wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

FlexRay 44-3
Header segment (5 bytes)
Payload segment
(0 ... 254 bytes)
Trailer segment
(3 bytes)
Reserved bit
Payload preamble indicator
Nullframe indicator
Sync frame indicator
Startup frame indicator
Frame identifier
Payload length Header CRC Cycle count Payload Frame CRC
FIGURE 44.2
Frame format.
44.2.2 Framing
A frame sent in either the static or dynamic segment consists of a header, the payload, and a trailer.
The 5-byte long header is made up of five control bits, an 11 bit long frame identifier, a payload length
field, a header cyclic redundancy checksum (CRC), and a cycle count field; cf. Figure 44.2. The header
CRC secures the sync frame indicator bit, the startup frame indicator control bit, the frame identifier,
and the payload length field. The payload holds an even number of bytes (between 0 and 254) that carry
the application data. When the payload preamble indicator control bit in the header is set, a part of
the payload in the static segment is reserved for network management information. Special cases are
nullframes that are identified by the nullframe indicator control bit set in the header; these frames have
all the payload bits set to zero. Null frames are sent autonomously by the network controller whenever
no new data are at hand from the node’s host. The trailer contains a CRC over the entire frame with a
Hamming distance of 6 for frame lengths up to 248 bytes payload and a Hamming distance of 4 for
frame lengths between 249 and 254 bytes payload. With a Hamming distance of 6, up to 5 arbitrary bit
flips can be detected.
44.2.3 Startup of a Cluster
When the cluster is not running (e.g., after power-up) the host of a dedicated, so-called coldstart node
may try to start the cluster. It sends a collision avoidance symbol and waits for a response within a configured
time. When no response is received, the node stops its start-up attempts and waits for an attempt
from another node to start the cluster. When the latter occurs, the leading coldstart node will reply and
synchronize its timebase. Both nodes will now make the transition to an active state and other noncoldstart
nodes may join the cluster, see [FE05] or [R08] for details.
44.2.4 Physical Layer
FlexRay employs a variant of the inverted non-return-to-zero (NRZ-I) coding for bit transmission.
A standard NRZ-I code represents a “One” with a change in the physical level whereas a “Zero” is with
no change. The modification of the employed coding consists of a byte-start sequence (the bit combination
“10”) that is prepended to every byte. The falling edge between these two bits is used for bitresynchronization.
Furthermore, every frame transmission is prepended by a transmission start and a
frame start sequence. The transmission start sequence consists of 3–15 bits and is used to activate star
couplers. Since activation takes some time, a few bits of the transmission start sequence are usually
consumed by them. Furthermore, the end of a frame is followed by a frame end sequence consisting of
the bits “01,” and an idle phase where the transmitter is turned off. Figure 44.3 illustrates the coding of
an entire FlexRay frame.
© 2011 by Taylor and Francis Group, LLC