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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 sufficient 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 differences, 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 (offset 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 offset, 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!
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