CAN FD protocol revision in detail

January 12, 2015 // By Julien Happich
Going beyond the 1 MBit/s bandwidth limit of traditional CAN, “CAN with Flexible Data-Rate”or CAN FD features a new frame format that supports the switch to faster bit rates or to a different data length coding, the payload per data frame increasing to 64 byte instead of 8 byte provided in Classical CAN.

While CAN FD has passed the first level of ISO balloting in summer 2014, some weaknesses in the data link layer protocol have been detected after the ISO voting, which have been fixed since then with the introduction of a stuff-bit counter and other protocol improvements (more details in Bosch's whitepaper ).

According to the organization CAN in Automation (CiA) GmbH, the changes in the CAN FD protocol are user-transparent, meaning the changes have nearly no impact on the usage. Therefore, the software and system development of CAN FD applications is not delayed.

But the chipmakers need to provide new CAN FD hardware. For FPGA implementations this is quite easy to do, but the micro-controller manufacturers have to do eventually a complete redesign. This may lead to some delay in the availability of micro-controllers featuring CAN FD.

“The year of 2014 was somehow challenging for the CAN FD community,” said Holger Zeltwanger, CiA Managing Director. “But it is better to detect problems before the serial production has started.”

Zeltwanger expects the next version of ISO 11898-1 standard and the related conformance test plan (ISO 16845-1), which includes the CAN FD protocol to be released in 2015.

The organization also plans to finalize the next version of ISO 11898-2, the high-speed CAN transceiver standard. It will merge the definitions, which were in the previous ISO 11898-2 standard as well as in ISO 11898-5 (low-power mode) and in ISO 11898-6 (selective wake-up procedure). This harmonized standard will support bit-rates higher than 1 Mbit/s as used in the data-phase of the CAN FD protocol.

CiA will organize further CAN FD plug-fest in Germany and USA, in order to prove the interoperability of CAN FD chips and modules. During plug-fest, the limits of different network topologies will also be tested. The results will be introduced into the CiA 601 series of CAN FD system design specifications and recommendations.

In addition, CiA is going to