Controller Area Network (CAN) Basics

Introduction

The Controller Area Network (CAN) is a robust serial communication protocol widely used in automotive, industrial, and embedded applications. Developed by Bosch in the 1980s, CAN allows microcontrollers and devices to communicate without a central host, making it ideal for real-time, high-reliability systems.

This article explores the fundamentals of CAN, including its architecture, message format, error handling, and practical applications.

1. CAN Architecture

Overview

CAN follows a multi-master, message-based communication model where multiple nodes can transmit and receive data over a shared bus.

Key Components:

1. CAN Nodes: Devices or microcontrollers connected to the CAN bus.
2. CAN Bus: A two-wire differential signaling bus (CAN_H, CAN_L) that reduces noise interference.
3. CAN Transceiver: Converts logic-level signals to differential CAN signals and vice versa.
4. CAN Controller: Manages message arbitration, error detection, and data transmission.

CAN Bus Communication Model:

• Uses a differential signal (CAN_H and CAN_L) to improve noise immunity.
• Follows the Carrier Sense Multiple Access/Collision Resolution (CSMA/CR) protocol for message arbitration.

2. CAN Message Format

Standard and Extended CAN Frames

CAN supports two frame formats:

• Standard CAN (11-bit identifier): 2¹¹ = 2048 unique message identifiers.
• Extended CAN (29-bit identifier): 2²⁹ = 536 million unique message identifiers.

Frame Structure:

1. Start of Frame (SOF): Indicates the beginning of a frame.
2. Identifier (ID): Unique message ID used for arbitration.
3. Control Field: Contains data length (DLC – Data Length Code).
4. Data Field: Contains 0 to 8 bytes of payload data.
5. CRC Field: Ensures data integrity.
6. ACK Field: Acknowledgment from receiving nodes.
7. End of Frame (EOF): Marks the frame’s end.

Message Prioritization:

• Lower identifier values indicate higher priority messages.
• The arbitration process ensures that only one node transmits at a time.

3. CAN Communication Mechanism

Arbitration Process

• CAN uses bitwise arbitration to resolve conflicts.
• A node with a dominant bit (0) wins arbitration over a recessive bit (1).
• The lowest identifier number (highest priority) gets bus access.

Error Handling in CAN

CAN has built-in error detection and correction mechanisms:

1. Bit Errors: Detects incorrect bit values during transmission.
2. Stuff Errors: Ensures proper frame synchronization by inserting stuffing bits.
3. CRC Errors: Validates data integrity using cyclic redundancy check (CRC).
4. Acknowledgment Errors: Ensures proper frame reception.
5. Form Errors: Detects incorrect format in fixed fields.

If an error is detected, the erroneous node sends an Error Frame and retransmits the message.

4. CAN Bus Implementation

Hardware Requirements

• Microcontroller with CAN Controller: STM32, ESP32, ATmega32M1, etc.
• CAN Transceiver: MCP2551, SN65HVD230, TJA1050.
• Termination Resistors (120Ω): Placed at both ends of the CAN bus to prevent signal reflection.

CAN Communication Example (C – STM32 HAL)

CAN_HandleTypeDef hcan;
CAN_TxHeaderTypeDef TxHeader;
CAN_RxHeaderTypeDef RxHeader;
uint8_t TxData[8];
uint8_t RxData[8];
uint32_t TxMailbox;

void CAN_Init() {
    hcan.Instance = CAN1;
    hcan.Init.Prescaler = 16;
    hcan.Init.Mode = CAN_MODE_NORMAL;
    hcan.Init.SyncJumpWidth = CAN_SJW_1TQ;
    hcan.Init.TimeSeg1 = CAN_BS1_8TQ;
    hcan.Init.TimeSeg2 = CAN_BS2_1TQ;
    HAL_CAN_Init(&hcan);
}

void CAN_SendMessage() {
    TxHeader.StdId = 0x123;
    TxHeader.DLC = 2;
    TxData[0] = 0xAB;
    TxData[1] = 0xCD;
    HAL_CAN_AddTxMessage(&hcan, &TxHeader, TxData, &TxMailbox);
}

void CAN_ReceiveMessage() {
    if (HAL_CAN_GetRxFifoFillLevel(&hcan, CAN_RX_FIFO0) > 0) {
        HAL_CAN_GetRxMessage(&hcan, CAN_RX_FIFO0, &RxHeader, RxData);
    }
}

5. CAN Variants and Extensions

CAN FD (Flexible Data Rate)

• Increases data payload up to 64 bytes.
• Supports variable bit rates for faster communication.

CANopen & J1939 Protocols

• CANopen: Industrial automation applications.
• J1939: Heavy-duty vehicle and automotive applications.

6. CAN Applications

1. Automotive Systems: Engine control units (ECUs), airbag systems, ABS.
2. Industrial Automation: Factory automation, motor control, robotic systems.
3. Medical Devices: Patient monitoring systems, diagnostic equipment.
4. Aerospace & Defense: Avionics communication, unmanned systems.

Conclusion

CAN is a powerful, reliable, and efficient communication protocol widely used in automotive and industrial applications. Its ability to handle multiple nodes, error detection mechanisms, and robust performance make it a preferred choice for real-time embedded systems.