Introduction to CAN FD frame structure

CAN FD frame structure

CAN FD nodes can receive and send CAN messages normally, but CAN nodes cannot receive and send CAN FD messages correctly because their frame formats are inconsistent.

Like CAN, CAN FD consists of 7 parts: frame start, arbitration segment, control segment, data segment, CRC segment, ACK segment and frame end.

2.1. Frame start

CAN and CANFD use the same SOF flag to mark the start of a message. The frame start consists of 1 dominant bit, marking the beginning of the message and playing a synchronization role on the bus.

2.2 Arbitration Section

Unlike CAN, CAN FD cancels the support for remote frames and replaces the RTR bit with the RRS bit, which is always dominant. IDE is used to distinguish between standard frames and extended frames.

The arbitration segment of a standard frame consists of an 11-bit ID, r1 bit (dominant), and IDE (dominant), for a total of 13 bits. The arbitration segment of an extended frame consists of a 29-bit ID, SRR (recessive), IDE (recessive), and r1 bit (dominant), for a total of 32 bits.

SRR: replaces the RTR bit in the CAN standard frame;

IDE: extended frame flag;

r1: reserved bit, dominant.

2.3 Control Segment

CAN FD has the same IDE, res and DLC bits as CAN, and adds three more bits: FDF, BRS and ESI.

FDF bit (Flexible Data Rate Format): A reserved bit in the original CAN data frame. Indicates whether it is a CAN message or a CAN-FD message. The FDF
bit is usually recessive (1), indicating a CAN FD message.

BRS bit (Bit Rate Switch): indicates bit rate switching. When BRS is dominant (0), the bit rate of the data segment is consistent with the bit rate of the arbitration segment (constant rate). When BRS
is recessive (1), the rate is variable (i.e., BSR to CRC is transmitted at the switching rate).

ESI bit (Error State Indicator): Sends node error status indication. It sends a dominant bit (0) when an active error occurs and a recessive bit (1) when a passive error occurs.

2.4 Data Segment

CAN FD is compatible with CAN data formats and supports a maximum of: 12, 16, 20, 24, 32, 48 and 64 bytes.

Like in Classical CAN, the CAN FD DLC is 4 bits and indicates the number of data bytes in the frame. To maintain a 4-bit DLC, CAN
FD uses the remaining 7 values ​​from 9 to 15 to indicate the number of data bytes used (12, 16, 20, 24, 32, 48, 64).

2.5 CRC segment

The cyclic redundancy check (CRC) in traditional CAN is 15 bits, while in CAN
FD it consists of fixed stuffing bits FSB (6/7 bits), stuffing bit count (4 bits), CRC (17/21 bits) and CRC delimiter (1 bit), for a total of 28 or 33 bits. In traditional CAN, the CRC can contain 0 to 3 stuffing bits, while in CAN
FD, there are always 4 fixed stuffing bits to improve communication reliability.

Fixed fill bit (FSB): Every 4 bits in the CRC segment are filled with a bit that is opposite to the upper bit.

When using CRC17, FSB is 6 bits;

When using CRC21, FSB is 7 bits.

Stuffing bit count: It consists of the stuffing bit count (3 bits) and the parity bit (1 bit).

CRC:

When the message length is less than 16, CRC17 is used, which consists of 17 bits;

When the message length is greater than 16, CRC21 is used, which consists of 21 bits.

CRC delimiter: fixed as recessive bit; after sampling from this bit, switch to arbitration domain baud rate.

2.6 ACK segment

ACK is followed by the CRC end marker bit. The difference is that CAN FD supports the recognition of 2 bits of ACK, which consists of the ACK bit and the ACK delimiter bit.

ACK: Receiving node acknowledgement bit, the receiving node should acknowledge the dominant bit;

ACK delimiter, fixed as implicit.

2.7. Frame End

Like CAN, the end of the CAN FD frame is also 7 consecutive recessive bits.