Controller Area Network (CAN)

Controller Area Network (CAN) is a robust, message-based communication protocol designed to allow electronic control units (ECUs), sensors, and microcontrollers to communicate with each other reliably without the need for a central host computer.

Wiring: Uses a twisted pair of wires (CAN High and CAN Low) to minimize interference.
Topology: A decentralized “bus” where every device can see every message.
Arbitration: A priority-based system where lower ID numbers get the right of way.
Reliability: Includes automatic retransmission of failed messages and fault confinement.

CAN Termination is critical for preventing signal reflections and ensuring the network remains stable.

CAN Logic Levels: CAN encodes bits using differential voltage: a dominant ‘0’ is created by driving CAN_H high and CAN_L low (~2 V difference), while a recessive ‘1’ leaves both lines at ~2.5 V (0 V difference), enabling robust noise immunity and bus arbitration.

CAN 2.0A Standard Frame

SOF (Start of Frame) – 1 bit

• Marks the beginning of a CAN frame
• Must be dominant (0) so every node can detect the frame start

Arbitration Field – 12 bits total
This decides which node wins if multiple ECUs transmit at the same time

11-bit Identifier (ID) – 11 bits

• The message name / priority number
• Lower numeric ID has higher priority on the CAN bus

RTR (Remote Transmission Request) – 1 bit

• 0 (dominant) = Data frame (normal frame carrying data)
• 1 (recessive) = Remote frame (requests data from another node)

Control Field – 6 bits total
Contains format and data length information

IDE / r1 – 1 bit

• In CAN 2.0A standard frame, IDE must be dominant (0), meaning 11-bit ID format
• Some diagrams label this bit as IDE, and also mention r1 (reserved usage)

r0 – 1 bit

• Reserved bit, must be dominant (0)

DLC (Data Length Code) – 4 bits

• 0 to 8 bytes in classic CAN

Data Field – 0 to 8 bytes

• The payload (actual content)
• Length is determined by DLC

CRC Field – 16 bits total
Used for error detection

CRC sequence – 15 bits

• CRC calculation result

CRC delimiter – 1 bit

• Must be recessive (1)

ACK Field – 2 bits total
Confirms that someone received the frame correctly

ACK slot – 1 bit

• Transmitter sends recessive (1)
• Any receiver that got the frame OK will overwrite it with dominant (0)

ACK delimiter – 1 bit

• Must be recessive (1)

EOF (End of Frame) – 7 bits

• Must be all recessive (1)
• Marks frame end and bus return to idle

CAN 2.0B Extended Frame

Arbitration Field (29-bit ID) – 32 bits total
This part is used for bus arbitration (priority) and message identification

11-bit Identifier (ID) – 11 bits

• The top 11 bits of the 29-bit identifier

SRR (Substitute Remote Request) – 1 bit

• Always recessive (1) in extended frames

IDE (Identifier Extension) – 1 bit

• IDE = 1 (recessive), this is an extended frame

18-bit Identifier (ID) – 18 bits

• Remaining 18 bits of the 29-bit identifier

RTR (Remote Transmission Request) – 1 bit

• 0 (dominant) = Data frame (normal frame carrying data)
• 1 (recessive) = Remote frame (requests data from another node)

Control Field – 6 bits total
Contains format and data length information

r1/r0 (reserved) – 1 bit

• Reserved bits must be dominant (0)

Reference

[1] CAN总线通讯协议