FlexRay Active Star with Bit-Reshaper: enabling extended network topologies and more reliable communication

June 08, 2011 // By Harald Gall, Christian Netzberger, Eric Schmidt, Monica Giardi
Susceptibility to electro-magnetic interference and signal quality issues with complex topologies have so far thwarted a breakthrough of the FlexRay data high-speed automotive bus. The article describes how to correct bit timing mismatches and thus significantly improving the overall signal quality in FexRay networks.

FlexRay is an automotive network technology conceived as a replacement for the CAN (Controller Area Network) standard, which cannot support the number of nodes and the sophistication of network-enabled functions that manufacturers want to build into today's vehicles.

FlexRay, however, has enjoyed limited success because systems designers have struggled to deliver reliable communications over extended cable lengths or in complex network topologies. At the root of the difficulties in FlexRay implementation is the phenomenon of asymmetric propagation delay, caused by, among other things, electro-magnetic interference (EMI) and extended cable lengths between nodes. The effect of asymmetric propagation delay is to shrink or expand the length of a bit to the point at which it cannot be decoded correctly at the receiver.

As a result, FlexRay has in practice delivered less fully on the promise of advanced and large-scale networking capability than its founders hoped when the FlexRay Consortium was founded in 2000.

A new bit-reshaping technique implemented in the AS8224 FlexRay transceiver from austriamicrosystems, however, corrects bit timing mismatches, ensuring that asymmetric delays are corrected at a central point in the network, where the effect of the corrections does most to improve network stability.

By using a bit-reshaping capability, systems designers can develop FlexRay networks that benefit from:

Enhanced and enlarged topologies, including topologies with more than one Active Star Longer cable runs More reliable operation in harsh conditions, such as the presence of high EMI The use of inexpensive unsheathed cable, as used today in CAN networks


Asymmetric delays

Asymmetric delay is a critical factor which limits the robustness and stability of FlexRay systems, and is far harder to correct than other effects such as propagation delays, signal truncations, ringing and reflections.

Static contributions to asymmetric delay are the sum of distortions caused by components and interfaces on the signal path. Asymmetric delays caused by components are specified by component vendors in their datasheets. Interface delays (eg between

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