Course Introduction. Purpose. Objectives. Content. Learning Time

Transcription

1 Course Introduction Purpose This training course provides an overview of Message Frames and hardware issues of the Controller Area Network (CAN) technology used to build networked, multiprocessor embedded systems. Objectives Discover the types of Message Frames in CAN bus traffic and the purpose of each. Learn the details of a CAN Data Frame. Understand hardware issues of implementing CAN networks. Content 23 pages 3 questions Learning Time 30 minutes 1

2 What is CAN? Controller Area Network Two-wire, bidirectional serial-bus communication method Economical solution for high-integrity networking in real-time embeddedcontrol applications Standardized internationally - CAN 2.0A: ISO11519 (low speed) - CAN 2.0B: ISO11898 (high speed) Widely used: >>100,000,000 nodes/year Key features - High reliability (essentially error free, even in noisy environments) - Low wiring cost and node-connection cost - Readily scalable - Off-the-shelf tools - Supported by many chips - Knowledge base growing 2

3 Types of Message Frames Data Frame - Carries the actual data (payload) Error Frame - Six consecutive dominant bits, sent anytime an error is detected by the hardware at any node Overload Frame - Requests a delay on the bus Interframe Space - Provides minimum spacing between data and remote frames; allows error frames to have levels of priority Remote Frame - Requests transmission of a Data Frame; carries no payload 3

4 Data Frames: Two Types Standard Frame (CAN 1.0 and 2.0A) Start of Frame Arbitration Field Control Field CRC Field Data Field End of Frame ACK Field Extended Frame (CAN 2.0B) 4

5 ERTIES ssing, 'Finish' button: iling, 'Finish' button: user to leave quiz: may view slides after quiz: may attempt quiz: Goes to Next Slide Goes to Slide At any time After passing quiz Unlimited times

6 Start of Frame Indicated by single dominant bit occurring when bus is idle All nodes synchronize timing to leading edge Start of Frame 6

7 Arbitration Field: IDs ID10 12 bits Standard ID (11 bits) ID0 RTR Arbitration Field Standard ID: CAN 2.0A and CAN 2.0B - Gives CAN 2.0B backward compatibility with CAN 2.0A - 11 bits allow up to 2032 unique IDs Extended ID: CAN 2.0B only (optional) - 29 bits allow over 500 million unique IDs - Extended ID with same 11-bit higher-order field has lower priority than Standard ID RTR bit following the ID is dominant for a Data Frame, recessive for a Remote Frame 32 bits ID28 Standard ID (High-order field, 11 bits) ID18 SRR IDE ID17 [Additional ID space] (Low-order field, 18 bits) ID0 RTR Extended ID (29 bits, CAN 2.0B) 7

8 Keeping Traffic Flowing CAN Arbitration Protocol: CSMA/CD-CR - Carrier Sense Nodes wait for period without bus activity before sending message - Multiple Access Every node has an equal opportunity to transmit message - Collision Detection Collision occurs if two nodes attempt to transmit at same time - Collision Resolution Non-destructive bitwise arbitration keeps messages intact; sends highest-priority message without delay; and subsequently allows retransmissions of lower-priority messages 8

9 Keeping Traffic Flowing Uses Dominant and Recessive bits - IDs with dominant bits get priority - Nodes attempting to send lower-priority messages try to transmit in each successive cycle, succeeding when they finally attain priority 9

10 Control Field Data Length Code (DLC) tells how much information the Data Field contains 2 bits of Control field are reserved 4 bits give DLC (# of bytes in Data Field) IDE RB0 Data Length Code (4 bits) DCL3 DCL0 Control Field Number of Data Bytes Data Length Code DLC DLC DLC DLC

11 Data Field Content: Payload of Data Frame Length: 0 to 8 bytes MSB is transmitted first Payload of Data Frame (0, 1, 2, 3, 4, 5, 6, 7 or 8 bytes) Data Field 11

12 CRC Field 15-bit Cyclic Redundancy Check (CRC) value followed by a recessive delimiter CRC polynomial: x 15 +x 14 +x 10 +x 8 +x 7 +x 4 +x 3 +1 Generated by transmitter node s hardware; verified in each receiver node s hardware If receiver-generated CRC matches transmitted CRC, receiver puts a dominant bit in ACK slot If no match, receiver sends NO vote AFTER upcoming ACK delimiter CRC Code 15 bits Del CRC Field 12

13 ACK Field ACK Slot + ACK Delimiter All receivers that receive valid messages report successful reception by placing a dominant bit in the ACK slot* Receivers that do not get a valid message (indicated by CRC mismatch) vote NO by placing an error flag after the delimiter ACK Del * Because every node acknowledges messages, the round-trip propagation delay is the primary distance limitation of CAN. ACK Field 13

14 End of Frame Delimits the end of a Data Frame Consists of seven recessive bits for Data Frame (also for a Remote Frame) Provides a short break in communication flow before the next frame, allowing time for Error Frames, etc. 7 bits End of Frame 14

15 ERTIES ssing, 'Finish' button: iling, 'Finish' button: user to leave quiz: may view slides after quiz: may attempt quiz: Goes to Next Slide Goes to Slide At any time After passing quiz Unlimited times

16 Types of Errors Five types of errors, detected locally: 1. Bit error Received bit at sender not equal to transmitted bit 2. Bit-stuffing violation More than five consecutive bits with same polarity 3. CRC error Checksum violation 4. Form error Bit pulse distorted; invalid bit at predetermined points in message frame 5. ACK error No dominant bit in ACK slot, so sender must retransmit Message Frame 16

17 Node Hardware Operating States Depending on error detection, hardware is in: Error Active State Normal operating state; messages can be transmitter and received. Error Passive State Despite frequent transmit and receive problems, messages still can be transmitter and received. Bus Off State Serious problems transmitting messages; no messages can be transmitted or received until hardware is reset. This prevents a faulty node from causing a bus failure. Summary of CAN Error Process 1. Error detected by node 2. Error frame immediately transmitted 3. Message cancelled at all nodes 4. Status of hardware at nodes updated 5. Message re-transmitted 17

18 Implementing CAN Systems Needed: Firmware CAN controller CAN transceivers A physical media Bus length (meters) km Maximum bit rate* 1Mbps 500kbps 250kbps 125kbps 10kbps 18 * CAN 2.0B

19 CAN Hardware Designs Two main CAN controller implementation strategies: CAN controller BasicCAN FullCAN Key differences between strategies: How decision is made on whether or not a message is of interest to a receiving node How remote frames are answered How messages are buffered Implementation determines amount of processing load put on host microcontroller 19

20 Features of BasicCAN Networks Transmit Receive Remote Frame Handling Overrun Philosophy Application fills complete Tx register, including ID, RTR, data length, and data; every ID can be transmitted Every CAN message can be received Normally two receive buffers in FIFO structure Global message filtering: Filter cannot be set up to pass only the interesting messages; final filtering must be done by the application Remote frames are answered by the application Keep oldest message; newer messages will be lost 20

21 Features of FullCAN Networks Transmit Receive Remote Frame Handling Overrun Philosophy Transmit mailboxes initialized once Only data bytes written before transmission Only messages with the IDs defined in receive mailboxes can be received No double-buffering for mailboxes Full acceptance filtering; only exact message IDs are let through Remote frames are answered automatically by the controller Keep newest message; older messages with the same ID will be lost 21

22 ERTIES ssing, 'Finish' button: iling, 'Finish' button: user to leave quiz: may view slides after quiz: may attempt quiz: Goes to Next Slide Goes to Slide At any time After passing quiz Unlimited times

23 Course Summary Types of CAN Message Frames Data Frame Details Bus Errors Implementation Strategies: BasicCAN vs. FullCAN 23