dsPIC30F Family Reference Manual
DS70074C-page 26-16 © 2004 Microchip Technology Inc.
B.5 CAN Bus Features
CAN has the following properties:
• Prioritization of messages
• Latency times ensured
• Configuration flexibility
• Multi-cast reception with time synchronization
• System wide data consistency
• Multi-master
• Error detection and signaling
• Automatic retransmission of corrupted messages
• Distinction between temporary errors and permanent failures of nodes and autonomous
switching off of defect nodes
1. Messages – information on the bus is sent in fixed format messages of different but limited
length. When the bus is free any connected unit may start to transmit a new message.
2. Information Routing – In CAN systems, a CAN node does not make use of any information
about the system configuration (e.g., station addresses).
3. System Flexibility – Nodes can be added to the CAN network without requiring any
change in the software or hardware of any node and application layer.
4. Message Routing – The content of a message is named by an ldentifier. The ldentifier
does not indicate the destination of the message, but describes the meaning of the data,
so that all nodes in the network are able to decide by Message Filtering whether the data
is to be acted upon by them or not.
5. Multicast – As a consequence of the concept of Message Filtering, any number of nodes
can receive and simultaneously act upon the same message.
6. Data Consistency – Within a CAN network, it is ensured that a message is simultaneously
accepted either by all nodes or by no node. Thus, data consistency of a system is
achieved by the concepts of multicast and by error handling.
7. Bit Rate – The speed of CAN may be different in different systems. However, in a given
system, the bit-rate is uniform and fixed.
8. Prioritization – The ldentifier defines a static message priority during bus access.
9. Remote Data Request – By sending a remote frame, a node requiring data may request
another node to send the corresponding data frame. The date frame and the
corresponding remote frame are named by the same ldentifier.
10. Multimaster – When the bus is free, any unit may start to transmit a message. The unit
with the message of higher priority to be transmitted gains bus access.
11. Arbitration – Whenever the bus is free, any unit may start to transmit a message. If 2 or
more units start transmitting messages at the same time, the bus access conflict is
resolved by bitwise arbitration using the ldentifier. The mechanism of arbitration ensures
that neither information nor time is lost. If a data frame and a remote frame with the same
ldentifier are initiated at the same time, the 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.
12. Safety – In order to achieve the highest safety of data transfer, powerful measures for
error detection, signaling and self-checking are implemented in every CAN node.
13. Error Detection – For detecting errors the following measures have been taken:
• Monitoring (transmitters compare the bit levels to be transmitted with the bit levels
detected on the bus)
• Cyclic Redundancy Check
•Bit Stuffing
• Message Frame Check
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