What’s all this CAN FD stuff anyway? Full duplex? Frequency domain? Fire department? No, actually it is the latest development in the CAN world. FD stands for Flexible Data-rate and was introduced to the world a couple years ago in a white paper from Bosch and is currently being standardized into an update of ISO11898-1.
Controller area network (CAN) is a common communications protocol and bus used to interconnect distributed applications where microprocessors need to communicate. It is widely known in automotive where it has its roots. In years of development it has grown and is now used in applications such as industrial control, field bus, white goods, aerospace and even coffee machines. The simplicity at the protocol level stems from the fact that most of the heavy lifting is done in hardware within the CAN controller peripheral. The flexibility of the bus topology also contributes directly to its widespread use.
As system complexity and inter-processor communication has grown, the available bandwidth within a CAN bus has decreased. Thus, the need for speed (or more bandwidth) has been growing. TI first addressed CAN timing margin and bandwidth on the physical layer side with the SN65HVD255, SN65HVD256 and SN65HVD257“turbo” CAN transceivers. These transceivers are optimized for large and highly-loaded CAN networks and optimize the loop delay timing within the transceivers. The benefit these transceivers offer is higher CAN data rates in the same network. The next step is to move to a CAN FD network, which my colleague, John Griffith, explained from a protocol advantage in his blog “The need for speed: Turbo-charged CAN”.
To go along with the new protocol, the transceivers and physical layer requirements are being addressed. Standardization work has begun within the ISO11898-2 framework. From a transceiver viewpoint, TI introduced a new family of “turbo” CAN transceivers optimized and specified for CAN FD, the SN65HVD265/6/7.
Within the classic CAN network, the two-way or round-trip loop delay of the system is the main limiting factor for the bit rate that is achievable. Since any node in the network may communicate during the arbitration phase of the frame, timing must account for the two farthest nodes, which could start communication at the same time while both of these nodes need to be able to detect each other. This leads to the round-trip delay requirements. CAN FD protocol removes the requirement of the round-trip delay during the data phase and allows for a second, higher bit rate. The constraint during the FD data phase from the network and physical layer is the symmetry of the bit on the bus (physical bit representation). The new parameter for CAN FD in the transceivers is called loop delay symmetry (tREC), shown in Figure 1. The new SN65HVD265/6/7“turbo” CAN family includes this timing parameter for CAN FD data rates.
Figure 1. Loop delay symmetry (tREC)
These devices have been specified for CAN FD up to 2Mbps across the full temperature and voltage range of the device. So, these turbo CAN devices allow higher data rates in the same network thanks to three improvements:
- Turbo CAN technology allows higher data rates during the arbitration phase of the CAN FD frame, limited by the “classic” CAN restrictions.
- More efficient data payload to overhead ratio from the CAN FD protocol that allows up to 64 bytes of data vs. the original 8 bytes.
- CAN FD specified data rate of 2Mpbs during the data phase of the CAN FD frame.
So, the effective bandwidth of CAN FD systems could be quadrupled or even better with respect to a classic CAN implementation! Thus the need for additional CAN buses, a gateway or bridge or alternative field bus technology is mitigated and will let CAN continue to grow and find new uses during the coming decade.
For more information:
Download the CAN FD datasheet
Buy the SN65HVD265 evaluation module