CAN FD is well established in the automotive industry. Many automakers have migrated from CAN CC (classic) networks to CAN FD. Other industries have started to use CAN FD, too. There are two reasons to do so: the larger data field (up to 64 byte) and the higher bit rate in the data phase. When the user only wants to benefit from the larger payload, the nominal bit rate and the data(phase) bit rate can be the same. You can achieve this by configuring only the nominal bit rate and not setting the BRS (bit rate switch) bit. Theoretically, you could configure both bit rates to the same value (e.g. 500 kbit/s) and set the BRS bit. To avoid any issues linked to bit rate switching and available bit timing register sizes, CiA recommends in these cases not to use the bit rate switching. Pay attention: Even if you do not intend to use the bit rate switching, initializing the bit timing registers for the data phase, with a suitable data phase bit rate can be beneficial. Implementations of CAN FD tools or devices may demand CAN FD usage with BRS. A network wide, suitable data phase bit rate setting in all devices, even if they are just temporarily connected, would avoid severe CAN error scenarios and would increase the robustness of your network.

Using CAN FD at low speed allows you to benefit from the large CAN FD data field and at the same time to support complex network topologies, with large network dimensions. Thus, requirements for embedded control networks used in e.g. lift applications, construction machineries, trains, or even cargo ships, can be met. Using the same bit rate in arbitration phase and data phase may put unacceptable latencies to your network. To avoid these latencies, a system designer could intend to switch e.g. from 125 kbit/s in arbitration phase to 500 kbit/s in the data phase. In theory, this should work. Practical experience has shown that several CAN FD controller implementations do not support an appropriate size in the bit timing registers, for adjusting this kind of bit rate combinations. This applies in particular, in case you want to achieve a bit timing stetting that results in robust network behavior and follows the rules for CAN FD node and system design, provided in CiA 601-2; available on the CiA website.

As several CiA members have been faced with these challenges, CiA has started to work on recommend bit timing settings for CAN FD networks, using very low bit rate combinations (e.g. 500 kbit/s, 250 kbit/s, and 125 kbit/s) in arbitration and data phase. The intent is to support device developers in their objective of providing interoperable products. System designers are assisted in setting up a robust CAN system. Finally, also semiconductor manufacturers get useful hints, which bit rate combinations are required in the field – e.g. based on the topology and cabling requirements.

The initiative is currently managed by the CiA IG02 CANopen FD. In case you like to join the initiative, please contact CiA office.

p.p. Reiner Zitzmann
General manager CAN in Automation GmbH