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Conquering EMI challenges in automotives

Posted: 31 Jan 2013 ?? ?Print Version ?Bookmark and Share

Keywords:Ethernet? on-board-diagnostics? MOST technology? ADAS?

The automotive industry widely recognises Ethernet as the preferred interface for on-board-diagnostics (OBD), and it has been deployed in various car models since 2008. Ethernet provides increased bandwidth speeds over traditional automotive buses, resulting in a reduction in software download times from hours to minutes compared to traditional methods. This adoption will be accelerated with the introduction of a standardised IP Diagnostics interface, as specified in ISO 13400, using Ethernet as the physical layer.

The expansion of Ethernet-based networks in the automotive industry has continued with next-generation infotainment and driver assistance systems. Originally found in only luxury high-end vehicles, such applications are fast becoming differentiating features on mid-range and even basic models. These new applications have, in tandem, generated demand for greater system bandwidth, which continues to rapidly rise, as shown in the timeline below:

???1981: CAN @1000Kbit/s
???2005: FlexRay @ 10Mbit/s
???2001: MOST @ 50Mbit/s
???2008: Ethernet @100Mbit/s
???Ethernet @ 1000Mbit/s (RTPGE IEEE802.3bp)

Advanced Driver Assistance Systems (ADAS) constitutes one of the fastest growing applications within the automotive market. Driven by government legislation and a desire for enhanced in-vehicle safety camera sensor networks are becoming commonplace. By 2017, camera-based module sales are expected to increase to 34 million in total, from 6.1 million in 2010 (IMS Research). This is in part due to the National Highway Traffic Safety Administration (NHTSA) ruling requiring every car sold in the United States from 2014 be fitted with at least a single rear view camera or sensor.

System costs are significantly lowered using Ethernet connectivity for multi-camera sensor networks in the car. Traditional proprietary methods are making way for open standard Ethernet. This has been reflected in the ISO 17215 Video Communication Interface for Cameras (VCIC) specification, defining Ethernet connectivity for use in vehicle camera and/or sensor applications.

Ethernet is emerging as the preferred network of choice for this new generation of networked vehicles, continuing to be the de facto networking bus for all other markets, thanks to an offering of ample bandwidth and open standardisation. True standardisation results in multiple interoperable supplier solutions, rapidly driving down costs.

This is in contrast to current popular infotainment networking technologies, for example MOST. The proprietary nature of MOST has often been blamed for failing to deliver the cost needs of car makers, as reported in the Hansen report, November 2008: "Suppliers and car makers think more access to SMSC's proprietary MOST technology would lower the cost of MOST and increase market acceptance. Some are looking at Ethernet as a possible alternative."

Ethernet provides many advantages over MOST technology, including increased bandwidths and flexible topology, but probably most importantly lower costs, due to the high volume deployment across multiple markets, accepted supply chain and multiple suppliers.

Challenges for automotive ethernet
The needs of more recent Ethernet-based applications, such as infotainment and ADAS, differ distinctly from the current applications, such as on-board-diagnostics in that these are applications operating in real time and while the car is moving. No matter what the circumstances might be, must meet car manufacturers' EMI limits. For Ethernet to be considered for any application in operation whilst the car is moving, it must fully comply with OEM EMI specifications.

Herein lies the challenge: the use of shielded cables would provide a solution to reducing radiated emissions within the car, but is usually undesirable. Shielded cable brings about complications in earth strategies, can adversely affect reliability, and add cost to production. Shielded cables cannot be manufactured in situ using wiring looms during production, but need to be pre-manufactured and purchased. Hence, the ultimate goal would be to operate Standard Ethernet over unshielded cable whilst meeting automotive OEM EMI limits. This solution dramatically reduces cabling costs, by up to 80%, over shielded counterparts, whilst maintaining interoperability with any other standard Ethernet device. The net result is lowest cost cable and silicon, with multiple suppliers.

Handling radiated emissions
Although the use of low cost unshielded cable is clearly desirable, the perception for some time has been that this was not possible with standard Ethernet 100BASE-TX PHY, and more proprietary means were required. When you observe the typical radiated emission characteristics of an Ethernet PHY, in figure 1, it is easy to understand where this perception originated. The high energy content at the lower frequency band due to the MLT3 coded 65MHz to 80MHz bandwidth typically 10dB to 15dB in excess of automotive OEM limits.

Figure 1: Example of 100Mbps ethernet board radiated emissions.

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