CAN Bus Protocol Decription - Digital Core Design.pdf

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needed to represent one bit. One characteristic of NRZ code is that the signal provides no edges that can be used for resynchronization whentransmitting a large number of consecutive bits with the same polarity. Therefore Bit stuffing is used to ensure resynchronization of all bus nodes -this means that during the transmission of a message, a maximum of five consecutive bits may have the same polarity. Whenever five consecutivebits of the same polarity have been transmitted, the transmitter will insert one additional bit of the opposite polarity into the bit stream beforetransmitting further bits. Receiver does the reverse operation that is called "destuffing" and removes additional bits from the bit stream.

Frame Formats

There are few frame formats existing:Data Frame;Remote Frame;

Error Frame;Overload Frame;Interframe Space.

The most important and composite is Data Frame which will be fully discussed. Other frames will be treated only briefly.

Data FrameA Data Frame is generated by a CAN node when the node wishes to transmit data.Start of Frame - the frame begins with a dominant Start Of Frame bit for hard synchronization of all nodes. Arbitration Field - the Start of Frame bitis followed by the Arbitration Field consisting of 12 bits: the 11-bit identifier, which reflects the contents and priority of the message, and theRemote Transmission Request bit. This bit is used to distinguish a Data Frame (RTR = dominant) from a Remote Frame (RTR = recessive). Control

Field - consisting of 6 bits. The first bit of this field is called the IDE bit (Identifier Extension) and is at dominant state to specify that the frame is aStandard Frame. The following bit is reserved and defined as a dominant bit. The remaining 4 bytes of the Control Field are the Data Length Code(DLC) and specify the number of bytes of data contained in the message (0-8 bytes). Data Field - the data is being sent in the Data Field which is of

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Phase Buffer Segment 2. For many CAN modules implementations, the Propagation Time Segment and Phase Buffer Segment 1 are combined, forease of programming, into one segment often called Timing Segment 1. Phase Buffer Segment 2 is then known as Timing Segment 2.

The original specifications (Versions 1.0, 1.2 and 2.0A) specify an 11 bit message identifier. This is known as "Standard CAN". Those DataFrames and Remote Frames, which contain 11-bit identifier are therefore called Standard Frames. Up to 2048 different messages can be identified.To remove this possible message number limitation specification V2.0A has since been update to version 2.0B. This version is referred to as"Extended CAN". Extended Frames contain a 29-bit identifier which allows over 536 Million message identifiers. The 29-bit identifier is made up ofthe 11-bit identifier ("Base ID") and the 18-bit Extended Identifier ("ID Extension") CAN specification Version 2.0B still allows message identifierlengths of 11 bits to be used. There are three different types of CAN modules available:

2.0A - handles 11-bit ID but considers 29-bit ID as an error;2.0B Passive - handles 11-bit ID, ignores 29-bit ID messages;2.0B Active - handles both 11 and 29-bit ID messages.

Care must be taken when mixing protocol versions.

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CAN Bus Protocol Decription - Digital Core Design file:///C:/Users/ganagaraj.r/Documents/MB/Download/CAN Bus/ashow.php.ht

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