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Grundlagen FlexRay BasicsV 1.1 36
• Frame-ID – also called Slot ID. Defines in which slot the frame should be
transmitted. The ID has a length of 11 bit and has a range from 1 to 2047, since 0 is
reserved for invalid frames.
• Payload Length – defines the number of bytes in the payload segment, divided by
two. It is only possible to configure payloads with an even number of bytes. Using its
seven bits, a range from 0 to cPayloadLengthMax = 127 can be configured, which
translates into a payload length of 0 to 254 bytes..
In any given network, the length of the payload in the static segment is identical. In
the dynamic segment, the length of the payload may differ from cycle to cycle. It may
also be different for the two channels.
• Header CRC – is a checksum, which is calculated over the area marked in red in
Figure 2.32. The Header Cycle Redundancy Check (CRC) polynome is:
( ) ( ) ( )
• Cycle Count – defines, in which cycle the Frame shall be send. It is possible to
address 0 to cCycleCountMax = 63 with this 6 bit value..
2.12.2 Payload
The payload area contains the actual data transmitted by the node. The length of the FlexRay
payload is 0 to 127 words, which translates to 0 to 254 bytes. A part of the payload can be
used for the Network Management Vector in the static segment or the Message ID in the
dynamic segment by setting the Payload Preamble Indicator.
2.12.3 Trailer
The trailer contains the CRC which is calculated over the entire length of the frame. With its
help, received data can be checked for validity. The polynome used in FlexRay has a
Hamming Distance of 6 at a frame length of 248 bytes. That means it is possible to detect up
to 5 bit transmission errors per frame. If the frame length is increased to 254 bytes, the
Hamming Distance decreases to 4.
Das polynome for the calculation is:
( ) ( )
( )
For the calculation of the CRC, both FlexRay channels are initialized using differ values. That
way, only data for the correct channel can be verified successfully.
2.12.4 Null frame
A null frame has the Null Frame Indicator bit set to zero, such a frame does not contain any
data. The payload section exists, but al data is filled up with zeros.
This frame is sent in the static segment, if the transmit buffer is blocked by the host, but the
node has to send in his slot. If such a case occurs in the dynamic segment, there is no frame
sent at all.
This method is important, especialy for Startup and Sync Fames. These actions are done by
the Communication Controller, and don’t require interaction from the host.
Bäurle 12.10.2012