FlexRay - The Next-Generation Automotive Communication Protocol

To satisfy the never-ending communication needs for improved automotive performance, various protocols have found existence.  FlexRay is one such next-generation, general-purpose high-speed protocol that offers safety-critical features. Combined multiple sensors, actuators, and electronic control units require synchronization to deliver high-end performance. Controller Area Network (CAN) falls short for such needs for growing Bandwidth within today's advanced vehicles. To meet these next-generation challenges in these advanced vehicles, the FlexRay protocol has proven significant in the world of tremendously growing automotive technology.

FlexRay Communication Protocol is a registered trademark of Daimler Chrysler AG. FlexRay's full use was introduced in 2008 by FlexRay Consortium who promotes the standardization of FlexRay as the next-generation in-car communication protocol. FlexRay was explicitly designed to meet the demands and challenges of the drive-by-wire (start-by-wire, brake-by-wire) and advanced device assistance systems (engine, transmission).

Structure of FlexRay Network:

FlexRay uses unshielded twisted pair cables to connect nodes. These have a cabling impedance of about 80-110 ohms that require termination at the end nodes. Just like CAN, resistors are connected between the pair of signal wires to achieve this.

FlexRay Structure.png

Differential signalling on each pair of cables reduces the effects of external noise on the network without expensive shielding. It supports single and dual-channel configurations, which consist of one or two pairs of wires, respectively.  Dual-channel configurations offer enhanced fault tolerance and increased Bandwidth. 

Features of FlexRay:

  • Reliable.

  • Faster.

  • Redundant.

  • High data rates up to 10 Mbps.

  • Flexible configuration.

  • Versatile topology options.

  • Fault-tolerant.

  • Event and time triggered.

  • Dual-channel system.

  • Can consist of 'n' number of nodes.

  • Handles a large variety of frames.

  • Error tolerant.

  • Collision free access.

  • Guaranteed message latencies.

  • Fixed communication latencies.

  • Global synchronous time for all ECUs.

  • Static and dynamic segments.

  • 20 times higher Bandwidth than CAN.

  • Manages multiple nodes using Time Division Multiple Access (TDMA).

Applications:

  • Electronic power steering. (Steering-by-wire).

  • Advanced driver assistance systems.

  • Power train.

  • Body control module.

  • Battery management system.

  • Antilock braking system.

  • Vehicle stability control (VSC).

  • Vehicle stability assist (VSA).

FlexRay network Topologies:

One of the significant advantages of FlexRay is that it can be laid inside the vehicle according to the vehicle's layout. It offers topologies similar to Ethernet, such as Bus (multi-drop passive), Star (active) connections or the connection of these two topologies termed as Hybrid topology of FlexRay networks. This helps the designer to increase performance, reliability and optimize cost for a given vehicle system design.

Multi-Drop Bus FlexRay Network:

FlexRay network topologies.png
  • Commonly used.

  • A single network cable connects multiple ECUs.

  • Only one ECU can transmit at a time.

  • Terminated end to eliminate signal reflection.

Star FlexRay Network:

Star topology of FlexRay.png
  • Individual links connect to a central active node.

  • They are used for more extended networks.

  • If one node fails, it does not impact other nodes.

  • The reduced amount of exposed wire helps increase noise immunity.  

Hybrid FlexRay Network:

Hybrid topology of Flexray.png
  • It is formed by combining the bus and star topologies.

  • Offers the best of both the topologies.

  • Cost-effective, reliable and easy to use.

FlexRay Protocol Communication Cycle: fixed at the time of network design (usually kept around 1-5ms). It contains four main segments, known as:

FlexRay Communication Cycle.png
  • Static Segment: slot reserved for deterministic data, arriving at a fixed period.

  • Dynamic Segment: accommodates various signals without slowing down the FlexRay cycle due to an excessive number of static slots. It allows occasionally transmitted data.

  • Symbol Window: involved in network maintenance and identification of unique cycles such as cold-start cycles.

  • Network Idle Time: used to maintain synchronization between node clocks by making adjustments for any drift that may have occurred during the previous cycle. It is always pre-defined to a known length by ECUs.

FlexRay Message Frame Format

For any static or dynamic segments, each slot consists of a FlexRay frame which is divided into three basic parts, as shown below:

Flexray Frame Format.png

Header:

Flexray message frame format.png
Flexray message format header.PNG

Payload:

Payload of flexray frame format.png

It contains the actual data that needs to be transmitted by the message frame. It has a length of 0 to 254 bytes. (30 times greater than that of CAN).

Message-ID (optional): - This ID uses the first two bytes of the payload segment for definition. It can be used as filterable data on the receiving side.

NW Vector (optional): - Network management vector must be 0 to 12 bytes long and common to all nodes.

 Trailer:

Trailer of flexray frame.png

It contains three 8-bits CRC values specified by the hardware to detect errors to prevent incorrect connections by changing the seed value on the connected channel.

Data Security and error handling:

A FlexRay network offers scalable fault tolerance. The option for allowing single and dual-channel communication makes it more secure. Using both the channels for connecting the devices on the Bus improves security. One can increase Bandwidth by using both channels for transferring non-redundant data.

The FlexRay protocol facilitates fast error detection, signalling and error containment within the physical layer using the Bus guardian mechanism that protects the channel from the interference caused by the communication not aligned with the cluster's communication Schedule.

Difference between CAN and FlexRay?

CAN & Flexray Difference.PNG

For every implementation, the FlexRay networks may be designed differently. Each node must be programmed with correct network parameters before participating on the Bus to achieve proper functioning. The FlexRay committee has standardized a format for the storage and transfer of these parameters in the engineering process to maintain network configurations between nodes.  The Field Bus Exchange Format or FIBEX file is an ASAM-defined standard that allows network designers, prototypers, validators, and testers to share network parameters easily and quickly configure ECUs, test tools hardware-in-the-loop simulation systems, and so on for easy access to the Bus. 

Influx brings you the Rebel CT FlexRay, a compact data logger ideal for development applications. It supports up to 7 CAN 2.0 networks, 2 FlexRay and additional LIN data. The Rebel CT FlexRay includes an internal 18Hz GNSS module and a 3D accelerometer and can be upgraded to include GNSS, 3D accelerometer, 3D Gyro, Wi-Fi and 4G LTE. For more details on this, please visit Rebel CT FlexRay.