CAN FD

The improved CAN data link layer protocol (also known as CAN FD) uses a second higher bit-rate in the data-phase (see Figure 1). In the arbitration phase and the ACK phase, the bit-rate is still limited to 1 Mbit/s depending on the network overall length. The in-bit-time detection is limited by the internal transceiver delays and the bus-line delay, which shall be smaller than the time segment before the sample-point (TSEG1). The “free” transmission in the data phase is limited by the transceiver-internal low-pass filter capability. In addition, the data-field as has been prolonged to up to 64 byte. Both improvements lead to higher throughputs in CAN networks. The improved efficiency (frame overall length divided by the payload) combined with the higher speed increases the throughput significantly

Using commercially available ISO 11898-2 compliant transceiver chips under laboratory

To distinguish between the classic CAN data frame and the improved CAN data frame, one of the reserved bits is used (EDL = extended data length). In order to code the larger payloads (data-fields with more than 8 byte), the remaining DLC (data length code) bit combinations are used. Data field length of 12, 16, 20, 24, 32, 48, and 64 are supported additionally. The control field comprises additional bits (see Figure 2). The RTR bit is reserved, meaning that there is no improved CAN remote frame. The EDL bit substituting the r1 in the classic CAN data frame is transmitted recessively indicating that the following bits shall be interpreted according to the improved CAN protocol. The next bit is reserved followed by the BRS (bit rate switch) bit. The BRS bit is used to switch to the higher configured bit-rate. The next bit (ESI = error state indicator) informs, if the transmitter is in error active (dominant) or error passive (recessive) state.

SOF = Start of frame (bit is always of dominant state)
ID = Identifier (frame priority and content indication)
RTR = Remote transmission request (dominant, if data frame)
IDE = ID extension (dominant for base frame format)
EDL = Extended data length (recessive, if data field is longer than 8 byte)
r0/1 = reserved bit BRS = Bit rate switch (recessive, if switched to alternate bit-rate)
ESI = Error state indicator (recessive, if transmitting node is in error passive state)
DLC = Data length code (indicates the length of the following data field)
CRC = Cyclic redundancy check (15-bit, 17-bit, or 21-bit)
D = Delimiter of CRC/ACK field (bit is always of recessive state)
ACK = Acknowledgment slot (correctly receiving node sends a dominant bit)
EOF = End of frame (all bits are always of recessive state)
IFS = Inter-frame space (the first two bits are always of recessive state)
IDLE = bus is in recessive state

In order to keep the reliability of the classic CAN protocol, the CRC polynomial are changed. The introduced 17-bit CRC polynomial is used for improved CAN data frames up to 16-byte payloads. For larger frames, the 21-bit CRC polynomial is used. This means the improved CAN controllers supporting the classic and the improved CAN protocol starts all three CRC polynomials. If the bit following the IDE bit is recessive, the classic 15-bit CRC polynomial is discontinued. After processing the DLC bits, the CAN controller stops calculating one of the other CRC polynomials. The improved CAN protocol is backwards compatible. This means, the support the classic CAN data and remote frames. However, if a CAN controller supporting just the classic CAN protocol sees an improved CAN data frame, it will send error flags. Therefore migration paths are necessary. One option is the usage of partial networking. Another is to separate nodes in two independent CAN network segments. It is also possible, to set the classic CAN controllers in listen-only mode.

Oscilloscope of an improved CAN data frame with an arbitration speed of 500 kbit/s and data transmission speed of 15 Mbit/s at a network length of 42 m.

For more technical details and first product information see the 13th iCC proceedings and the CAN Newsletter Online as well as the printed CAN Newsletter.