CAN FD Explained Simple (2022)
Updated: Mar 29, 2022
By the time the automobile industry entered into advancement, CAN bus had started to rule the industry with the wide range of functionality it could offer. It had so many good features at a very reasonable cost that it soon became the core communication protocol to be used. But, with time, technological developments outgrew CAN's ability to function. The growing number of technologically superior electronics modules that called for continuously rising data required more flexible support. The then so extensive list of features started falling short, and this demanded a solution. Also, with this explosion in data and bandwidth needs, just increasing more Can wires was not an option. So, as a solution, in 2011-2012, Bosch developed and released an extension to the original CAN-Bus protocol known as CAN Flexible Data rate (specified in ISO 11898-1:2015).
CAN FD is practically a more capable CAN. Flexible signal transmission provided automotive electronics' communication with increased bandwidth and all the required functionality very cost-effectively. Moreover, it offers upgradation to almost every feature CAN hold and is a better alternative for more advanced needs of data and bandwidth requirements.
The classical CAN 2.0 bus has many features that make it an ideal choice for application where the number of ECU's is more, and the bandwidth utilization is less. Classical CAN bus supports a maximum message payload of 8 bytes per frame at a maximum data rate of 1Mbps. Also, standard/extended CAN 2.0A allows 11Bit/29Bit data transmission, respectively. Whereas CAN FD supports a flexible message payload, ranging from 0, 8, 12, 16, 20, 24, 32, 48, 64 bytes per frame at 2, 5 and 8 Mbps of data rates. Looking at the Frame Formats for classical CAN Bus and CAN FD, they may not seem very different. But there are a few added fields in a CAN FD frame format that are not present in classical CAN bus.
RRS: Remote Request Substitution (always a dominant 0). The remote frames are not at all supported in CAN FD. (In classical CAN, there is RTR (Remote Transmission Request) for identifying the data frames and remote frames) FDF: Flexible Data Rate Format (always a recessive 1) used to indicate Flexible data frame format usage. EDL: Extended Data Length (always a recessive 1) for managing larger payloads and faster bit-rates in CAN FD. BRS: Bit Rate Switch helps determine the bit rate of a data frame.
Dominant 0 signifies that the arbitration rate for the CAN FD data frame is up to 1Mbit/sec.
Recessive 1 signifies a higher/faster arbitration rate for the CAN FD data frame ranging up to 5Mbit/sec.
ESI: Error State Indicator A dominant 0 indicates the error-active mode. A recessive 1 indicates the error-passive mode. DLC: Data Length Code is a 4-bit code in CAN FD which denote the number of data bytes in the frame. (DLC values ranging from 1001 to 1111 are used to specify the data lengths of 12, 16, 20, 24, 32, 48, and 64 bytes). CRC: The Cyclic Redundancy Check is 17 bits long for up to 16 bytes of data or 21 bits for 20-64 bytes. Its length depends upon the length of EDL and DLC bits. CAN FD always uses 4 fixed stuff bits that improve communication reliability.
For an explanation on other fields, please refer to articles https://www.influxbigdata.in/can-error-basics https://www.influxbigdata.in/can-bus-basics The addition of such fields to CAN FD enables it to perform accordingly with the automotive industry's needs. It finds application in automotive communication where more data needs to be communicated in less time with an increased speed. One can use the same bandwidth to accumulate more extensive data. Also, just like the classical CAN bus, CAN FD is also loaded with many advantages. Some being:
Up to 30 times more efficient and faster communication between multiple ECUs.
Decreased number of undetected errors with advanced CRC.
Flexibility to switch between faster and slower data rates.
Allows more data to fit into a single message.
Improved network bandwidth.
Increased Protocol efficiency.
Reduced protocol overhead.
CAN FD had found existence a few years back and is still being adopted by the automotive industries. The significant advantage that CAN FD has over classical CAN bus is the backward compatibility that enables it to be used with the circuits using the classical CAN bus. But, if we consider the rate at which the technology is advancing for automobiles, it seems pretty evident that CAN FD will prove to be a better alternative to classical CAN. The major advantages of CAN FD being capable of transmitting flexible data at a higher speed using lesser bandwidth make it stand apart and strong. At Influx, we bring you ReXgen and Rebel CT4 CAN FD, equipped with 2 CAN FD buses. Which enable the user to send more signals at a flexible data rate. For more information on our CAN FD products, please Click here.