Image Processing Reference
In-Depth Information
SOF
Arbitration ield
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Node 1
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Node 2
Bus
level
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t
he
arbitration
phase
starts
Only node
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FIGURE .
CAN arbitration phase with two nodes starting transmitting simultaneously. Node detects that a
frame with a higher priority than its own is being transmitted when it monitors a level (i.e., dominant level) on the
bus while it has sent a bit with a level (i.e., recessive level). Aterwards, Node immediately stops transmitting. (From
Navet, N., Song, Y.Q., Simonot-Lion, F., and Wilwert, C.,
Proc. IEEE
., (), , . With permission.)
Any CAN node may start a transmission when the bus is idle. Possible conflicts are resolved
by a priority-based arbitration process, which is said nondestructive in the sense that, in case of
simultaneous transmissions, the highest priority frame will be sent despite the contention with lower
priority frames. The arbitration is determined by the arbitration fields (identifier plus RTR bit) of
the contending nodes. An example illustrating CAN arbitration is shown in Figure .. If one node
transmits a recessive bit on the bus while another transmits a dominant bit, the resulting bus level is
dominant due to the “and” operator realized by the physical layer. herefore, the node transmitting a
recessive bit will observe a dominant bit on the bus and then will immediately stop transmitting.
As the identifier is transmitted “most significant bit first,” the node with the numerically lowest
identifier field will gain access to the bus. A node that has lost the arbitration will wait until the bus
becomes free again before trying to retransmit its frame. CAN arbitration procedure relies on the
fact that a sending node monitors the bus while transmitting. he signal must be able to propagate
tothemostremotenodeandreturnbackbeforethebitvalueisdecided.hisrequiresthebittime
to be at least twice as long as the propagation delay which limits the data rate: for instance, Mbit/s
is feasible on a m bus at maximum while kbit/s can be achieved over m. To alleviate the
data rate limit, and extend the lifespan of CAN further, car manufacturers are starting to optimize
the bandwidth usage by implementing “traffic shaping” strategies that are very beneficial in terms of
response times (see, for instance, Ref. []).
CAN has several mechanisms for error detection. For instance, it is checked that the CRC transmit-
ted in the frame is identical to the CRC computed at the receiver end, that the structure of the frame
is valid, and that no bit-stuffing error occurred. Each station which detects an error sends an “error
flag” which is a particular type of frame composed of six consecutive dominant bits that allow all the
stations on the bus to be aware of the transmission error. he corrupted frame automatically reenters
into the next arbitration phase, which might lead it to miss its deadline due to the additional delay.
