CAN FD (flexible data rate) - The basic idea

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

Because of the bandwidth requirements of the automotive industry, the CAN data link layer protocol needed to be improved. In 2011, Robert Bosch GmbH started the CAN FD (flexible data rate) development in close cooperation with carmakers and other CAN experts. The improved protocol overcomes two CAN CC (classic) limits: You can transmit data faster than with 1 Mbit/s and the payload (data field) is now up to 64 byte long and not 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.

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

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.

CAN FD – Some protocol details

In order to distinguish between CAN CC data frames and CAN FD data frames, one of the formerly reserved bits is used. This bit is called FD frame (FDF) 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 CAN CC data or remote frame. In the newly introduced bit rate switch (BRS) 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 CAN CC data frames, but some extensions have been introduced (* stuff-bits are not considered; ** with fixed stuff-bits)
Field Description
SOF start of frame
CRC cyclic redundancy check
ACK acknowledgement
EOF end of frame
IMF intermission field

The CAN FD protocol controller also supports CAN CC frames. CAN CC as well as CAN FD are internationally standardized together in ISO 11898-1:2024. CAN FD data frames with 11-bit identifiers use the FD base frame format (FBFF) and those with 29-bit identifiers use the FD extended frame format (FEFF). The CAN FD protocol does not 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 is ignored
Field Description  
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 CAN CC data frames. The FDF bit indicates the usage of FD frame formats. At the sample-point of the BRS bit, the bit rate switch is performed. This guarantees a maximum of robustness. The following error state indicator (ESI) bit provides information about the error status: a dominant value indicates an error active state.

Extended control field: The DLC values are not used by CAN CC but used as shown by CAN FD
Field Description
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
DLC data length code
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)

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 with the ISO 11898-1. They don’t implement the above-mentioned additional safe guard features.

Title Details
Published
Size
Status
Action
CiA 601-1 version 2.0.0 CAN FD node and system design - Part 1: Physical interface implementation
Scope

This set of documents specifies and recommends the usage of CAN FD hardware implementations. It consists of the following parts: Part 1: Physical interface implementation, Part 2: Controller interface specification, Part 3: System design recommendation, Part 4: Ringing suppression. This part provides specifications and guidelines on the physical layer interface design for automotive electronic control units and non-automotive devices supporting the CAN FD data link layer as defined in ISO 11898-1 and the CAN high-speed physical layer as defined in the harmonized ISO 11898-2 standard, which includes the specification of high-speed transceivers with low-power mode (formerly known as ISO 11898-5) and/or with selective wake-up functionality (formerly known as ISO 11898-6).

 2021-03-08 818 KB TR Login
CiA 601-2 version 1.0.0 CAN FD node and system design - Part 2: Protocol controller interface
Scope

This set of documents specifies and recommends the usage of CAN FD hardware implementations. It consists of the following parts: Part 1: Physical interface implementation, Part 2: Controller interface specification, Part 3: System design recommendation, Part 4: Ringing suppression. This part describes the specified interface between the protocol controller implementing the CAN FD data link layer protocol and the host controller including necessary configuration registers. Such a harmonized interface reduces the effort for adapting the low-level driver software to different implementations.

 2019-09-06 479 KB DS Login
CiA 601-6 version 1.0.0 CAN FD node and system design - Part 6: Cable
Scope

This set of documents specifies and recommends the usage of CAN FD hardware implementations. It consists of the following parts: Part 1: Physical interface implementation, Part 2: Controller interface specification, Part 3: System design recommendation, Part 4: Signal improvement, Part 6: CAN FD cable. This part specifies the requirements and test methods of the mechanical and electrical parameters for cables to be used in CAN FD networks.

 2019-09-06 227 KB DS Login
CiA 601-4 version 2.1.0 CAN FD node and system design - Part 4: Signal improvement
Scope

This document specifies additional requirements for CAN transceivers compliant with ISO 11898-2:2016 for the purpose to reduce differential and common-mode ringing on the CAN_H and CAN_L wires, namely the CAN SIC transceiver. Furthermore, this document specifies EMC tests for the CAN SIC transceiver.

2023-09-01 77 KB withdrawn Login
CiA 601-1 version 2.0.1 CAN FD node and system design, Part 1: Physical interface implementation
Scope

This part provides specifications and guidelines on the physical layer interface design for automotive electronic control units and non-automotive devices supporting the CAN FD data link layer standardized in ISO 11898-1:2024 and the CAN FD physical layer standardized in ISO 11898-2:2024.

 2024-07-18 855 KB WD Login
CiA 601-3 version 1.0.4 CAN FD node and system design Part 3: System design recommendations
Scope

 2024-08-30 937 KB WD Login
CiA 601-3 version 1.0.0 CAN FD node and system design - Part 3: System design recommendations, CAN FD bit-timing configuration and evaluation tool, version 1.0 as an attachment to the CiA 601-3 v. 1.0.0 TR682 KiB
Scope

This set of documents recommends the usage of CAN FD hardware implementations. It consist of the following parts: Part 1: Physical interface implementation, Part 2: CAN controller interface specification, Part 3: System design recommendation, Part 4: Signal improvement. This part provides recommendations for CAN FD network design, especially, bit-timing setting rules and guidelines are given. This part also includes the spreadsheet that helps to perform the CAN clock tolerance calculation, the bit-timing evaluation and optimization, the bit asymmetries calculation, and the evaluation of the robustness.

 2021-03-08 682 KB TR Login
CiA 602-1 version 0.0.6 CAN FD for commercial vehicles – Part 1: Physical layer
Scope

 2015-10-30 264 KB WD Login
CiA 603 version 1.1.0 CAN network time management
Scope

This document specifies the time-stamping, when transmitting and receiving CAN data frames in classical CAN and CAN FD formats. This time synchronization can be used in particular in AUTOSAR environments. Therefore, a link to the AUTOSAR time synchronization is described.

 2023-09-01 458 KB DS Login