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AT89C51CC02CA-RATUM 数据表(PDF) 74 Page - ATMEL Corporation |
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AT89C51CC02CA-RATUM 数据表(HTML) 74 Page - ATMEL Corporation |
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74 / 156 page  74 AT/T89C51CC02 4126J–CAN–05/06 number of following data bytes in the "Data field". In a remote frame, the DLC contains the number of requested data bytes. The "Data field" that follows can hold up to 8 data bytes. The frame integrity is guaranteed by the following "Cyclic Redundant Check (CRC)" sum. The "ACKnowledge (ACK) field" compromises the ACK slot and the ACK delimiter. The bit in the ACK slot is sent as a recessive bit and is overwritten as a domi- nant bit by the receivers which have at this time received the data correctly. Correct messages are acknowledged by the receivers regardless of the result of the acceptance test. The end of the message is indicated by "End Of Frame (EOF)". The "Intermission Frame Space (IFS)" is the minimum number of bits separating consecutive messages. If there is no following bus access by any node, the bus remains idle. CAN Extended Frame Figure 33. CAN Extended Frames A message in the CAN extended frame format is likely the same as a message in CAN standard frame format. The difference is the length of the identifier used. The identifier is made up of the existing 11-bit identifier (base identifier) and an 18-bit extension (identi- fier extension). The distinction between CAN standard frame format and CAN extended frame format is made by using the IDE bit which is transmitted as dominant in case of a frame in CAN standard frame format, and transmitted as recessive in the other case. Format Co-existence As the two formats have to co-exist on one bus, it is laid down which message has higher priority on the bus in the case of bus access collision with different formats and the same identifier / base identifier: The message in CAN standard frame format always has priority over the message in extended format. There are three different types of CAN modules available: – 2.0A - Considers 29 bit ID as an error – 2.0B Passive - Ignores 29 bit ID messages – 2.0B Active - Handles both 11 and 29 bit ID Messages Bit Timing To ensure correct sampling up to the last bit, a CAN node needs to re-synchronize throughout the entire frame. This is done at the beginning of each message with the fall- ing edge SOF and on each recessive to dominant edge. Bit Construction One CAN bit time is specified as four non-overlapping time segments. Each segment is constructed from an integer multiple of the Time Quantum. The Time Quantum or TQ is the smallest discrete timing resolution used by a CAN node. 11-bit base identifier IDT28..18 Interframe Space CRC del. ACK del. 15-bit CRC 0 - 8 bytes SOF SOF SRR IDE ACK 7 bits Intermission 3 bits Bus Idle Bus Idle (Indefinite) Arbitration Field Arbitration Field Data Field Data Frame Control Field Control Field End of Frame CRC Field ACK Field Interframe Space 11-bit base identifier IDT28..18 18-bit identifier extension ID17..0 18-bit identifier extension ID17..0 Interframe Space 4-bit DLC DLC4..0 CRC del. ACK del. 15-bit CRC SOF SOF SRR IDE r0 4-bit DLC DLC4..0 RTR RTR r0 r1 r1 ACK 7 bits Intermission 3 bits Bus Idle Bus Idle (Indefinite) Remote Frame End of Frame CRC Field ACK Field Interframe Space |
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