Analog Tip GMSL: Pixel And Tunnel Modes for Better System Performance

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Gigabit Multimedia Serial Link is a proprietary SERDES technology from Analog Devices that was developed for the high-speed transmission of video data over long distances using a coaxial cable or two shielded twisted-pair cables. An overview of the possibilities.

Gigabit Multimedia Serial Link (GMSL) was originally developed for automotive applications. Today, GMSL is also used in other areas such as industry, agriculture, medicine and many others.

Three GMSL generations are currently available: GMSL1, GMSL2 and GMSL3. Figure 1 shows an example of a GMSL-capable network. The SERDES (serializer/deserializer) interfaces are placed between a video source and a video sink, allowing much greater distances to be achieved between the two while reducing the number of cables required.

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The video sources can be either sensors or processor units, while the video sinks are either displays or other processors.

High-Speed Connection of Video Interfaces: CSI-2

CSI-2 is a high-speed video interface standardized by the MIPI (Mobile Industry Processor Interface) alliance. It is used in many applications such as automotive technology, mobile communications, drones, robotics, etc. As a rule, the CSI-2 video source consists of sensors (imagers, radar devices, LIDARs) and the video sink consists of a processor unit, a system on chip (SoC), a microcontroller, etc.

CSI-2 is a protocol that defines how the image data is formatted, received and transmitted and uses D-PHY or C-PHY as the hardware layer. These are two different physical layers defined by the MIPI Alliance that describe the electrical properties, signal integrity and timing characteristics required for high-speed data transmission.

C-PHY supports applications with higher bandwidth than D-PHY. Each PHY uses different data and clock channel topologies that require different layout considerations. The CSI-2 packet structure consists of three main segments: Header, Payload and Footer. With minor differences, this structure applies to both C-PHY and D-PHY.

The header contains secure information about the content of the payload (usable data). This tells the recipient what type of data they need to prepare for. The payload contains the most important information that needs to be transmitted. The footer secures the payload information using CRC checksums.

SERDES Video Data Transmission

In a GMSL system, the CSI-2 packets are received by the serializer, encoded into the GMSL packet format and transmitted via the cable to a pairing deserializer, which unpacks the video and sends the CSI-2 information to a local processor.

With the further development of the GMSL-CSI-2 components, two different modes for video data transmission have emerged. Both ensure secure and reliable transmission of video content via the GMSL connection. The two transmission modes are pixel mode and tunnel mode.

Pixel mode is the older variant that was introduced with the first product families of GMSL2. As Figure 2 shows, in this mode the header and footer of the incoming CSI-2 packet are removed and the usable data is converted to pixel format to be sent over the GMSL connection. The conversion from CSI-2 format to pixel format is done in the serializer, while on the other side of the connection, at deserializer level, the CSI-2 structure is reconstructed and a new header and footer are added to the structure.

In tunnel mode, on the other hand, the entire CSI-2 data structure is repackaged, as shown in Figure 3. For this reason, tunnel mode is sometimes referred to as CSI-2 forwarding and does not support pixel-based processing. The availability of one or both transmission modes varies between the GMSL components and is specified in the respective data sheets.

For successful operation of the GMSL connection, both the serializer and the deserializer must be supported and configured in either pixel or tunnel mode. If the modes of the serializer and deserializer do not match, video data will not be transmitted. Other peripheral interfaces or protocols supported by the GMSL components such as I²C, UART, GPIOs etc. are not affected. (kr)

According to documents from Analog Devices.

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