Image Sensor Communication Protocols
Before reviewing image sensors, we need to understand the protocols which they use to communicate. Longer distance protocols will be discussed later.
I2C is a serial (1 data bit wide) communications protocol.
Serial Camera Control Bus (SCCB) runs on top of I2C and is used for controlling cameras. It is the OmniVision-specific term for standard I2C.
MIPI-CSI Camera Command Set (CCS) is a simple scheme for configuring registers over an I2C bus, supporting various register address sizes.
Serial Peripheral Interface Protocol (SPI) is a parallel interface. It can be 1,2,4 or 8 bits wide. FPGAs use SPI for external Flash Memory and often external RAM access. The Himax cameras can be controlled using either SPI or I2C. In some cameras, images can be transmitted over SPI, albeit slowly. The SPI protocol is not ideal for video as it lacks dedicated lines for essential timing signals like Vertical Sync (VSync) and Horizontal Sync (VSync).
Digital Video Port . View
Digital Video Port (DVP), also known as Camera Interface (CAMIF) or Camera Parallel Port, is a parallel interface using single ended wires.
DVP should not be confused with the older Silicon Graphics Digital Video Port standard, which uses differential signaling. The modern meaning for DVP originated with Omnivision, but is now widely used by other vendors. Different DVP versions can have different bit widths, color spaces, colors transmitted per clock, and signal polarity. While many DVP cameras have a limited bit depth, there are 24 bit DVP cameras. There will be a lot of wires to connect, which may require a more expensive board. Additional wires may be needed to control the camera using an SPI, I2C or SCCB interface. The longer the wires are the lower the maximum frequency. At 100 Mhz, they can provide up to 1.66 Mpixels @60FPS.
DVP is considered a poorer cousin to MIPI (described later). But it has three advantages over MIPI.
All FPGAs support DVP.
DVP can have greater bandwidth than Lattice NX with MIPI hard cores.
DVP runs at much lower frequencies than MIPI, so it is possible to do debugging using less expensive logic analyzers.
DVP has no licensing fees.
Differential Signalling Protocols. View
MIPI, USB, HDMI and DVI are all based on differential signalling. Differential Signaling uses two wires carrying complementary voltages.
The differential signalling receiver detects the difference of the two voltages. Compared to DVP, differential signaling uses a smaller voltages swing, consumes less power, has better noise immunity, can reach higher frequencies, and can travel further. Here are some of the differential signaling standards used in cameras.
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