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FlexRay - Protocol | Cypress Semiconductor

FlexRay - Protocol

 Background |  Overview |  Features |  In-car Communication Protocol |  Target applications |  Data transfer method |  Difference with CAN |  Internal configuration of a node |  Electronic signals |  Bus Guardian |  Topology |  Segment configuration |  Protocol |  Status transition |  Error control |

FlexRay ASSP

Frame format

The frame of FlexRay has the following features:

  • The header segment Header CRC is calculated and specified by the host.
  • The trailer segment CRC is calculated by the hardware.
  • The CRC also changes initial values on the connected channel to prevent incorrect connection.
 FlexRay frame format

FlexRay frame format

Header segment

  • Reserved bit
    This is a reserved bit for future expansion.
  • Payload preamble indicator
    This bit indicates the existence of vector information in the payload segment of the frame.
    At static frame, it indicates NWVector, and at dynamic frame, it indicates Message ID.
  • Null frame indicator
    This bit indicates whether or not the data frame in the payload segment is NULL
  • Sync frame indicator
    This bit indicates the existence of synchronous frame.
  • Startup frame indicator
    This bit indicates whether or not the node sending frame is the start-up node.
  • Frame ID
    An ID is assigned to each node at system designing. (Valid range: 1 to 2047)
  • Length
    It specifies the data length of the payload segment part.
  • Header CRC
    It specifies the CRC calculation values of Sync Frame Indicator, Startup Frame Indicator, Frame ID, and Length that are calculated by the host.
  • Cycle
    Cycle counter. It indicates the cycle count of the node that transfers the frame during the frame transfer time.

Payload segment

  • Data
    Data. Valid range is from 0 to 254 bytes.
  • Message ID
    Optional. It uses the first two bytes of the payload segment for definition, and it can be used as the filterable data on the receiving side.
  • NWVector
    Optional. The network management vector length must be from 0 to 12 bytes and common to all nodes.

Trailer segment

  • CRC
    It is calculated and specified by the hardware.
    It changes the seed value on the connected channel to prevent incorrect connections.

Timing of configuring protocol

FlexRay communication cycles consists of four timing levels.

 FlexRay timing levels

FlexRay timing levels

Timing levels and their functions

Timing level Function
Communication cycle level Frame scheduling
Arbitration grid Arbitrated slot (time-sharing multiple control)
Macrotick level Synchronized service
Microtick level Segmentalized clock for the synchronized service

The Macrotick level and the Microtick level act as a clock to be the basis of configuration of each segment.
Synchronization is performed on the basis of this clock.

  • Macrotick:
    A global synchronizing clock that manages the entire network time.
  • Microtick:
    A node-specific clock. Minimum units of the FlexRay management time.

The following figure shows an example of clock division at 10 Mbps of the transfer rate.

 Example of relationship between the clock division and slot length

clock cycle

- Note -

Some parts of this site show only the outlines of the actual FlexRay standards to make them easily understood.