Overview of FlexRay communication protocol

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In order to meet the endless communication needs of improving vehicle performance , Various agreements already exist .FlexRay It is such a next-generation general high-speed protocol that provides security critical features . Combined multiple sensors 、 The actuator and electronic control unit need to be synchronized to provide high-end performance . Controller area network ( CAN ) Unable to meet the growing demand for bandwidth in today's advanced vehicles . In order to meet these next generation challenges of these advanced vehicles ,FlexRay The agreement has proved to be of great significance in the world of rapid development of Automotive Technology .

FlexRay The communication protocol is a registered trademark of DaimlerChrysler Corporation .FlexRay The full use of is made by FlexRay Alliance in 2008 , launched in , The alliance will promote FlexRay It is standardized as the next generation vehicle communication protocol .FlexRay Designed to meet drive by wire （ Start by wire 、 Brake by wire ） And advanced equipment auxiliary system （ The engine 、 transmission ） Designed for the needs and challenges of .

1 FlexRay Network structure

FlexRay Use unshielded twisted pair to connect nodes . Their cable impedance is about 80-110 ohm , It needs to be terminated at the end node . It's like CAN equally , A resistor is connected between the pair of signal lines to achieve this .

The differential signal on each pair of cables can reduce the impact of external noise on the network , Without expensive shielding . It supports single channel and dual channel configurations , It consists of one or two pairs of lines . Dual channel configuration provides enhanced fault tolerance and increased bandwidth .

2 FlexRay Characteristics

• reliable .

• Hurry up .

• Redundant .

• the height is 10 Mbps High data rate .

• Flexible configuration .

• Multiple topology options .

• Fault tolerance .

• Event and time trigger .

• Dual channel system .

• Can be “n” The nodes make up .

• Deal with various frameworks .

• They tolerate mistakes .

• Collision free access .

• Ensure message delay .

• Fixed communication delay .

• all ECU Global synchronization time .

• Static and dynamic segments .

• Bandwidth ratio CAN high 20 times .

• Time division multiple access (TDMA) Manage multiple nodes .

application ：

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• Electric power steering .（ Steer by wire ）.
• Advanced driving assistance system .
• powertrain .
• Body control module .
• Battery management system .
• Anti lock braking system .
• Vehicle stability control (VSC).
• Vehicle stability assist (VSA).

3 FlexRay Network topology

FlexRay One of its significant advantages is that it can be arranged inside the vehicle according to the layout of the vehicle . It provides a topology similar to Ethernet , For example, the bus （ Multipoint passive ）、 Stars （ Active power ） Connection or connection of these two topologies , be called FlexRay Hybrid topology of the network . This helps designers improve the performance of a given vehicle system design 、 Reliability and optimized cost .

3.1 Multipoint bus FlexRay The Internet

• Commonly used .
• One network cable connects multiple ECU.
• There can only be one at a time ECU transmitted .
• End to end to eliminate signal reflection .

3.2 star FlexRay The Internet

• Each link is connected to the central active node .
• They are used for more extended networks .
• If a node fails , It will not affect other nodes .
• Reducing the number of bare wires helps to improve the anti noise ability .

3.3 blend FlexRay The Internet

• It is composed of bus and star topology .
• Provide the best of the two topologies .
• Cost effective 、 Reliable and easy to use .

**FlexRay Protocol communication cycle ：** Fixed in network design （ Usually keep at 1-5ms about ）. It consists of four main parts , be called ：

• Static segment ： Time slot reserved for deterministic data , Arrive in a fixed period .
• Dynamic segment ： Adapt to various signals , It will not slow down because of too many static slots FlexRay cycle . It allows the occasional transfer of data .
• Symbol window ： Involve network maintenance and identify unique cycles , Such as cold start cycle .
• Network idle time ： It is used to maintain synchronization between node clocks by adjusting any drift that may occur in the previous cycle . It always consists of ECU Predefined as a known length .

4 FlexRay Message frame format

For any static or dynamic segment , Each slot consists of a FlexRay The frame of , The frame is divided into three basic parts , As shown below ：

4.1 HEADER

4.2 Payload

It contains the actual data that needs to be transmitted by the message frame . Its length is 0 To 254 Bytes .（ yes CAN Of 30 times ）.

Message-ID（ Optional ）： this ID Define using the first two bytes of the payload segment . It can be used as filterable data at the receiving end .

NW vector （ Optional ）： The length of the network management vector must be 0 To 12 Bytes , And it is common to all nodes .

4.3 Trailer

It contains three hardware specified 8 position CRC value , Used to detect errors by changing the seed value on the connection channel to prevent incorrect connections .

4.4 Data security and error handling

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FlexRay The network provides scalable fault tolerance . The option of allowing single channel and dual channel communication makes it more secure . Using these two channels to connect devices on the bus can improve security . Bandwidth can be increased by using two channels to transmit non redundant data .

FlexRay The protocol uses bus protection mechanism to promote rapid error detection in the physical layer 、 Signaling and error control , This mechanism protects the channel from interference caused by communication inconsistent with the cluster communication scheduling .

5 CAN and FlexRay The difference between

For each embodiment ,FlexRay The design of the network may be different . Before joining the bus , Each node must be programmed with the correct network parameters , To achieve normal operation .FlexRay The Committee standardized the storage and transmission format of these parameters in the process of Engineering , To maintain the network configuration between nodes . Fieldbus exchange format or FIBEX File is ASAM The standard of definition , Allow network designers 、 Prototype designer 、 Verifiers and testers can easily share network parameters , And quickly configure ECU、 Test tools, hardware in the loop simulation system, etc .