CAN FD: From theory to practice

The international standardization of CAN Flexible-Data Rate (FD) is settled. The next step is the development of recommendations and specifications, or how to design CAN FD networks.


Figure 1: The CAN data link and physical sub-layers are standardized in the ISO 11898 series with the AUI as interface (TxD and RxD) between a CAN controller and transceiver and the MDI as interface between transceiver and bus-wires. Courtesy: CiAThe CAN FD (CAN Flexible-Data Rate) data link layer submitted to the international organization for standardization (ISO) has passed the Draft International Standard (DIS) balloting without negative votes. This means, after implementing the observed comments, the ISO 11898-1 standard will be published. More than 100 comments, mainly of editorial nature, are already observed and implemented. Now, it is just a matter of time, for the ISO 11898-1 document to be published as the International Standard.

This standard also specifies a part of the physical layer, the physical coding sub-layer, according to the open system interconnection (OSI) reference model. The CAN FD physical media attachment (PMA) sub-layer describing the transceiver characteristics is internationally standardized in ISO 11898-2. This document has been submitted for DIS balloting. It also comprises the optional low-power mode (formerly in ISO 11898-5) and the optional selective wake-up functionality (formerly in ISO 11898-6). System-related specifications have been deleted. The new ISO 11898-2 standard specifies just the transceiver characteristics. The physical media dependent sub-layer is not in the scope of ISO 11898 series. It is highly application-specific and might be specified by other ISO standards or other associations (such as CiA, IEC, or SAE).

ISO also standardizes the conformance test plans for ISO 11898-1 and ISO 11898-2 implementations. The related standards, ISO 16845-1 and ISO 16845-2 are under development. ISO 16845-1 has been submitted for DIS balloting, and ISO 16845-2 is in committee draft (CD) voting. The test houses C & S Group and IHR are already developing conformance test prototypes. Most of the automotive chipmakers are in the process to integrate CAN FD cores into their micro-controllers. Many of them have licensed the IP module from Bosch. There are also several vendors providing a self-implemented CAN FD ASIC/FPGA. Engineering samples of CAN high-speed transceivers qualified for 2 Mbit/s and 5 Mbit/s are already available from several companies.

Next step: Using the longer data frames

The standardization of the CAN FD data link layer and physical layer is settled. Even the first higher layer protocols (transport layer and application layer) make use of the longer CAN FD data frames. This includes the so-called ISO transport layer as standardized in ISO 17765-2. The first implementations by Vector and Volkswagen were tested last October during a CAN FD plugfest organized by CAN in Automation (CiA). Also the XCP calibration protocol version 1.2 specified by the nonprofit ASAM association makes use of the 64-byte data fields. The automotive open system architecture (Autosar) version 4.2.1 also supports CAN FD.

Figure 2: The proposed protocol structure for commercial vehicles complies with Autosar (Source: CiA 602-2). Courtesy: CiAThe CiA CANopen special interest group (SIG) application layer currently develops the CiA 301 version 5.0, which will be based on the CAN FD data link layer. Most of the CANopen protocols will remain as they are for more than 20 years. However, the public domain operating system (PDOS) will be prolonged to 64 bytes. The number of mapping entries will be still 64. This means it is not possible to map more than 8 byte bit-wise. Byte-wise mapping is preferred. The bigger change regards the SDO protocol. It will be completely changed: The Universal SDO protocol will be structured better and will be easier to implement. It is still under development.

The SAE is discussing, in its J1939 CAN FD task force, how to make use of the longer frames. In parallel, the CiA IG (interest group) commercial vehicles prepare the CiA 602-2 application layer proposing a mapping of J1939-71 parameter groups into CAN FD data frames. It provides an optional safety/security field. The CAN identifier contains the source address as specified in SAE J1939-21. It is intended to reuse the 8-byte parameter groups as specified in J1939-71. In the future, longer or shorter parameter groups could be specified and mapped into frames as specified in CiA 602-2.

Learn more about designing a physical CAN FD network and configuring bit-timing

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