Automotive Ethernet Standards: Present and Future Design Requirements
Ethernet has been around for over 40 years, but it hasn’t made headway outside the office or industrial settings until recently. Ethernet networking in cars makes this old yet powerful technology mobile and provides an easy way to transfer data around a vehicle quickly. Anyone following the auto industry knows that modern vehicles are moving computers, and they now leverage more data than ever before.
The reliance on more sensors, more data, and more processing power puts greater reliance on high-speed data transfer protocols within a vehicle. Ethernet is now the backbone of in-vehicle networking, and designers will need to accommodate higher data rates under the automotive Ethernet standards for new designs. Here are some of the current and upcoming design challenges in automotive Ethernet and how these filter down to PCBs for new vehicles.
Greater Data Rates for ADAS and Driverless
Current automobiles are more software-driven than ever before, and all that embedded software (and hardware) needs to transfer a lot of data. ADAS systems, ECUs, lane assist, pedestrian detection, and many other features all exchange plenty of data. At some point, digital CAN networks can’t provide the throughput required to orchestrate so many different portions of a system.
The IEEE 802.3bw standard (100Base-T1 BroadR-Reach) is a 100 Mbps automotive Ethernet standard used in newer automobiles as it provides longer reach while still satisfying vehicle EMI metrics. It uses staggered PAM-3 signaling (27 MHz vs. 62.5 MHz in 100Base-T) and advanced encoding to reduce the required link transmission capacity. Going from 1 Mbps (high end of CAN networks) to 100 Mbps was sufficient to provide enough data throughput for various features in new cars, particularly live video transmission (e.g., backup cameras).
With sensor data being analyzed and used within complex AI/ML models, autonomous cars require even more data throughput. The 1000Base-T1 and 1000Base-RH standards specify two physical layers that can be used within automotive Ethernet. Still, the most advanced vehicles with multiple cameras, radar modules, GPUs, lidar/sonar, and other sensors will rely on 2.5, 5, or 10 Gbps automotive Ethernet. The IEEE 802.3 standards group is already preparing for speeds beyond 10G Ethernet for in-vehicle networking.
The move to faster data rates brings greater signal integrity requirements, power integrity, and EMI/EMC. Companies like Molex are already releasing products like shielded connectors/cables designed for 20 Gbps PHY in automotive Ethernet. The intrepid automotive PCB designer will also need to adapt to working at these higher data rates over differential channels.
Changing everything to automotive Ethernet under 100Base-T1 or higher standards also eliminates the need for CAN, LIN, and other protocols within the vehicle. This then removes the need for Ethernet/CAN and other gateways for interfacing between different protocols. It also reduces the total hardware weight in the vehicle. This is great for car designers and gas mileage, but it puts a greater onus on PCB designers to consider the higher data rates during layout and routing.
MII and Higher Routing Standards
Routing between MAC, PHY, and switches uses MII or one of its variants. 100Base-T1 uses MII, RMII, RGMII, or SGMII for routing between the MAC on a CPU/FPGA/MCU, switch, and PHY on the board. Automotive-grade RJ45 connectors are available, but other plug styles are rated as automotive grade, which can support data transfer beyond 10 Gbps (see these connectors for Rosenberger). The exact routing standard used in your board will depend on what your PHY transceiver supports.
MII is rather straightforward and operates at low MHZ frequencies: a PAM3 signal is transmitted over the physical layer at 33.3 MHz over twisted-pair copper (CAT 5e or better). RGMII and SGMII are best for gigabit applications. RGMII uses four parallel bits for data transmission, so signals are transferred using PAM5 signaling at 1 Gbps (625 MHz carrier). SGMII transfers data over LVDS at 1 Gbps (i.e., SerDes). Designing for each type of signaling standard requires paying attention to signal integrity in each frequency domain.
The image above shows a typical layout and signaling scheme for RGMII and SGMII for 100Base-T1 automotive Ethernet PHYs. The same layout applies to gigabit and faster automotive Ethernet devices. Be sure to follow the Ethernet routing standards in your PCB when routing between the MAC, PHY, and connectors. You’ll find newer MCUs with integrated MAC/PHY units in the chip to reduce board size. Newer semiconductor companies are also producing automotive-grade MAC/PHY chips for use in automotive Ethernet applications.
Stackups for Automotive Ethernet and Noise
You also need to consider best practices for layer stackup design and grounding, just as you would for other devices with Ethernet ports as your stackup influences noise-induced or conducted into the channel. I say to always run the analog ground plane in the board right up to the RJ45 connector. You need to carefully plan a return path for other components to prevent interference between digital and analog board sections. Note that common-mode noise dominates Ethernet channels, including automotive Ethernet, and proper grounding will suppress common-mode noise from being conducted into the channel.
Although changing the distance between power/ground planes does not influence common-mode noise intensity on a victim trace, it influences interplane capacitance. This is critical for power integrity in the board as fast digital ICs need stable power. The MCU/CPU on a board that supports time-sensitive networking (TSN standard) for ADAS and other safety systems will be a fast IC and power ripple can induce jitter in the output data. Jitter always propagates downstream from its source and adds in quadrature throughout the system. Keeping spacing between power and ground planes low provides sufficient decoupling to reduce the PDN impedance up to reasonably high frequencies, so you should use a stackup with adjacent power/ground planes to ensure power integrity.
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