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Fielding robust FlexRay in five steps

Posted: 02 Apr 2007 ?? ?Print Version ?Bookmark and Share

Keywords:FlexRay applications? suspension control? five steps to FlexRay?

FlexRay comes onto the road with single-channel high-speed power-train, driver-assistance and comfort automotive electronics applications. On the new BMW X5, FlexRay is used in suspension controlthis allows for a gentle learning phase with low risk for engineers and developers before applying the fault-tolerant, deterministic protocol to safety-relevant driving functions using two communication channels and bus guardian supervision.

In developing FlexRay applications, there are five basic steps that design engineers can apply to achieve a robust network topology.

1. The quantity of nodes and their tentative locations in the vehicle chassis have to be defined first. Then the cable length can be determined, which would be needed to realize a passive bus without stubsa "daisy-chain" topologywith termination at its cable ends (Figure 1). If the cable length is shorter than 10m, a topology has been found, which can be considered to be used in series production.

2. In case the identified cable length is longer than 10m, the use of an "active star" should be considered (Figure 2). If the cable length ranges above 20m, the use of an active star is a must. The simplest active star features only two branches, splitting the wiring harness into two electrically decoupled parts. Since it is possible to build up active stars with NXP's TJA1080 transceiver, the total number of required transceivers is increased by one only.

3. When the application shall be able to continue operation also after a vehicle crash, the crash-sensitive nodes of the system should be arranged on a separate branch (Figure 3). Thus, in case the wires get squeezed and clamped to a differential voltage, data transfer becomes impossible only on the affected branch. The active star will allow the nodes on the unaffected branches to continue communication without detriment.

Figure 1

Figure 2

Figure 3

4. Because of resonance rise, nodes or wiring exposed to very harsh RF fields should also be located on a separate branch (Figure 4). Using a split-termination (FlexRay Electrical Physical Layer specification v2.1 Revision B) at the cable's both ends diverts the induced RF current into the ground potential. This results in lower common-mode voltage amplitudes on the wires without affecting nodes connected to other branches. Consequently, jitter in the received data stream can stay within acceptable limits.

5. Optional nodes shall not be the end nodes of a trunk cable to ensure that an appropriate (split-) termination at both cable ends is always present (Figure 5). Shifting the electrical position of a node along the cable must not result in cable length far above 10m (first step). Short stubs (

Figure 4

Figure 5

Validation, optimization
Following these five steps helps in fielding robust FlexRay topologies in terms of their electrical properties. Simulations are recommended for further validation and optimization of the defined topology. Monte-Carlo simulation methods may be used to consider manufacturing tolerances of the wiring harness, production spread, and temperature dependence of transceivers and active stars.

In addition, the FlexRay consortium has delivered a sophisticated cable model including the skin effect for the wiring harness. More information about split termination, cables and connectors for FlexRay applications is given in the FlexRay Electrical Physical Layer specification v2.1 Revision B. Additionally, some topology layout recommendations can be found in the Electrical Physical Layer Application Note v2.1 Revision B. Both documents can be retrieved from the FlexRay Consortium Website.

- Bernd Elend
System Engineer, NXP Semiconductor

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