2 The Limitations of Classical CAN and Regular CAN FD

The first-generation CAN protocol, ISO 11898-2, also known as classical CAN, was released around 1993. The protocol allowed only 8 bytes of payload data transfer, and a maximum specified data-rate of 1Mbps. These limitations were quickly realized in automotive applications, where vehicles have a number of electronic nodes that communicate with each other using the CAN bus.

The CAN FD protocol specification was released around 2015, which increased the payload length to 64 bytes and the maximum signaling rate in the data phase to 5Mbps. However, the arbitration phase signaling rate was still limited to 1Mbps for backwards compatibility with classical CAN.

While CAN FD brought the advantages of a faster data-rate and a longer payload, these advantages were not sufficient to keep pace with the ever-increasing number of ECUs added to vehicle CAN bus networks. Designers realized that harnessing the real potential of CAN FD transceivers was not possible, as bus ringing (resulting from complex star networks) affected correct signal communication. Figure 2-1 is an example of star topology.

Figure 2-1 CAN Nodes Connected in a Star Network

In complex star topologies with multiple stubs, a signal traveling on the bus experiences an impedance mismatch which causes reflections. These reflections distort the CAN bus and cause oscillations, resulting in an incorrect CAN bus level and RXD at the sampling point. Although these network effects were not specific to CAN FD networks, at the lower-speed operation of classical CAN, the bit duration was longer, and the bus ringing diminished such that sampling the correct bit was possible (as shown in Figure 2-2) resulting in correct communication.

Figure 2-2 CAN Bus Ringing and RXD Glitch for Classical CAN Speeds

For a 5Mbps CAN FD operation, a 200ns bit duration was much too small for the ringing in complex star topologies to disappear, hampering reliable data communication. This deterred system designers from using CAN FD at 5Mbps.

With an increase in the exchange of network data and faster throughput demands in modern-day vehicles, CAN SIC paves the way for a next-generation in-vehicle communication bus technology that is faster and provides more network flexibility and scalability.