Three minutes to learn about the latest CAN FD protocol

For China Manufacturing 2025 and the development direction of the automotive industry, new energy and intelligence have been the two themes discussed. In the process of intelligent car, the CAN FD protocol has received extensive attention due to its superior performance. This article will discuss the CAN FD protocol with everyone.

In today's society, cars have become an indispensable part of our lives. People hope that cars are not only a means of transportation, but also an extension of the scope of life and work. In the car, just like staying in your own office and home, you can call, surf, entertain, and work.

Therefore, in order to improve the competitiveness of products, auto manufacturers will integrate more and more functions into automobiles. The ECU (Electronic Control Unit) has increased in a large amount to increase the bus load rate sharply, and the conventional CAN bus has become increasingly incapable. Therefore, the CAN FD (CAN with Flexible Data-Rate) protocol was born.

It inherits the main features of the CAN bus, improves the network communication bandwidth of the CAN bus, improves the false frame miss detection rate, and maintains most of the hardware and software of the network system, especially the physical layer. This similarity allows ECU suppliers to upgrade their car communication networks without major modifications to the software portion of the ECU.

First, the improvement made by CAN FD

CAN FD uses two methods to improve the efficiency of communication: one way is to shorten the bit time and increase the bit rate; the other way is to lengthen the data field length and reduce the number of messages to reduce the bus load rate. Three polynomials are used in the CRC check segment to ensure data reliability under high speed communication.

(1) CAN with Flexible Data-Rate

From the BRS bit in the control field to the ACK field (including the CRC delimiter) is the variable rate, and the rest is the rate used by the original CAN bus. Each of the two rates has a set of bit time definition registers. In addition to using different bit time units TQ, the allocation ratios of the bit time segments can also be different.

(2) New data field length

CAN FD greatly expands the length of the data field. The DLC supports a maximum of 64 bytes. When the DLC is less than or equal to 8, it is the same as the original CAN bus. When it is greater than 8, there is a nonlinear growth, the largest data. The field length can be up to 64 bytes. The nonlinear relationship between the DLC value and the number of bytes is shown below.

(3) CRC check field

In the process of standardizing the CANFD protocol, the reliability of communication has also been improved. Due to the different lengths of DLCs, CAN FD has selected two new BCH-type CRC polynomials when the DLC is greater than 8 bytes.

Second, CAN FD frame structure analysis

(1) CAN FD data frame at frame start (SOF) and arbitration segment (ArbitraTIon Field)

Compared with the traditional CAN, the CAN FD cancels the support of the remote frame, and replaces the RTR bit with the RRS bit, which is often dominant. The IDE bit is still the standard frame and extended frame flag. If the standard frame and the extended frame have the same first 11-bit ID, the standard frame will have the priority of the bus because the IDE bit is 0.

RRS (remote request subsTItuTIon) remote request replacement bit: the RTR bit in the traditional CAN;

SRR (subsTItute remote request) replaces the remote frame request bit: the SRR bit is reserved and has no effect;

IDE (Identifier Extension) Extended frame flag: dominant (0) = 11-bit ID, recessive (1) = 29-bit ID.

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Guangzhou Ysure Electronic Technology Co., Ltd. ,