CAN FD - The basic idea

Because of the bandwidth requirements of the automotive industry, the CAN data link layer protocol needed to be improved. In 2011, Bosch started the CAN FD (flexible data-rate) development in close cooperation with carmakers and other CAN experts. The improved protocol overcomes to CAN limits: You can transmit data faster than with 1 Mbit/s and the payload (data field) is now up to 64 byte long and limited to 8 byte anymore. In general, the idea is simple: When just one node is transmitting, the bit-rate can be increased, because no nodes need to be synchronized. Of course, before the transmission of the ACK slot bit, the nodes need to be re-synchronized.

CAN FD data frames can be transmitted with two different bit-rates: In the arbitration phase the bit-rate depends on the network topology and is limited to 1 Mbit/s; in the data phase the bit-rate is limited by the transceiver characteristics

Using a ratio of 1:8 for the bit-rates in the arbitration and data phase leads to an approximately six-times higher throughput considering that the CAN FD frames use more bits in the header (control field) and in the CRC field.

Faster and longer: Larger payloads improve the protocol efficiency and lead to a higher throughput

CAN FD – Some protocol details

In order to distinguish between Classical data frames and CAN FD data frames, one of the formerly reserved bits is used. This bit is called FDF (FD frame) bit. If it is of recessive value, the following bit sequence is interpreted as a CAN FD data frame. If it is of dominant value, it is a Classical data or remote frame. In the newly introduced BRS (bit rate switch) bit, the second bit-rate is applied, when it is of recessive (r) value. If it is of dominant (d) value, the arbitration phase bit-time setting is used in the data phase, too.

Structure of CAN FD data frames: The fields are the same as in Classical CAN data frames, but some extensions have been introduced (* stuff-bits are not considered; ** with fixed stuff-bits)

SOF = start-of-frame, CRC = cyclic redundancy check, ACK = acknowledgement, EOF = end-of-frame, IMF = intermission field

The CAN FD protocol controller has to also support Classical CAN frames. Both CAN protocols (Classical as well as CAN FD) are internationally standardized in ISO 11898-1:2015. CAN FD data frames with 11-bit identifiers use the FBFF (FD base frame format) and those with 29-bit identifiers use the FEFF (FD extended frame format). The CAN FD protocol doesn’t support remotely requested data frames.

Two CAN FD frame formats: The IDE bit is recessive in FEFF, the RRS bit is always dominant, and the value of the SRR bit doesn’t matter

RRS = remote request substitution, SRR = substitute remote request, IDE = identifier extension, FDF = flexible data rate format, d = dominant, r = recessive, r0 = reserved

The control field comprises additional bits not provided by the Classical CAN data frames. The FDF (FD format) bit indicates the usage of FD frame formats. At the sample-point of the BRS (bit-rate switch) bit, the bit-rate switch is performed. This guarantees a maximum of robustness. The following ESI (error state indicator) bit provides information about the error status: a dominant value indicates an error active state.

Extended control field: The DLC (data length code) values not used by the Classical CAN protocol are shown as used by the CAN FD protocol

IDE (identifier extension), FDF (flexible data rate format), BRS (bit rate switch; recessive, if alternate bit-rate), ESI (error state indicator; recessive, if error passive)

During the standardization process of the CAN FD protocol, some additional safe guards were introduced in order to improve the communication reliability. This is why the CRC field comprises 17-bit (for frames with payloads up to 16 byte) or 21-bit (for frames larger than 16 byte) polynomials and an 8-bit stuff-bit counter plus a parity bit. The CRC field use fixed-stuff bits (FSB) with an opposite value of the previous bit. All these safe guards guarantee that all single failures are detected under all conditions. Even the possibility to detect multiple failures has been improved.
Medium-term, non-ISO CAN FD controllers might also be on the market – these are not compliant to the ISO 11898-1 standard. They don’t implement the above-mentioned additional safe guard features.

Stuff-bit counter: The 3-bit stuff-bit counter is grey-coded and it is protected by a parity bit as well as by the following fixed stuff-bit (FSB)

CAN FD – Further readings

The CAN Newsletter magazine has published several articles about CAN FD, which can be downloaded as PDF files: