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Cars sag under weighty wiring

Posted: 16 Jan 2006 ?? ?Print Version ?Bookmark and Share

Keywords:rick demeis? automotive designline? networking? car?

The typical electronics content of cars is burgeoning; estimates are that electronics and software account for 40 percent of a vehicle's value. Design engineers face increasing functionality requirements for automotive systems, such as infotainment and multimedia, engine and emissions controls, and safety systems ranging from smart airbags to stability control. For these developers, network bus architectures that reduce discrete, dedicated wiring runs while facilitating the addition of features can be a godsendprovided that they are implemented wisely.

With more autoelectronics engineers needing to know the nuances of integrating components and their systemswhich must play together with other vehicle electronicssome basic considerations have to be addressed before any design is undertaken.

The first question, simply put by Matt Ruff, a member of the bit systems engineering team at Freescale Semiconductor Inc., is, "Does it make sense to network?" The answer will vary case by case, Ruff said, "with cost and complexity as the main drivers."

He noted some trade-offs that must be made regardless of the network protocol under consideration. In automotive, cost always tops the list. "If you have a working solution that is not complex, then you can save the cost of making a change by not going to networking," says Ruff. But given consumers' expectations for added features at minimal added cost, opting to network may prove cost-effective in the long term because it establishes a foundation for future flexibility.

"With small, simple systemsa controller connected to an actuator by a short, discrete wirenetworking may not make sense," Ruff said. But as wiring length and the desired control level rise, the list of considerations could expand to include where to end the network and transition to discrete wiring, along with how to partition the various networks themselves.

Tooling up
Another trade-off consideration is the experience of the design team, said Ruff. Lack of a track record with complex networking systems might mean forgoing or outsourcing their development. Thanks to expert design tools, however, help is available to ease the task.

"Engineers tend to want to design from scratch, which is not always the best course with complex systems," said Warren Savage, CEO of IPextreme. The company's XPack software packages intellectual property in EDA-tool-neutral formats. IPextreme recently announced availability of Freescale's FlexRay automotive network communications controller for licensing onto customer "machines" (chips).

Networking price
Hidden network costs can be a moving target. "It's slippery, with lots of aspects, not just BOM," said Ruff. While networking in general cuts wiring, freeing up internal spaces and easing bulkhead pass-through requirements, there are the costs of implementing the network; testing and test software; manufacturing, labor and installation expenses; and maintenance and long-term reliability influences warrantee expenses.

While the network physical wiring may seem mundane, "wiring harnesses are typically the second most expensive component, next to the engine," noted Larry Anderson, marketing director for Mentor's automotive networking business unit. So the reductions in wiring offered by networking can yield significant material and fabrication savings.

Other wiring cost factors, highlighted by Enrique Ortega, Mentor's transportation sector market director, include how to "break" the harnesses for ease of installation in a vehicle and accounting for the use of a common harness in specific vehicle models, as well as in both left- and right-hand-drive versions of a car.

A matter of choice
In choosing a specific network, throughput is a key factor. But the data buses under consideration also have other important characteristics, noted Dean Mahoney, strategic marketing engineer for Texas Instruments Inc.'s mixed-signal automotive business unit. They include what the data is used for, the location in the vehicle and the electromagnetic compatibility requirements. All can affect network choice and cost, Mahoney said.

EMC performance breaks down into emissions (radiating into the environment) and immunity (robustness to electric fields). "It comes down to developing a fundamental understanding of single-wire and differential-pair data transmission and signal characteristics over copper and PHY-layer interconnect media," said Mahoney. A single-wire LIN bus is more susceptible to outside emissions than two-wire twisted-pair, where the electric fields are coupled. But for a low-data-rate application, such as in a door for actuating windows or locks, EMC would not be as much of an issue with LIN as it would be for the engine compartment near the ignition system or in the central-dashboard electronics stack.

Robustness also encompasses reliability. Communications integrity hinges on the adequacy of the bus message structure and on error checking for the application, with the goal of delivering the correct message to the right node.

The experts questioned for this primer on automotive networking offered hints on the development aspects that may require particular attention.

Don't overload the system, cautioned Freescale's Ruff. "Don't run or design at 100 percent bus load. You'll always need overhead on the network for special, low-probability or unforeseen situations, especially if you're not designing all the components yourself. They may not play well together."

- Rick DeMeis
Site Editor

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