|
Over the last few years, automotive electronics have increasingly defined the driving experience of modern vehicles. Starting in engine management and car audio, electronics have now penetrated all major systems in the vehicle ranging from power train, body, chassis, driver assistance systems, and active and passive safety systems.
The trend to network these systems started in the mid-80s with the introduction of the controller area network (CAN). At that time every electronic control unit (ECU) still represented an autonomous functional unit in the vehicle. As the number of ECUs has increased along with the technical abilities that electronic control can provide, the trend has shifted from networked ECUs to distributed systems where functions are spread across multiple ECUs.
To address the increasing demands of this trend, the automobile's communications network needs to provide not only high-speed data transfer, but also data transfer that is deterministic and fault-tolerant—and thus capable of supporting advanced distributed control systems. Over the last few years, an industry consortium of more than 120 companies has developed the FlexRay communications system, a time-deterministic protocol with a data rate of 10MBit/s for advanced control systems in vehicles.
The development of FlexRay started in 2000 in the context of an industry consortium that was founded by the four companies BMW, DaimlerChrysler, and the then Philips Semiconductors (now NXP) and Motorola (now Freescale Semiconductor). By 2003 Robert Bosch, GM, and VW joined, bringing the number of core partners to its current number of seven. The consortium quickly attracted numerous members throughout the automotive industry, bringing its membership numbers beyond 120 by the end of 2005.
Technical characteristics
FlexRay communication is configured in recurring communication cycles at a data rate of 10 MBit/s. Each communication cycle contains a static communication segment, a dynamic communication segment, and the network idle time, which is a communication-free period that concludes each communication cycle. In addition, each communication cycle can contain an optional symbol window that can be used to run-time test an ECU's interconnection to the physical network.
The FlexRay communications cycle features static and dynamic segments.
View a full-size image
The figure above shows an example of a FlexRay communication cycle. The static communication segment accommodates data communication that requires bounded latency and small latency jitter. In order to achieve this, the static communication segment utilizes a TDMA-based communication scheme based on static communication slots. In combination with a static schedule that is calculated off-line during the design of the system, FlexRay is able to address highly deterministic distributed applications, such as closed-loop control applications where the control loop is closed over the network.
In the static segment, the progression of static communication slots occurs in lockstep on both channels. ECUs may send frames with the same or with different content on each of the two channels within the same static communication slot. It is also possible to allocate the channels to different ECUs within one and the same static communication slot or to leave slots empty. In the dynamic segment, the pattern of dynamic communication slots unfolds independently on the two channels depending on whether dynamic communication slots are used or left empty.
The temporal characteristics of the FlexRay communication cycle are defined at design time and stored statically in each ECU. ECUs that require greater bandwidth and those that need shorter update intervals for messages are assigned more slots than those that require less bandwidth or those that allow for longer up-date intervals for messages.
|
