8.3 CAN bus communication

CAN bus communication is a robust protocol for connecting electronic control units in vehicles and industrial systems. It uses a multi-master architecture, allowing any node to initiate communication when the bus is free, and supports data rates up to 1 Mbps for real-time applications.

CAN employs differential signaling for reliable data transmission in noisy environments. It defines four frame types for data transmission and network management, and uses a non-destructive bitwise arbitration mechanism to determine bus access priority among multiple nodes.

CAN Fundamentals

Overview of CAN

• CAN (Controller Area Network) is a robust, message-based protocol for communication between electronic control units (ECUs) in vehicles and industrial automation systems
• Utilizes a multi-master bus architecture, allowing any node to initiate communication when the bus is free
• Supports data rates up to 1 Mbps, making it suitable for real-time applications (automotive systems, industrial control)

Physical Layer Characteristics

• CAN uses differential signaling to ensure reliable data transmission in noisy environments
• The physical layer consists of two wires: CAN High and CAN Low
  • CAN High carries the dominant (logical 0) and recessive (logical 1) states
  • CAN Low carries the inverted signal of CAN High
• The differential voltage between CAN High and CAN Low determines the logical state of the bus (dominant: CAN High > CAN Low, recessive: CAN High ≈ CAN Low)

CAN Nodes

• A node in a CAN network is any device connected to the bus, such as an ECU or sensor
• Each node has a unique identifier (ID) that determines its priority during arbitration
• Nodes can transmit and receive messages on the CAN bus, allowing for distributed control and monitoring (engine control module, airbag control unit)

Data Transmission

CAN Frame Types

• CAN defines four frame types for data transmission and network management:
  1. Data Frame: Carries data from a transmitting node to one or more receiving nodes
  2. Remote Frame: Used to request data from another node
  3. Error Frame: Transmitted by any node that detects an error on the bus
  4. Overload Frame: Used to introduce a delay between data or remote frames
• The structure of a CAN frame includes fields for arbitration (ID), data length, data, CRC, and acknowledgment

Arbitration and Priority

• Arbitration is the process of determining which node gains access to the bus when multiple nodes attempt to transmit simultaneously
• CAN uses a non-destructive bitwise arbitration mechanism based on the message ID
  • The lower the ID value, the higher the priority of the message
  • During arbitration, each node compares the ID it is transmitting with the state of the bus
  • If a node transmits a recessive bit while the bus is in a dominant state, it loses arbitration and backs off
• This arbitration mechanism ensures that the highest priority message is always transmitted first without corruption

Bit Stuffing and Timing

• CAN employs bit stuffing to maintain synchronization between nodes and to detect errors
• After five consecutive bits of the same polarity (five dominant or five recessive), a bit of the opposite polarity is inserted (stuffed) into the bit stream
• The receiving nodes remove the stuffed bits to restore the original data
• Bit stuffing helps maintain clock synchronization and provides a means for error detection (stuff error)
• CAN also defines time segments for synchronization, propagation delay compensation, and sample point configuration (Sync_Seg, Prop_Seg, Phase_Seg1, Phase_Seg2)

Error Handling

Error Detection Mechanisms

• CAN implements several error detection mechanisms to ensure data integrity:
  1. Bit Monitoring: Each transmitting node monitors the bus state and compares it with the transmitted bit
  2. Bit Stuffing: Violations of the bit stuffing rule indicate an error
  3. Frame Check: Errors in the fixed-form fields of a frame (CRC delimiter, ACK delimiter, End of Frame)
  4. Cyclic Redundancy Check (CRC): A 15-bit CRC is calculated and transmitted with each frame for error detection
  5. Acknowledgment (ACK): Receiving nodes acknowledge the correct reception of a frame
• If any of these error detection mechanisms fail, the node detecting the error transmits an Error Frame

Error Confinement and Recovery

• CAN uses an error confinement mechanism to prevent faulty nodes from disturbing the network
• Each node maintains two error counters: Transmit Error Counter (TEC) and Receive Error Counter (REC)
  • The counters are incremented and decremented based on the detected errors and successful transmissions/receptions
  • Nodes can be in three states depending on the counter values: Error Active, Error Passive, and Bus Off
• Error Active nodes can actively participate in error signaling and have a TEC and REC below 128
• Error Passive nodes have a TEC or REC between 128 and 255 and can still transmit messages but do not actively signal errors
• Bus Off nodes have a TEC greater than 255 and are disconnected from the bus until a reset occurs
• The error confinement mechanism ensures that a faulty node does not continuously disrupt the network and allows for automatic recovery when the fault condition is resolved