38.4 CAN FD Message Frames

The ISO11898-1:2015 describes the different CAN message frames in detail. Figure 38-7 through Figure 38-12 explain and summarize the construction of the messages and fields.

There are four different CAN data/remote frames (see Figure 38-8):
  • CAN Base Frame: Classic CAN 2.0 frame using Standard ID
  • CAN FD Base Frame: CAN FD frame using Standard ID
  • CAN Extended Frame: Classic CAN 2.0 frame using Extended ID
  • CAN FD Extended Frame: CAN FD frame using Extended ID

There are no remote frames in CAN FD frames; therefore, the RTR bit is replaced with the RRS bit (see Figure 38-8). The RRS bit in the CAN FD base frame can be used to extend the SID to 12 bits. When enabled, it is referred to as SID11; it is the Least Significant bit (LSb) of SID[11:0].

Figure 38-9 specifies the control field of the different CAN messages. Before CAN FD was added to the ISO11898-1:2015, the FDF bit was a reserved bit. Now the FDF bit selects between Classic and CAN FD formats.

The BRS bit selects if the bit rate needs to be switched in the data phase of CAN FD frames. Figure 38-12 illustrates the error and overload frames. These special frames do not change.
Note: If an error is detected during the data phase of a CAN FD frame, the bit rate will be switched back to the Nominal Bit Rate (NBR). Error frames are always transmitted at the arbitration bit rate.