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AT89C51CC02CA-RATUM 数据表(PDF) 73 Page - ATMEL Corporation |
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AT89C51CC02CA-RATUM 数据表(HTML) 73 Page - ATMEL Corporation |
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73 / 156 page  73 AT/T89C51CC02 4126J–CAN–05/06 CAN Controller The CAN Controller provides all the features required to implement the serial communi- cation protocol CAN as defined by BOSCH GmbH. The CAN specification as referred to by ISO/11898 (2.0A & 2.0B) for high speed and ISO/11519-2 for low speed. The CAN Controller is able to handle all types of frames (Data, Remote, Error and Overload) and achieves a bitrate of 1-Mbit/s at 8 MHz 1 Crystal frequency in X2 Mode. Note: 1. At BRP = 1 sampling point will be fixed. CAN Protocol The CAN protocol is an international standard defined in the ISO 11898 for high speed and ISO 11519-2 for low speed. Principles CAN is based on a broadcast communication mechanism. This broadcast communica- tion is achieved by using a message oriented transmission protocol. These messages are identified by using a message identifier. Such a message identifier has to be unique within the whole network and it defines not only the content but also the priority of the message. The priority at which a message is transmitted compared to another less urgent mes- sage is specified by the identifier of each message. The priorities are laid down during system design in the form of corresponding binary values and cannot be changed dynamically. The identifier with the lowest binary number has the highest priority. Bus access conflicts are resolved by bit-wise arbitration on the identifiers involved by each node observing the bus level bit for bit. This happens in accordance with the "wired and" mechanism, by which the dominant state overwrites the recessive state. The com- petition for bus allocation is lost by all nodes with recessive transmission and dominant observation. All the "losers" automatically become receivers of the message with the highest priority and do not re-attempt transmission until the bus is available again. Message Formats The CAN protocol supports two message frame formats, the only essential difference being in the length of the identifier. The CAN standard frame, also known as CAN 2.0 A, supports a length of 11 bits for the identifier, and the CAN extended frame, also known as CAN 2.0 B, supports a length of 29 bits for the identifier. Can Standard Frame Figure 32. CAN Standard Frames A message in the CAN standard frame format begins with the "Start Of Frame (SOF)", this is followed by the "Arbitration field" which consist of the identifier and the "Remote Transmission Request (RTR)" bit used to distinguish between the data frame and the data request frame called remote frame. The following "Control field" contains the "IDen- tifier Extension (IDE)" bit and the "Data Length Code (DLC)" used to indicate the 11-bit identifier ID10..0 Interframe Space 4-bit DLC DLC4..0 CRC del. ACK del. 15-bit CRC 0 - 8 bytes SOF SOF RTR IDE r0 ACK 7 bits Intermission 3 bits Bus Idle Bus Idle (Indefinite) Arbitration Field Data Field Data Frame Control Field End of Frame CRC Field ACK Field Interframe Space 11-bit identifier ID10..0 Interframe Space 4-bit DLC DLC4..0 CRC del. ACK del. 15-bit CRC SOF SOF RTR IDE r0 ACK 7 bits Intermission 3 bits Bus Idle Bus Idle (Indefinite) Arbitration Field Remote Frame Control Field End of Frame CRC Field ACK Field Interframe Space |
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