CAN Bus Workshop_Version 03__06-2008_EN.pdf - 第34页
1 - 8 S tudent Gu ide CAN BUS Wor kshop 2 Commun icatio n and C ontrol Editio n 06/200 8 8 2.2.1.1 CAN Bus protocol Fig. 2.2 - 5 CA N-bus protocol Start : This bit ind icates t he begi nning o f a teleg ram and i s a dom…
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Student Guide CAN BUS Workshop
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2.2.1 General structure
The CAN Bus is a decentral multi-master bus. The data are transmitted via the differential voltage
of the two CAN_High and CAN_Low lines, which are each fitted with a terminating resistance of
120 Ohm.
Fig. 2.2 - 3 CAN Bus structure
Fig. 2.2 - 4 CAN Bus -Controller and Microcontroller
Legend
– Microcontroller: Exchanges data with the CAN controller
– CAN controller: Adds the data frame, establishes the connection and manages errors.
– Transmitter/receiver: Adjusts the level (driver levels)
Each bus node has a CAN controller, which can transmit and receive data if the bus is free.
This CAN controller communicates with a microcontroller. The microcontroller steers and controls
the relevant CAN bus nodes.
A CAN Bus node can only transmit if the bus is free i.e. if there is no communication taking place
with other nodes. Access to the CAN BUS is fixed in the CAN protocol (identifier). This results in
differing priorities among the individual CAN bus nodes.
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Student Guide CAN BUS Workshop
2 Communication and Control Edition 06/2008
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2.2.1.1 CAN Bus protocol
Fig. 2.2 - 5 CAN-bus protocol
Start: This bit indicates the beginning of a telegram and is a dominant bit. After this bit is set, no
other user of the CAN bus is able to send.
Address field (11 bit identifier): The 11 bit address identifier value determines the bus access.
The lower value has the highest priority.
Control field: The 4 lowest bits in the 6 bit field show the data length of the following data field in
bytes (DLC: Data Length Code.
Data field: Contains the information actually required and can be from 0 byte to 8 byte. The trans-
fer of a byte begins with the most significant bit (the bit with the highest value).
Data control field CRC: Consists of a 15 bit check sequence (CRC sequence + CRC delimiter =
CRC Field - Cyclic Redundancy Check) and a recessive delimiter bit. The redundant information
in the control sequence allows the receiver to check whether the message received has been fal-
sified by interference.
End: Each data telegram is terminated by a sequence of 7 recessive bits.
2.2.1.2 11 Bit Identifier
Fig. 2.2 - 6 11 bit identifier
The CAN bus system is using the 11 Bit identifier for addressing the different CAN objects
An 11 Bit identifier (address) identifies the type, priority, source and /or target of the message.
This identifier also controls the bus access (arbitration).
start
address
(11 bit identifier)
control
information.
data (0-8 bytes user information) CRC
end
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Student Guide CAN BUS Workshop
Edition 06/2008 2 Communication and Control
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2.2.1.3 CAN Bus Arbitration
Arbitration (arbitration means decision)
In CAN networks, there is no addressing of subscribers or stations in the conventional sense, but
instead, prioritized messages are transmitted. Whenever the bus is free, any unit may start to
transmit a message. In general, a subscriber can only occupy the bus if this is free. The bus
subscriber can detect the bus occupation state by analyzing a certain time period within which the
bus must be inactive.
When multiple nodes begin to send a message at the same time, a selection phase (arbitration
phase) is used to decide which node may remain on the bus.
Bus access conflicts are resolved by including a message arbitration field (as a default the 11 bit
identifier is used).
The basis of bit-wise arbitration is the differentiation of 2 physical bus levels, a dominant one (low)
and a recessive bit (high).
A free bus is always on the recessive level. A DATA FRAME prevails over the REMOTE FRAME.
During arbitration every transmitter compares the level of the bit transmitted with the level that is
monitored on the bus. If these levels are equal the unit may continue to send.
When a recessive level is sent and a dominant level is monitored, the unit has lost arbitration and
must withdraw without sending one more bit. At the end of arbitration, the only subscriber left on
the bus is the one whose message has the lowest identifier value (logical zero is a dominant level).
The lower the identifier value is, the higher the priority of a message is.
When the bus is free any unit may start to transmit a message. The message sent by this
subscriber is not destroyed here i.e. it is a loss-free arbitration.
Fig. 2.2 - 7 flow chart bus arbitration
waiting if bus is free
bit SoF
bus in receiving state
1st bit of arbitration
transmitted
compare transmitted bit
level with bus level
bus in error state
arbitration lost?
recessive bit on dominant bus state
all arbitration bits are transmitted,
send control field and data field
next bits
START: Any member
will send a message