosensitive element on the CCD has the ability to store charge and is arranged in a matrix. When light is felt on the surface, the charge will be reflected on the element, and the signals generated by all the light-sensing components on the entire CCD constitute a completed picture.
Definition: Complementary metal-oxide semiconductor. It is regarded as the imaging device of the future, because the CMOS structure is relatively simple and the same as the existing large-scale integrated circuit production process, so the production cost can be reduced. In principle, the CMOS signal is a charge signal in units of dots, which is more sensitive, faster, and saves power. The current advanced CMOS is not worse than the general CCD, but the CMOS process is not very mature. The ordinary CMOS has low resolution and poor imaging, and it is too prone to noise points.
Generally speaking, the image quality of CCD sensors is better than that of CMOS sensors. However, recently, CMOS has been comparable to CCD in low-brightness performance. In high-brightness applications, system product designers still prefer to use CCD sensors. Facing future mainstream applications, if you want to capture high-quality images in a low-brightness environment, or apply to the image input requirements of light, thin and short personal mobile products, the advantages of CMOS sensors are unmatched by CCD sensors.
The function of DSP is mainly to receive the data transmitted by the Sensor. When the Sensor receives the light source data, it will output the data, and then it will be received by the DSP, and then the DSP will exchange this signal into a USB signal and send it to the computer.
Quite a lot of sensors have ISP (Image Signal Processor). Such sensors can process images by themselves, so the data sent to DSP are all processed images. DSP only needs to convert the sensor’s YUV signal into Just use USB, or compress the image and convert it to Motion JPEG compression format.
Connect the computer, ammeter and module with the test cable to check whether the standby current and working current of the module are within the normal range, as shown on the right. After opening the image and check whether the screen is normal. If there is an LED light, check whether it lights up after opening the image.
Use 40 times “computer microscope” to check, and use a dust-free wiping cloth with a little alcohol to clean the Sensor surface. After confirming that the sensor surface is free of dirt, oil, lint, or scratches, install the cleaned lens. As shown on the right.
In the light box, place the Module in the fixed fixture and aim at a certain distance of the focus chart (Chart), and start the software IQC Focus to watch the image.
Align the center of the image with the center of the sun chart and adjust the focus, as shown on the right. At the same time, according to the black and white card, check whether the image is bad. The brightness of the light source at the center of the focus chart is between 450 Lux and 550 Lux.
Use the dispensing bottle to place a small drop of screw on the left and right sides of the joint between Lens and Holder and the four sides of the joint between Holder and PCB. After the glue is dispensed, send the module to the drying room for 3 hours and wait for the screw fixing glue to completely solidify before proceeding to the next step.
Remove the copper foil and paste it on the back of the PCB. Fold the copper foil with Mylar on the front of the PCB and fold the copper foil on the other side.
Place the Module in the fixed fixture and aim at the sun chart at a certain distance, start the software on the PC to watch the image, check whether the focal length is adjusted, check whether the image is normal according to the black and white card, as shown on the right. The brightness of the light source at the center of the sun image is between 680 Lux and 780 Lux.
Use test fixtures and software to perform recording test judgment on the finished module, and use headphones to listen to the recording to detect whether the recording is sound and whether there is noise.