A2B and Ethernet in Automotive Applications: What, When, and How: Page 2 of 3

January 23, 2020 //By Matteo Crosio, Analog Devices
A2B and Ethernet in Automotive Applications: What, When, and How
The number of electronic systems in cars has increased in recent years, reaching higher levels of complexity with the adoption of new technologies for infotainment and advanced driver-assistance systems along with multiple sensors for many different purposes. This results in much higher amounts of data zooming across the vehicles. Carmakers and electronics developers therefore have to be aware of new networking technologies.

It is always possible to provide slave nodes with a local power supply, in case the power budget provided by the master is insufficient. The bus allows a bidirectional, master-to-slave and slave-to-slave communication, bringing up to 32 channels downstream and upstream (12, 16, and 24 bits). Most importantly, a 2-cycle latency is guaranteed, providing latency-sensitive applications such as ANC/RNC with a deterministic support. The bus is capable of transporting I2C massages, allowing the configuration over distance of ADCs/ DACs at slave nodes.

What really simplifies the configuration of an A2B network is SigmaStudio, a graphical design environment supporting SigmaDSP and SHARC DSP families. An A2B transceiver (AD2428, AD2427, and AD2426) provides I2S and PDM interfaces. Typically, an I2S interface is used for connecting to ADCs and DACs, while digital microphones use PDM.

One of the major concerns in automotive applications is related to electromagnetic compatibility (EMC). A2B has passed the most stringent automotive EMC and electromagnetic interference (EMI)-compatibility tests using just a 2-wire UTP cable. RNC applications require accelerometers and microphones distributed around and inside the vehicle. The use of analog parts is cost-prohibitive as it would require additional circuitry (analog-to-digital converters), wiring, and connectors. The A2B technology simplifies this architecture with a novel approach to audio sources and sensors.

Ethernet in Automotive

Ethernet is a very popular networking technology and possesses a large ecosystem. However, its use in the automotive field is limited so far to a few applications such as diagnostics, in-vehicle infotainment systems, and connectivity to sensors. Its rival for in-car applications is likely MOST, which can compete in terms of speed.

Although Ethernet has the potential to become the definitive answer to the huge demand for bandwidth arising with the newest technologies (radar and lidar, for example), there are several aspects still limiting its adoption in cars.

Traditional Ethernet cables used for 100-Base-TX, based on two differential pair wires and isolated by transformers, are too expensive for automotive applications. Moreover, a Cat-5 cable doesn’t meet automotive EMI standards, making the 100-Base-TX Ethernet useless for in-car communications other than diagnostics and firmware updates.

For vehicle-to-vehicle (V2V) or vehicle-to-everything (V2X) communications, in-car data transport must be supported in terms of synchronization, traffic shaping, and fixed latency. Ethernet doesn’t have this kind of support, unless new protocol stacks are implemented.

Let’s consider the physical layer first.

With the aim of meeting the requirements in terms of weight, EMI, and cost, the Institute of Electrical and Electronics Engineers (IEEE) defined a new standard called 802.3bw, also known as 100-Base-T1. IEEE 802.3bw is a 100 Mbps standard based on a UTP cable that is bidirectional and that meets strong automotive emissions requirements. EMI is reduced by using the basic principles of superposition, specific encoding, and scrambling schemes.

Weight and cost are lower when an unshielded 2-wire cable is used instead of the traditional Cat-5 cable. Technologies like power over Ethernet (PoE) provide power along with data, sharing the same wires. However, PoE requires at least two pairs of wires for providing power, clearly in contrast with the need to reduce the number of wires.

This is why IEEE defined the standard 802.3bu, also known as power over data lines (PoDL). PoDL can provide power over a single pair of wires, adding some complexity to the transceiver schematics.

Figure 2. In-car cabling for audio systems simplified by A2B technology.

As already stated, to support automotive applications, Ethernet needs additional SW providing determinism. This can be achieved by the Audio Video Bridging (AVB) protocol, developed within the IEEE 802.1, the organization in charge of the second layer in the ISO/OSI model.

AVB is a SW technology providing time synchronization and traffic shaping. With these fundamental concepts, Ethernet can bring audio and video content reliably. AVB led to the definition of a set of protocols known as time-sensitive networking (TSN), focused on the industrial and automotive markets, providing Ethernet with real-time support.

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