Understanding CAN Bus for Embedded Systems

Why Does CAN Bus Matter?

When designing embedded systems, especially in automotive, industrial automation, and robotics, a major challenge is ensuring reliable, efficient communication between multiple microcontrollers and sensors. Early systems relied on point-to-point wiring, leading to complex, heavy, and expensive wiring harnesses. As the number of electronic control units (ECUs) increased in automobiles and industrial machines, a better solution was needed.

CAN (Controller Area Network) Bus was developed to address this by providing:

• A robust, fault-tolerant communication protocol that works in noisy environments.

• Reduced wiring complexity by allowing multiple devices to communicate over a shared two-wire network.

• Collision-free communication using arbitration.

• Real-time capability with prioritized message handling.

Originally designed by Bosch in 1986 for automobiles, CAN is now widely used in industrial automation, medical equipment, aerospace, and robotics.

How CAN Bus Works

CAN is a multi-master, broadcast-based serial communication protocol that follows the OSI model’s Data Link Layer (Layer 2). It allows multiple nodes to communicate without a central master, making it scalable and fault-tolerant.

• Bus Topology: Uses differential signaling (CAN_H and CAN_L) over a twisted-pair cable.

• Message-Based Communication: Unlike UART, SPI, or I2C, which are address-based, CAN uses message identifiers instead of addresses, meaning any node can process a message based on its identifier.

• Arbitration Mechanism: Messages are prioritized using IDs. Lower ID values have higher priority and can interrupt lower-priority messages using a non-destructive bitwise arbitration.

• Error Handling & Detection: Includes CRC checks, acknowledgment bits, and error frames to maintain data integrity.

1. Basics of the CAN Protocol

2. CAN Bus Frame Structure

A CAN frame consists of the following fields:

• SOF (Start of Frame) (1 bit): Marks the beginning of a message

• Identifier (11 or 29 bits): Unique ID (lower ID = higher priority)

• Control (6 bits): DLC (Data Length Code), specifies data size (0-8 bytes)

• Data (0-64 bits): Actual payload (max 8 bytes in classic CAN, 64 in CAN FD)

• CRC (15 bits): Error-detecting cyclic redundancy check

• ACK (1 bit): Acknowledgment from receiving nodes

• EOF (End of Frame) (7 bits): Marks end of transmission

There are two main types of CAN:

1. Classical CAN: Limited to 8 bytes per frame.

2. CAN FD (Flexible Data-Rate): Supports up to 64 bytes per frame and higher data rates.

3. CAN Bus Arbitration (Collision Avoidance)

CAN bus handles multiple devices trying to communicate simultaneously using bitwise arbitration. Since the bus is wired-AND, a dominant (0) bit always wins over a recessive (1) bit.

Example:

• Node A sends ID: 101000

• Node B sends ID: 110100

• At the first bit where Node A sends a dominant (0) and Node B sends a recessive (1), Node B loses arbitration and stops transmitting.

• This ensures that the highest-priority message always gets through first, with no data corruption.

4. Physical Layer & Wiring

• CAN Transceiver (e.g., MCP2551, TJA1050): Converts digital signals to differential signals.

• Two-Wire Differential Signaling (CAN_H & CAN_L):

  • Logic 1 (Recessive) → CAN_H = CAN_L (~2.5V)

  • Logic 0 (Dominant) → CAN_H = 3.5V, CAN_L = 1.5V

• Termination Resistors (120Ω): Required at both ends of the bus to prevent signal reflections.

Real-World Examples of CAN Bus

1. Automotive

Modern cars have 50+ ECUs communicating over multiple CAN buses for:

• Engine control (ECU)

• Anti-lock braking system (ABS)

• Airbags, power steering, infotainment

• Diagnostic communication (OBD-II over CAN)

🔹 Fact: Since 2008, all cars in the U.S. must use CAN for OBD-II diagnostics.

2. Industrial Automation

• PLC-to-PLC communication

• Robotic arms in factories

• Elevator control systems

🔹 Case Study: In manufacturing plants, CAN networks reduce downtime by allowing real-time monitoring of motors, sensors, and conveyors.

3. Aerospace & Medical

• Boeing 787 Dreamliner uses CAN for avionics.

• Medical equipment (MRI, ventilators, infusion pumps) relies on CAN for safe and precise operation.

🔹 Case Study: NASA’s Mars Rovers use CAN bus for subsystem communication due to its reliability in harsh environments.

Interesting Facts & Takeaways

1. F1 Race Cars Use CAN: Formula 1 cars use high-speed CAN for real-time telemetry to monitor engine, tire pressure, and aerodynamics.

2. CAN in Submarines: Military submarines use CAN networks for pressure monitoring, ballast control, and sonar systems.

3. CAN Hacking Risks: Since all messages are broadcasted, hackers can exploit CAN (e.g., Jeep Cherokee hack in 2015) to control vehicles remotely.

Conclusion

CAN Bus is a critical backbone in embedded systems, enabling reliable, real-time communication with minimal wiring. Its fault tolerance, arbitration mechanism, and broad adoption make it ideal for safety-critical applications.

If you're developing embedded systems, learning CAN bus will open up opportunities in automotive, robotics, aerospace, and industrial automation.