How does the camera work？

According to a certain resolution, the points on the image are collected by interline scanning. When a certain point is scanned, the gray level of the image at the point is converted into a voltage value corresponding to the gray level through the image sensor chip, and then the voltage value is output through the video signal end. Specifically (see Figure 5-1), the camera continuously scans a line on the image, and the output is a continuous voltage signal, and the ups and downs of the voltage signal reflect the changes in the gray level of the image. When a line is scanned, the video terminal outputs a level below the minimum video signal voltage (e.g. 0.3V) and holds it for a period of time. This corresponds to a voltage "notch" immediately after each line of image signal, which is called a line synchronization pulse and is a sign of a scanned line feed. Then, after skipping a line (because the camera is interlaced), start scanning a new line, and so on, until the video signal of the field has been scanned, and then a field blanking area will appear. There are several compound blanking pulses in this area, and one of them is much wider (that is, the duration is much longer) than the other blanking pulses, called the field synchronization pulse, which is the symbol of the scanning field change. The field synchronization pulse marks the arrival of a new one, but the field blanking zone happens to span the end of the last one and the beginning of the next one, and the next video signal will not really arrive until the field blanking zone has passed. The camera scans 25 images per second, and each picture is divided into two odd and even fields, the first odd field and then even field, so it scans 50 images per second. Only odd lines in the image are scanned in odd fields, and only even lines are scanned in even fields.

Cameras have two important metrics: resolution and effective pixels. The resolution is actually the number of synchronization pulses per line, because the more line synchronization pulses, the more lines are scanned per image. In fact, the resolution reflects the vertical resolution of the camera. Effective pixels are often written in the form of multiplication of two numbers, such as "320×240", where the former value represents the fine degree of a single line video signal, that is, the line resolution; The latter value is resolution, so effective pixels = row resolution × resolution.

The addresses of the OV7620 function registers are 0x00 ~ 0x7C (many of them are reserved registers). By setting the corresponding registers, the OV7620 can be made to work in different modes. For example, to set OV7620 to continuous scanning and RGB raw data 16-bit output mode, you need to set the following Settings: I2CSendByte () is a register write function, its first parameter OV7620 is the chip address 0x42 defined by the macro, the second parameter is the on-chip register address, and the third parameter is the corresponding register setting value.

The OV7620 is controlled using the SeriaICameraControlBus (SCCB) protocol. SCCB is a simplified I2C protocol, SIO-l is a serial clock input line, SIO-O is a serial bidirectional data line, respectively equivalent to SCL and SDA of I2C protocol. The bus timing of SCCB is basically the same as that of I2C, and its response signal ACK is called the 9th bit of a transmission unit, which is divided into Don't Care and NA. The Don 'tcare bit is generated by the slave machine. The NA bit is generated by the host. Since SCCB does not support multi-byte read and write, the NA bit must be high. In addition, SCCB has no concept of repeated start, so in the SCCB read cycle, the host must send the bus stop condition after sending the on-chip register address. Otherwise, the slave will not be able to generate a Don 'tCare response signal when the read command is sent.