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Progress in CAN FD standardization

For a couple of months, experts in ISO have been discussing the standardization of the CAN FD protocol. In addition, CiA has started to develop recommendations for the usage of CAN FD networks.


WITHIN THE ISO, A TASK FORCE IS IN THE PROGRESS of standardizing the CAN FD protocol. It will be introduced in the next revision of the ISO 11898-1 standard. A first work draft has been discussed ISO internally. The general concepts of the CAN FD protocol as proposed by Bosch have not been changed. However, implementation-related specifications will be withdrawn from the standard as far as possible. Also the wording and terminology needs to be improved, in order to avoid misinterpretations and misunderstandings. For example the "transceiver delay compensation" is renamed to "transmitter delay compensation". In December, the responsible ISO task force will start the development of the CAN FD conformance test plan to be implemented in the ISO 16845-1 standard. The ISO 11898-1 committee draft for voting is planned to be submitted at the beginning of next year.

Because the ISO 11898-1 standard addresses mainly the chipmakers, additional specifications and recommendations for the proper usage are necessary. "All that is above 1 Mbit/s is RF technology," explained Holger Zeltwanger, CiA Managing Director. "We have to consider physical effects, which we can ignore when we are using data rates lower than 1 Mbit/s." A typical example is the shortening of recessive bits when the temperature is very low. That is the reason why CAN FD networks require a much more precise design of the physical layer. One recommendation is to use the same clock frequency in all nodes (20 MHz, 40 MHz, or 80 MHz will be recommended). CiA has started the development of CAN FD recommendations in the Interest Group "CAN FD" (CiA 601 series). It is intended to provide a cookbook for users containing some specifications for bit-timings and recommendations for calculating the necessary parameters and limits. "This will make life easier for system designers," said Reiner Zitzmann from CiA.

On the international CAN Conference, several papers will provide detailed information on the research results regarding CAN FD networking. These results will be introduced in the CiA documents, in particular the calculations and recommendations for the physical layer. In addition, there will be proposals for migration paths from classic CAN networks to CAN FD networks.

The CiA Special Interest Group (SIG) CANopen application layer has started to adapt the improved CAN data link layer. The experts discussed the migration to CAN FD. In particular, the improvement of SDO services to payloads up to 64 byte is not yet finalized. The PDO extension to 64 byte will require a byte-wise mapping, meaning that a complete bit-wise mapping of more than 8 byte is not possible. This is due to the number of mapping parameters (64 sub-indexes). The other topic is the CANopen bit-timing and other physical layer specifications considering an open network based on CAN FD. "We expect to have stable CANopen specifications for CAN FD," said Reiner Zitzmann, "when the first CAN FD silicon is available in volumes."

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