The flex ray protocol
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This document discusses the FlexRay protocol. It was developed for automotive applications as CAN was insufficient for new electronic systems with real-time requirements. FlexRay supports higher data rates, flexible topologies, and fault tolerance. It divides communication into static and dynamic segments for hard and soft real-time messages. Nodes synchronize clocks to support time-critical applications. FlexRay enables next-generation in-car control systems with speeds up to 20Mbps.
The flex ray protocol
- 1. . . . The FlexRay protocol Master Of Control Systems Wissam Kafa June 17, 2014
- 2. 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 . . .Wissam Kafa | HIAST 2/26
- 3. 1 Introduction • FlexRay: A Communication Protocol in distributed systems within automotive 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. . . .Wissam Kafa | HIAST 3/26
- 4. 2 Why FlexRay? • X-by-wire Technique steer-by-wire, brake-by-wire,. . . • Hydraulic steering and braking is replaced by an electronic system of sensors and actuators. • Over years these new tasks have increased the requirements of the communication between control units. • CAN is not sufficient any more. Real-time capabilities are not supported because of bit arbitra- tion . . .Wissam Kafa | HIAST 4/26
- 5. 3 Network Topology Figure 1: some possible FlexRay Network topologies (a) Passive bus. (b) Active star. (c) Hybrid topology . . .Wissam Kafa | HIAST 5/26
- 6. Passive Bus Topology Figure 2: Passive Bus Topology A node can be connected to one or both channels A and B. . . .Wissam Kafa | HIAST 6/26
- 7. 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. . . .Wissam Kafa | HIAST 7/26
- 8. Hybrid Topology Figure 4: Hybrid Topology . . .Wissam Kafa | HIAST 8/26
- 9. 4 Structure of a FlexRay Node Figure 5: Structure of a FlexRay Node . . .Wissam Kafa | HIAST 9/26
- 10. FlexRay Node: Host Controller - Processor to execute the main application. - It processes the received data. - Decides what to do, and what to be sent to the communication controller . . .Wissam Kafa | HIAST 10/26
- 11. FlexRay Node: Communication Controller - Realizes all functions of the FlexRay pro- tocol. - Receives data that should be sent from the host controller. - Decides what to do, and what to be sent to the communication controller. - Handles the data according to the FlexRay protocol, and sends them to the bus driver . . .Wissam Kafa | HIAST 11/26
- 12. 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 the FlexRay. - 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 . . .Wissam Kafa | HIAST 12/26
- 13. FlexRay Node: Bus Driver - Responsible for the connection between the FlexRay nodes and the bus. - Sends Data to the Bus. - Receives Data from the Bus. . . .Wissam Kafa | HIAST 13/26
- 14. 5 FlexRay configuration: Cycle Segments Figure 6: FlexRay Cycle Segment . . .Wissam Kafa | HIAST 14/26
- 15. FlexRay configuration: Cycle Segments - The communication on the bus passes in cycles. - Each cycle can be divided into three seg- ments: - Static segment. - Dynamic segmentand. - Symbol segment. - A cycle is terminated by a network idle time, the NIT. - A typical FlexRay cycle takes about 2.5ms. . . .Wissam Kafa | HIAST 15/26
- 16. 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 by guaranteed latency. - No delays or collisions could occur. - A node can be allocated to more than one slot by clever distribution of slot IDs. - A Hard real-time application which should be realized in the static segment: - Explosion of the airbag. . . .Wissam Kafa | HIAST 16/26
- 17. 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 with the ID of the control unit, then the control unit is allowed to send data. - If a longer message has to be sent, the time slot of the next node shifts backwards. - An application for the dynamic segment: The control of the wipers depending on the rain sensor. . . .Wissam Kafa | HIAST 17/26
- 18. Cycle Segments: The symbol segment - In the symbol segment: FlexRay sends internal control infor- mation (starting the network). . . .Wissam Kafa | HIAST 18/26
- 19. 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. . . .Wissam Kafa | HIAST 19/26
- 20. 6 Clock Synchronization - Large temperature differences, voltage changes and production tolerances have a negative influence on the accuracy of the clocks. - A regular synchronization, especially for real-time and time-critical applications is essential. - For correct Operation, each node has to know the start time, the end time 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 algorithm and is most likely the midpoint algorithm. . . .Wissam Kafa | HIAST 20/26
- 21. 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 with 80MHz. One tick of the oscillator correspond 0.0124 us. A micro-tick is typical twice this time. - The synchronization of the offset, as well as the rate correction uses micro-ticks as smallest time unit. . . .Wissam Kafa | HIAST 21/26
- 22. 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. . . .Wissam Kafa | HIAST 22/26
- 23. Summary-Conclusion • The CAN network has reached its highest performance levels with a 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. . . .Wissam Kafa | HIAST 23/26
- 24. Summary-Conclusion Figure 8: Vehicle-Network-Standards . . .Wissam Kafa | HIAST 24/26
- 25. Summary-Conclusion Figure 9: Comparision (LIN, CAN, FlexRay) . . .Wissam Kafa | HIAST 25/26
- 26. . . . Thanks!