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The FlexRay Protocol

Transcript of The flex ray protocol

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    The FlexRay protocol

    Master Of Control SystemsWissam Kafa

    June 17, 2014

  • Outline

    1 Introduction

    2 Why FlexRay?

    3 Network Topology

    4 Structure of a FlexRay Node

    5 FlexRay configuration: Cycle Segments

    6 Clock Synchronization

    7 Summary-Conclusion

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  • 1 Introduction

    FlexRay: A Communication Protocol in distributed systems withinautomotive context.

    developed by the FlexRay consortium (BMW, DaimlerChrysler, Mo-torola, Philips) founded in 1999.

    since 1999 many well-known companies joined (e.g. Bosch, GM,VW, Mazda, etc.)

    aim: fast, flexible, fault-tolerant communication protocol.

    FlexRay was used for the first time in BMW X5 model in 2007.

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  • 2 Why FlexRay? X-by-wire Technique

    steer-by-wire, brake-by-wire,. . .

    Hydraulic steering and braking is replaced by an electronic systemof sensors and actuators.

    Over years these new tasks have increased the requirements of thecommunication between control units.

    CAN is not sucient any more.

    Real-time capabilities are not supported because of bit arbitra-tion

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  • 3 Network Topology

    Figure 1: some possible FlexRay Network topologies (a) Passive bus. (b)Active star. (c) Hybrid topology

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  • Passive Bus Topology

    Figure 2: Passive Bus Topology

    A node can be connected to one or both channels A and B.

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  • Active Star Topology

    Figure 3: Active Star Topology

    - free of closed rings.- Received Signal is driven to all connected nodes.- A node could be connected to a maximum of two star couplers.

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  • Hybrid Topology

    Figure 4: Hybrid Topology

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  • 4 Structure of a FlexRay Node

    Figure 5: Structure of a FlexRay Node

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  • FlexRay Node: Host Controller

    - Processor to execute the main application.

    - It processes the received data.

    - Decides what to do, and what to besent to the communication controller

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  • FlexRay Node: Communication Controller

    - Realizes all functions of the FlexRay pro-tocol.- Receives data that should be sent from thehost controller.- Decides what to do, and what to be sentto the communication controller.- Handles the data according to the FlexRayprotocol, and sends them to the bus driver

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  • FlexRay Node: Bus Guardian- Changes in the supply of a node could oc-cur.

    This could cause defects on the bus.- Important for the fault-tolerance of theFlexRay.- Bus guardian could prevent these defects.- It organizes sending the data on the bus.- It prevents the node from sending and re-ceiving outside its time slots.- It can Recognize synchronization and com-munication errors

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  • FlexRay Node: Bus Driver

    - Responsible for the connection betweenthe FlexRay nodes and the bus.

    - Sends Data to the Bus.

    - Receives Data from the Bus.

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  • 5 FlexRay configuration: Cycle Segments

    Figure 6: FlexRay Cycle Segment

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  • FlexRay configuration: Cycle Segments

    - The communication on the bus passes incycles.- Each cycle can be divided into three seg-ments:

    - Static segment.- Dynamic segmentand.- Symbol segment.

    - A cycle is terminated by a network idletime, the NIT.- A typical FlexRay cycle takes about 2.5ms.

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  • Cycle Segments: Static Segment- The static segment is time triggered.- It is divided into time slots, Each slot has:

    - A fixed length.- ID assigned to a specific control unit.

    - Hard real-time requirements possible byguaranteed latency.- No delays or collisions could occur.- A node can be allocated to more than oneslot by clever distribution of slot IDs.- A Hard real-time application which shouldbe realized in the static segment:

    - Explosion of the airbag.

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  • Cycle Segments: Dynamic Segment

    - For reacting flexibly on specific events.- Event triggered segment.- It is also divided into slots with IDs.-If the ID of the actual slot corresponds withthe ID of the control unit, then the controlunit is allowed to send data.- If a longer message has to be sent, thetime slot of the next node shifts backwards.- An application for the dynamic segment:

    The control of the wipers dependingon the rain sensor.

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  • Cycle Segments: The symbol segment

    - In the symbol segment:FlexRay sends internal control infor-

    mation (starting the network).

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  • Cycle Segments: NIT and Frames

    Figure 7: FlexRay-Frame

    - The Network Idle Time is used for the synchronization of the clocks.- Each slot corresponds to one frame.- A frame can contain up to 254 bytes of data.

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  • 6 Clock Synchronization

    - Large temperature dierences, voltage changes and productiontolerances have a negative influence on the accuracy of the clocks.- A regular synchronization, especially for real-time and time-criticalapplications is essential.- For correct Operation, each node has to know the start time, the endtime and the number of the actual slots.- Therefore, all nodes need a common time base.- The Data rate also depends on the synchronization.- The synchronization of FlexRay is an internal synchronization algorithmand is most likely the midpoint algorithm.

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  • Clock Synchronization- The problem is divided into two aspects:

    - Nodes have to compound on a common time (oset correction).- Nodes have to adjust the time deviation between them (rate

    correction).- Each nodes communication controller has a local clock.

    counts in micro-ticks.- For example a FlexRay network with 10MBit/s scans the bus with80MHz. One tick of the oscillator correspond 0.0124 us. A micro-tick istypical twice this time.- The synchronization of the oset, as well as the rate correction usesmicro-ticks as smallest time unit.

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  • 7 Summary-Conclusion

    FlexRay focuses on a set of core needs for todays automotive indus-try.

    Higher data rates than previous standards.

    very flexible network topology.

    fault-tolerant operation.

    FlexRay thus delivers the speed and reliability required for next-generation in-car control systems.

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  • Summary-Conclusion

    The CAN network has reached its highest performance levels witha maximum speed of 1 Mbps.

    With a maximum data rate of 10 Mbps available on two channels,A gross data rate of up to 20Mbit/sec.

    Time and Event Triggered Protocol.

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  • Summary-Conclusion

    Figure 8: Vehicle-Network-Standards

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  • Summary-Conclusion

    Figure 9: Comparision (LIN, CAN, FlexRay)

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    Thanks!

    IntroductionWhy FlexRay?Network TopologyStructure of a FlexRay NodeFlexRay configuration: Cycle SegmentsClock SynchronizationSummary-Conclusion