APIX: The underlying technology
Serial APIX (Automotive PIXel link), based on Inova Semiconductors proven GigaSTaR - Gigabit/s serial transmit and receive technology - was developed for uncompressed, real-time video and data communication with actual up to 3 Gbps data transmission rate especially for automobiles and their high EMC demands. It is a uniquely designed multichannel SerDes (Serializer-Deserialzer) architecture to meet the bandwidth and distance requirements of today’s and future automotive connectivity designs. Using CML based differential transmission technologies; video data, several independent full duplex ommunication channels and even the power can be connected by only using a simple shielded copper cable (STP) with two differential pairs in the vehicle.
APIX is based on a proprietary line code with a minimum overhead of 11%, allowing content independent robust video transmission.
The major application is stable and reliable transmission of video and data between a head unit and numerous indicating and information displays of the dashboard instrumentation and rear seats. APIX is also used to transport video and control data from imaging sensors, which are appearing in automobiles in larger numbers. These include cameras for round view systems, passenger occupation, road sign recognition, lane departure warning and night vision.
Current Mode Logic
A substantial advantage of an AC coupled physical layer is the freedom to operate with different termination and supply voltages at the transmitter and the receiver. Therefore the link is optimized for DC balanced, AC coupled transmissions, requiring external coupling capacitors of typically 100nF in the transmission line, with a differential impedance of 100Ω through connectors and cable. The Physical layer interfaces incorporate flexible front-ends, to optimize the signal for different cable characteristics. To enable different vendors to use different technologies, the analog supply voltage may range from 1.2V up to 3.3V.
The APIX technology features a fundamental advantage to pixel clock driven devices by implementing a dedicated clock system for the high speed serial link. With this, the serial link is not influenced by any variances or jitter at the pixel clock which lowers the risk for uncontrolled EMI emission. In addition, the link offers „hot-plug“ capabilities, as the high speed link synchronizes immediately on power up or connect and independently from video data. The APIX transmitter features two separate clock domains for the video stream, which decouples the pixel clock of the link clock driving the high speed serial line. Video data consisting of parallel pixel data for colour information and pixel control data for the framing of the image are registered in a video data buffer. The data buffer performs the clock domain crossing of the pixel clock domain and the system clock domain. At the receiver, the link clock is recovered from the serial data stream and serves as reference for the main system clock. The recovered data are deframed, decoded and pushed into the receiver video data buffer. Finally, these data are provided at the pixel data interface, synchronous to the also recovered pixel clock.
AC coupling, DC balancing
All APIX transmitters and receivers are always AC coupled to the cable on transmitting and receiving side. Therefore all APIX family devices feature a “DC free” line code, a pre-requisite for the AC coupling of the APIX transmitter and receiver to the transmission media. Besides the known advantages of AC coupling, such as avoiding common-mode errors, and protection against erroneous input-voltage levels, it enables the possibility to implement different ways of a DC path through the cabling, for transmitting DC supply power from one end to the other end of the cable.
A proprietary line code enables the generation of a constant, serial bit stream that is entirely independent of the type and content of transmitted data, and consequently shows no spectral lines caused by video content. As a result APIX is virtually invisible in terms of electromagnetic radiation even at a maximum transmission rate of 3 Gbps downstream and 187.5 Mbit/s upstream.
The immunity against interference is also very high. Measurements show robust video and data transmission at up to 350mA RF interference current – a critical value being specified by some car OEMs.
The combination of adjustable driver characteristics, selectable operating modes and spread spectrum-clocking enables the optimum combination of minimal EMI, maximum transmission distances, and lowest power consumption.
The bandwidth of an APIX link is currently scalable up to 3 Gbps allowing cost and feature optimized integrated circuits for specific applications.
The serial link interface consists of the downstream link, capable of supporting bandwidth modes of 500 MBit/s, 1 Gbps, 1.5 Gbps and 3 Gbps and an optional upstream link, providing bandwidth modes of 62.5 MBit/s and 187.5 MBit/s. The underlying multichannel packet oriented architecture allows totally independent transmission of several channels of video, audio, data and even Ethernet.
Current APIX Interface devices provide parallelTTL or openLDI video interfaces, I²S Audio interfaces, I²C and SPI data communication and even an MII or RMII interface for an embedded 100 Mbit Ethernet channel. Beyond point to point data links also "daisy chaining" of two receiver devices is possible, saving components and cost in advanced infotainment architectures in the car.
All APIX devices are software configurable to be adapted to the application specific requirements of bandwidth, video interfaces and control signals.
Inova also provides the AShell technology which is a higher level communications protocol to enable easy implementation of secure and error resistant communication links over the sideband channels. The APIX Automotive Shell called “AShell” is an abstraction layer for the data communication. The AShell allows a secure and error free data exchange on the bi-directional full duplex communication channels of the APIX link. Apart from the error control functions, the AShell is a wrapping layer, ensuring data integrity, information about transmission link status, as well as simple error messaging, in both directions.
Currently Inova and other semiconductor companies offer a wide range of products based on the APIX and APIX2 technology. With the introduction of APIX2 it was possible to introduce HD (High Definition) display resolutions into the car.
APIX2 is fully backward compatible with the first generation of APIX products. This allows highly flexible use of all available products allowing uncompromised operation of all available devices. Customer’s have complete flexibility and can do independent upgrades of transmitter and receiver subsystems from one to the other APIX technology generation.
The next step – the introduction of APIX NEXT technology to support even UHD (Ultra High Definition) type of displays and other cable media is in development. First products will be introduced into the market soon, featuring also advanced video, audio and data communication interfaces in line with advancements in imager, display and SoC technologies of Inovas Semiconductor partners.