CAN-FD (CAN with Flexible Data-Rate) Protocol Tutorial

What is CAN-FD? CAN-FD (Controller Area Network with Flexible Data-Rate) is an enhanced high speed version of the traditional CAN protocol. Exploring the Benefits of CAN FD in Automotive vehicles.

As the name suggest CAN-FD has a flexible data rate and it is used to overcome the challenges faced by CAN protocol i.e. the speed limitation. With the advancement of automotive vehicles, the number of ECUs in a vehicle also increased and therefore there was a need for higher data transfer rates. Therefore, in close collaboration with automotive manufacturers and CAN specialists, Bosch GmbH initiated the development of CAN FD (Flexible Data-Rate) in 2011. It addresses two key limitations of traditional CAN i.e. achieving faster data transmission beyond 1 Mbit/s and extending the payload (data field) upto 64 bytes, removing the previous max 8-byte constraint in CAN.

CAN Frame Structure

The Standard CAN frame format is the basic structure used for data transmission in a CAN network. It defines how information is organized and transmitted between ECUs within a vehicle. The Standard CAN frame format consists of several key components:

1. Start of Frame (SOF): The SOF field signals the beginning of a CAN frame. It is a fixed 1-bit dominant (0) followed by 11 recessive bits (1). This unique bit pattern helps synchronize the receiving nodes.
2. Identifier (ID) Field: The ID field contains the message identifier, which serves to prioritize and identify different messages on the network. Standard CAN uses 11 bits for the ID, allowing for up to 2^11 (or 2048) possible unique identifiers (CAN-IDs).
3. Remote Transmission Request (RTR) Bit: The RTR bit, located within the ID field, indicates whether the message is a data frame (RTR = 0) or a remote frame (RTR = 1). Data frames carry actual payload data, while remote frames are used to request data from other nodes.
4. Control Field: The control field includes several bits, such as the IDE (Identifier Extension) bit, which indicates whether an 11-bit or 29-bit ID is used. Additionally, there is an SRR (Substitute Remote Request) bit in case a node supports both standard and extended frames.
5. Data Field (Payload): The data field carries the actual payload of the message. In a Standard CAN frame, this field can contain up to 8 bytes (64 bits) of data.
6. CRC (Cyclic Redundancy Check): The CRC field contains a 15-bit cyclic redundancy check code, allowing nodes to verify the integrity of the received message.
7. ACK (Acknowledgment): The ACK field is present in CAN and indicates the acknowledgment status of the frame. It includes two bits, namely:
   • ACK Slot: The ACK slot is used for the acknowledgment process.
     • ACK Delimiter: A delimiter that marks the end of the ACK slot.
8. The ACK field indicates whether a transmitting node has received an acknowledgment for its message.
9. End of Frame (EOF): The EOF field marks the end of the CAN frame. It consists of 7 recessive bits.
10. Interframe Space (IFS): The IFS is a period of time between consecutive frames to allow for proper separation.

CAN-FD Frame Structure

A CAN FD frame structure is an extended version of the traditional CAN (Controller Area Network) frame structure. The primary enhancements in CAN FD include the support for higher data rates and larger data payloads. Here’s an overview of the CAN FD frame structure:

1. Start of Frame (SOF): The SOF field marks the beginning of the frame. It consists of a dominant bit (0) followed by several recessive bits.
2. Arbitration Field: Similar to Standard CAN, the arbitration field contains the Identifier (ID) bits. In CAN FD, the ID can be either 11 bits or 29 bits.
3. Control Field: The control field includes several key bits:
   • DLC (Data Length Code): Specifies the number of bytes in the Data Field.
   • EDL (Extended Data Length): Indicates whether the frame is CAN FD.
   • BRS (Bit Rate Switch): Informs the receiver to switch to a higher data rate if supported.
4. Data Field: The Data Field carries the actual payload of the message. In CAN FD, this field can contain up to 64 bytes of data.
5. CRC (Cyclic Redundancy Check): The CRC field includes a 15-bit CRC sequence for error checking.
6. ACK Field: The ACK field is present in CAN FD and indicates the acknowledgment status of the frame. It includes two bits, namely:
   • End of Frame (EOF): Similar to Standard CAN, the EOF field signifies the end of the frame.
   • Interframe Space (IFS): The IFS is a period of time between consecutive frames.
7. ACK Slot: The ACK slot is used for the acknowledgment process.
8. ACK Delimiter: A delimiter that marks the end of the ACK slot.
9. End of Frame (EOF): The EOF field marks the end of the CAN frame. It consists of 7 recessive bits.
10. Interframe Space (IFS): The IFS is a period of time between consecutive frames to allow for proper separation.

Enhancements of CAN-FD over CAN

CAN-FD provides the following enhancements over the traditional CAN protocol:

Flexible Data Rate: As the name suggests, CAN-FD allows for flexible data rates, which means it can achieve higher data transfer speeds than the original CAN protocol. The data rate can go beyond 1 Mbps, reaching up to 8 Mbps or more, depending on the specific implementation and hardware used.

Increased Frame Length: With CAN a max of 8 bytes of data can be transferred. Now, CAN-FD allows for larger data frames, accommodating more data in a single transmission. CAN-FD can support payload lengths up to 64 bytes or more, depending on the implementation.

Backward Compatibility: Although CAN-FD introduces enhancements, it is designed to be backward compatible with the traditional CAN protocol (CAN 2.0). This means that CAN-FD controllers can communicate with CAN 2.0 controllers on the same bus, but at the original CAN data rates and frame lengths.

CAN-FD has found extensive use in modern automotive applications, where the demand for higher data transfer rates and more complex data exchanges between ECUs is on the rise. With CAN-FD, automotive systems can efficiently handle the increasing data volumes required for advanced driver assistance systems (ADAS), infotainment systems, and other safety-critical applications.

Additionally, CAN-FD is also adopted in industrial automation, aerospace, and other domains where the advantages of higher data rates and larger frame lengths are beneficial for communication between distributed electronic components.