1. Main reason: With the increasing number and complexity of automotive functions, the load rate of traditional CAN bus (CAN2.0) is getting higher and even up to 95%.

2. Limitations of traditional CAN:

The maximum transmission rate is 1Mbit/s (typically ≤ 500Kbit/s), and the actual usage rate in the automotive field is up to 500Kbit/s, which cannot meet the increasing data throughput.

The additional overhead of traditional CAN data frames exceeding 50% (overhead>50%)

The effective data field of each frame message is 8 bytes, accounting for less than 50% of the entire frame message information

It is difficult to cope with the threats of new vehicle buses such as FlexRay and Ethernet in terms of performance.

3. Main features of CAN FD:

CAN FD leverages the advantages of CAN and compensates for its shortcomings.

Adopting the same event triggering mode as CAN communication, the software is easy to develop and port.

The maximum data transmission rate reaches 5Mbit/s, better meeting the requirements of high real-time and high data transmission rate applications

The effective data field of each frame message is 64 bytes, accounting for more than 70% of the entire frame message information

Compared to emerging buses such as FlexRay and Ethernet, the cost is lower.

4. Historical factors: It is impossible for automobiles to directly abandon the current traditional CAN bus technology. In order to solve the problem of CAN bus load rate while being compatible with CAN bus and not having too high research and development costs, the latest revised version of ISO 11898-1 (classical CAN and CAN FD) was released in 2015.

3、 Comparison between CAN FD and CAN bus

(1) The main differences between CAN and CAN FD are: different transmission rates, different data field lengths, different frame formats, and different ID lengths.

1. Variable rate: CAN FD adopts two bit rates, from the BRS bit in the control field to the ACK field (including CRC delimiter), which is the variable rate, and the rest is the rate used by the original CAN bus.

2. Increase in data field length: CAN FD has greatly expanded the length of the data field, with DLC supporting a maximum of 64 bytes. When DLC is less than or equal to 8 bytes, it remains the same as the original CAN bus. Due to non-linear growth when DLC is greater than 8 bytes, the maximum data field length can reach 64.

The maximum length of CAN standard frame ID is 11 bits, and the length of CAN FD standard frame ID can be extended to 12 bits.

10. DATE Field Data Field

CAN FD has improved in terms of security. To avoid the impact of bit padding on CRC, CAN FD has added a stuff count in the CRC field to record the number of padding bits corresponding to the modulus of 8, represented by Grey Code, and also added parity check bits. FSB (fixed stuff bit) is fixed as the complement of the previous digit.

②CRC

With the expansion of the data field, in order to ensure the quality of information transmission. The CRC calculation of CAN FD should not only include the bits of the data segment, but also the Stuff Count and padding bits from SOF. By comparing the calculation results of CRC, it can be determined whether the receiving node can receive normally.

In CAN, the number of CRC bits is 15, while in CAN FD, the CRC field is extended to 21 bits.

When transmitting data of 16 bytes or less: CRC 17 bits

When transmitting data exceeding 16 bytes: CRC 21 bits

③ CRC delimiter

The CRC delimiter indicates the end of the CRC check field and is a 1-bit normal recessive bit. However, in CAN FD, considering the distance between nodes, a maximum of 2 bits of time are allowed at the receiving end. The data field (variable speed segment) of the CAN FD frame is the first sampling point of the CRC delimiter.

12. ACK response field

The ACK response field of CAN FD includes response gaps and response delimiters, which have the same composition as CAN. Differently, the length of the CAN response field is 1 bit, and the CAN FD receiving node recognizes it as a valid response using 2 bits of time.