Notes / Communication Protocols
CAN FD: 64 bytes and a faster data phase
Arbitration at the old speed, the data phase faster. What CAN FD changes to get past the 8-byte limit, and design cautions.
Classic CAN is limited to 1 Mbps and 8 bytes, which made it hard to carry the data volume of ADAS and firmware updates. CAN FD changed two things.
What changed
| CAN | CAN FD | |
|---|---|---|
| Data length | Up to 8 bytes | Up to 64 bytes |
| Data-phase speed | Same as arbitration (≤1 Mbps) | Faster (2 to 5 Mbps is common) |
| CRC | 15 bits | 17 / 21 bits |
Two speeds
In the arbitration phase every node has to compare bits at the same time, so it uses a slow speed that fits the bus length (for example 500 kbps). Once arbitration is over and the sender is decided, only the data phase switches to the fast speed, starting at the BRS bit.
Design cautions
- The transceiver has to support CAN FD speeds.
- As the data-phase speed rises, the sampling point and transmitter delay compensation (TDC) settings become important.
- Classic CAN nodes see FD frames as errors. To mix on one bus, every node must understand FD.
- Wiring rules such as termination and stub length become stricter.