What is CAN FD?
CAN FD (Controller Area Network with Flexible Data-rate) is an extension of the Classical CAN protocol that enables higher data rates and larger payload sizes. Developed by Bosch and standardized in ISO 11898-1:2015, CAN FD addresses the growing bandwidth demands of modern automotive electronics while maintaining backward compatibility with existing CAN infrastructure.
CAN FD Key Improvements Over Classical CAN
| Feature | Classical CAN | CAN FD |
| Data Rate (Arbitration) | Up to 1 Mbps | Up to 1 Mbps |
| Data Rate (Data Phase) | Up to 1 Mbps | Up to 8 Mbps |
| Payload Size | 8 bytes max | 64 bytes max |
| CRC | 15-bit | 17-bit or 21-bit |
CAN FD Protocol Details
Frame Format
CAN FD introduces two new control bits in the frame format:
- FDF (FD Format): Distinguishes CAN FD from Classical CAN frames
- BRS (Bit Rate Switch): Enables switching to higher data rate
- ESI (Error State Indicator): Indicates transmitter error state
Bit Rate Switching
CAN FD uses two bit rates within a single frame:
- Arbitration Phase: Standard CAN bit rate (up to 1 Mbps) for arbitration
- Data Phase: Higher bit rate (up to 8 Mbps) for data transmission
CAN FD Frame Structure:
┌─────────────────────────────────────────────────────────────────┐
│ SOF │ ID │ RTR │ IDE │ FDF │ BRS │ ESI │ DLC │ DATA │ CRC │ ACK │
└─────────────────────────────────────────────────────────────────┘
├────── Arbitration Rate ──────┤├── Data Rate ──┤├─ Arb ─┤
Data Length Code (DLC)
CAN FD extends DLC encoding for larger payloads:
| DLC Value | Classical CAN Bytes | CAN FD Bytes |
|---|---|---|
| 0-8 | 0-8 | 0-8 |
| 9 | 8 | 12 |
| 10 | 8 | 16 |
| 11 | 8 | 20 |
| 12 | 8 | 24 |
| 13 | 8 | 32 |
| 14 | 8 | 48 |
| 15 | 8 | 64 |
Hardware Implementation Considerations
Bit Timing Configuration
CAN FD requires separate bit timing configurations for arbitration and data phases:
- Nominal Bit Time: Used during arbitration phase
- Data Bit Time: Used during data phase (shorter)
- Transmitter Delay Compensation (TDC): Required for high data rates
Transceiver Requirements
High-speed CAN FD operation requires compatible transceivers:
- Support for data rates up to 5 Mbps or 8 Mbps
- Fast edge rates with controlled slew rate
- Short loop delay for TDC
- Examples: TJA1044, MCP2558FD, TCAN1044
CRC Calculation
CAN FD uses different CRC polynomials based on payload size:
- CRC17: For payloads ≤16 bytes (polynomial: x17 + x16 + x14 + ...)
- CRC21: For payloads >16 bytes (polynomial: x21 + x20 + x13 + ...)
Automotive Applications
Why CAN FD for Modern Vehicles?
- ADAS (Advanced Driver Assistance Systems): Sensor fusion requiring high bandwidth
- ECU Reprogramming: Faster flash updates over CAN
- Powertrain Control: More data per message, fewer frames
- Gateway Applications: Routing between different networks
Network Architecture
Modern vehicles often combine multiple protocols:
- CAN FD: Powertrain, chassis, body control
- Automotive Ethernet: ADAS, infotainment backbone
- LIN: Low-speed sensors and actuators
- FlexRay: Safety-critical applications (legacy)
Migration from Classical CAN to CAN FD
Backward Compatibility
CAN FD is designed for coexistence with Classical CAN:
- CAN FD nodes can transmit Classical CAN frames
- Classical CAN nodes will see CAN FD frames as errors
- Mixed networks require gateway or protocol conversion
Migration Strategy
- Phase 1: Replace critical ECUs with CAN FD-capable nodes
- Phase 2: Use CAN FD for new messages, Classical CAN for legacy
- Phase 3: Gradually convert all messages to CAN FD format
CAN FD Controller IP Core Features
A comprehensive CAN FD controller IP should include:
- Full ISO 11898-1:2015 compliance
- Support for both Classical CAN and CAN FD frames
- Configurable message RAM and filtering
- Transmitter Delay Compensation
- Error management and bus-off recovery
- Time-triggered operation support
- APB/AXI host interface options
Conclusion
CAN FD represents a significant evolution for automotive networking, providing the bandwidth and payload capacity needed for next-generation vehicles while preserving investment in existing CAN infrastructure. Its adoption continues to grow across the automotive industry and is expanding into industrial automation applications.
Vcores offers ISO 11898-compliant CAN FD controller IP cores supporting data rates up to 8 Mbps, with configurable message objects, hardware filtering, and comprehensive error handling. Contact us for evaluation or integration support.
Technical References
- ISO 11898-1:2015 - Road vehicles - Controller area network (CAN)
- CAN FD Specification Version 1.0 - Robert Bosch GmbH
- SAE J2284 - High Speed CAN for Vehicle Applications
