The typical electronics content of cars is burgeoning; estimates are that the electronics and software account for 40 percent of a vehicle's value. Design engineers face increasing functionality requirements for such automotive systems 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 godsend-provided they are implemented wisely.
With more auto electronics engineers needing to know the nuances of integrating components and their systems-which must play together with other vehicle electronics-some basic considerations have to be addressed before any design is undertaken.
The first question, simply put by Matt Ruff, a member of the 8-Bit Systems Engineering Team at Freescale Semiconductor Inc. (Austin, Texas), is, "Does it make sense to network?" The answer will vary case by case, Ruff said, "with cost and complexity be-ing 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 systems-a controller connected to an actuator by a short, discrete wire-networking 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 for expertise
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 (Campbell, Calif.). 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).
"Integrating such a proven IP block onto a chip can be done faster, with fewer problems [than developing from scratch], cutting system size, power, interference effects and cost," Savage said. Being able to place such an IP core within a chip wouldn't be possible without the maturity of modern EDA tools and smaller physical features, he noted.
"We've captured the IP developers' knowledge so the designer can configure and verify using the GUI without having to dive into the code," said Savage. "The designers work in more 'readable' forms. The EDA software translates the high-level hardware description language into gates and transistors, with the hardware looking like software code."
The build-or-buy decision for bus communications software is complicated by the need to integrate different automotive OEMs' electronic control units, said Niklas Gustafson, product manager for Enea (Stockholm). His company's Element tool formulates middleware and system software to configure network interfaces and communications to build reliability into automotive networks.
The Element package borrows from Enea's experience in telecom. "There, a router can't be taken down from service to correct a problem or upgrade," Gustafson said of the reliability needed. So systems are monitored and the necessary diagnostics generated, but problems are allowed to exist and are isolated until software updates can be implemented.
Carrying over this approach into automotive-network applications, the resulting fault-tolerant framework is allowed to "failover." Simply put, because CPUs are not directly connected to sensors, the physical interface can be reconfigurable (reprogrammed). So if a CPU fails, its services can be migrated to another processor.
That capability also allows for balancing the processing load among system CPUs. The reconfigurable middleware permits the system to be reprogrammed at a dealer not only to correct problems but also to install new features and functions that might become available. That decouples shorter electronics development times from the longer product cycles in the automotive industry.
Tools also help choose the network architecture itself, based on the given application. Mentor Graphics Corp.'s Volcano package, developed by a then-Volvo subsidiary, analyzes network traffic in terms of the number of signals and how often they need to be sent to each device on the network in order to choose the appropriate data bus. The breakdown is as follows: a single-wire Local Interconnect Network (LIN) for low speeds, Control Area Network (CAN) for higher speeds, FlexRay "deterministic CAN," and Media Oriented Systems Transport (Most) for large files for multimedia entertainment.
"The software tests for 'corner cases,' to tell if all signals can be accommodated by the network topology," noted Larry Anderson, marketing director for Mentor's Automotive Networking Business Unit. This not only avoids instances where a combination of two or three actions produces an unwanted result (such as a trunk's opening when the lights and horn are activated) but, more important, maximizes bus bandwidth, which lowers cost.
For example, Anderson cited Volvo's bandwidth utilization of only 30 to 40 percent prior to Volcano to allow for headroom. With the tailoring possible with the software, utilization is up around 80 percent, accommodating more devices on the same network.
The semiconductor companies also provide networking design help. Freescale's Virtual Garage team will take an automotive OEM's architecture and see how it interacts. "The team determines if it is sized correctly, where any bottlenecks are and if the networks are correctly partitioned," said Ruff. "They will suggest if any functions should be moved from one processor to another." And the approach can be used to gain insight to existing networks and to factor in cost considerations.
The price of networking
Hidden network costs can be a moving target. "It's slippery, with lots of aspects, not just BOM [the bill of materials]," 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, Mentor's Anderson noted, "the wiring harnesses are typically the second most expensive component, next to the engine." 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 (for example, status and control, actuation or video), 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 physical-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 then may not play well together."
In a similar vein, Kevin Tanaka, world- wide automotive marketing and product-planning manager for FPGA maker Xilinx Inc. (San Jose, Calif.), naturally warns about overbuilding a system with microcontroller chips. Programmable arrays, he noted, allow integrated architectures that can be scaled up or down readily, controlling both cost and complexity.
Don't neglect the fusing strategy (for powering multiple networked devices), inrush-current handling or schemes for locating circuit protection devices, advised Mentor's Ortega. Pay attention to grounding to avoid ground shifts.
Freescale's Ruff and TI's Mahoney echoed the latter warning and added that sensors and actuators may be affected by the power available from the electrical system, as well as temperature and the engine compartment environment. Voltage, resistance and ground shifts can alter throughput, they noted.
Finally, Mahoney and Ruff mentioned the standards issue. Although there are various national and international networking standards, OEMs may have their own requirements within and between networks for such aspects as bus length, capacitance, number of nodes and length of stubs. Mahoney noted that standards for electrostatic discharge include requirements for handling components and modules during assembly.
"A conformance test is not always a conformance test," said Ruff. "It depends on what it is testing conformance to, where in the design process it occurs" and whether it is for a component alone or for data linked to a software-loaded controller, for instance.
For engineers entering the growing world of automotive networks, the opportunities are numerous and increasingly versatile. With the appropriate attention to detail, goals should come to fruition.
For more on the companies mentioned in this story, see www.freescale.com, www.ip-extreme.com, www.enea.com, www.mentor.com, www.ti.com and www.xilinx.com.
Rick DeMeis (firstname.lastname@example.org) is site editor for AutomotiveDesignLine.com.
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