EECS 373 Controller Area Networks Samuel Haberl Russell Kuczwara Senyuan Zhong

Presentation Outline  CAN history and basics  CAN standard message frame & arbitration  Error checking & noise protection  CAN implementation  Application strengths

CAN History  Developed by Bosch in 1983  Presented at SAE Conference in Detroit in 1986  Designed for automotive uses  Allows multiple controllers and sensors to communicate  2008: all US vehicles required to implement CAN for OBD  Multiple CAN implementations and variations

Where is CAN Used?  Automotive industry  Electronic control units (ECUs)  Sensors and switches  Multimedia systems  Factory automation  Low-cost machinery and electronics  Michigan Hybrid Racing Electric Drive System Photo credit: Sam Haberl

CAN Basics  Multi-master, broadcast serial bus  Real Time Arbitration  Robust error-checking and noise protection  No extra wires for clock or arbitration  Cheap tm

Masters and Targets  Each node can be a master and a target  Arbitration field indicates message type  Nodes listen to messages that they care about  Addresses composed of dominant and recessive bits  0 – Dominant, 1 – Recessive  Lowest address (0x000) is most dominant

Standard Message Frame  Start of frame  Arbitration (identifier)  Control (data length, reserved bits)  Data field  CRC (cyclical redundancy check)  ACK (acknowledgement field)  End of frame  Intermission (time between frames) Image from 11 or 29 Bit

Arbitration 11 or 29 Bit

Error Checking  Form Errors  Proper message formatting is enforced  Bit Errors  Transmitter detects wrong bit while transmitting.  CRC Checksum Errors  Checksum calculated, compared to transmitted checksum  Acknowledgement Errors  ACK field must be set dominant by a receiving node  Bit stuffing Errors  Any time there are 5 of the same bits, one opposite bit sent  Error triggered by 6 identical bits

Error Handling  Any node on the bus can raise an “Error frame.”  Holds bus dominant for 6 bits  Other nodes realize this is wrong, raise their error frames  Node that was transmitting re-sends message  Each node keeps individual error count  Node goes into “passive” mode at 127 errors  Node removes itself from bus after 255 errors Image from frame.php?navanchor=

Timing  No bus clock signal, each node has its own clock  Each bit is divided into four sections  Synchronization, Propagation, Phase 1, Phase 2  Bit sampled between phase 1 and 2  Receiving nodes synchronize by adjusting phases

CAN Implementations Variants  CAN architecture can be implemented in many different approaches  Standards for single-wire, twisted pair, four-wire  Twisted-pair CAN is most commonly used  Twisted pair uses CAN_H and CAN_L signals  No additional wires needed  Signal generated by transceiver, not by processor content/themes/networkcablinglosangeles/images/unshielded_cable.jpg

Electrical Noise  Lots of Noise in Automotive Application  Only getting worse with more automation for luxury

Differential Signals  Two-Wire differential signal  Value represented by difference between wires, not the absolute value of one wire  Difference creates immunity to noise 10? 00

From Master to Bus (2-Wire Implementation)  Processor uses logic-level signal  Signals sent over 2-wire bus  Use transceiver as bridge

Full Message Example  Sends the message with identifier  Sends One Byte with value “1”  15 bit CRC field  Bit Stuffing is handled by can transceivers and is not visible

A CAN Bus For Everyone  Allows multiple bitrates and buses  Bitrates affect robustness and bus length  1 Mbit/s – High-priority  i.e. Airbags  10 kbit/s – Low-priority  i.e. Audio System changes (volume, audio source)

Multiple CAN Bus Example

Application Strengths  Arbitration with no delay or extra wires  High-priority devices don’t wait  No extra weight or cost from wires  Robust error correction  Faulty nodes remove themselves from bus  Minimizes need for software correction  Noise immunity from twisted pair CAN  Inexpensive  56 2,500 qty for SN65HVD1040DR CAN Transceiver (DigiKey)

References  Robert Bosch GmbH   KVaser CAN Solutions   Dearborn Group Technologies   Softing CAN Bus Products  bus/index.php?navanchor= bus/index.php?navanchor=  Vector CANtech  GsAIw&et_rp=1 GsAIw&et_rp=1

Questions?