39.6.2.3 CAN FD Operation

There are two variants in the CAN FD frame format, first the CAN FD frame without bit rate switching where the data field of a CAN frame may be longer than 8 bytes. The second variant is the CAN FD frame where control field, data field, and CRC field of a CAN frame are transmitted with a higher bit rate than the beginning and the end of the frame.

The previously reserved bit in CAN frames with 11-bit identifiers and the first previously reserved bit in CAN frames with 29-bit identifiers will now be decoded as FDF bit. FDF = recessive signifies a CAN FD frame, FDF = dominant signifies a Classic CAN frame. In a CAN FD frame, the two bits following FDF, res and BRS, decide whether the bit rate inside of this CAN FD frame is switched. A CAN FD bit rate switch is signified by res = dominant and BRS = recessive. The coding of res = recessive is reserved for future expansion of the protocol. In case the CAN receives a frame with FDF = recessive and res = recessive, it will signal a Protocol Exception Event by setting bit PSR.PXE. When Protocol Exception Handling is enabled (CCCR.PXHD = ‘0’), this causes the operation state to change from Receiver (PSR.ACT = “10”) to synchronizing on CAN communication (PSR.ACT = “00”) at the next sample point. In case Protocol Exception Handling is disabled (CCCR.PXHD = ‘1’), the CAN will treat a recessive res bit as a form error and will respond with an error frame.

CAN FD operation is enabled by programming CCCR.FDOE. In case CCCR.FDOE = ‘1’, transmission and reception of CAN FD frames is enabled. Transmission and reception of Classic CAN frames is always possible. Whether a CAN FD frame or a Classic CAN frame is transmitted can be configured via bit FDF in the respective Tx Buffer element. With CCCR.FDOE = ‘0’, received frames are interpreted as Classic CAN frames, witch leads to the transmission of an error frame when receiving a CAN FD frame. When CAN FD operation is disabled, no CAN FD frames are transmitted even if bit FDF of a Tx Buffer element is set. CCCR.FDOE and CCCR.BRSE can only be changed while CCCR.INIT and CCCR.CCE are both set.

With CCCR.FDOE = ‘0’, the setting of bits FDF and BRS is ignored and frames are transmitted in Classic CAN format. With CCCR.FDOE = ‘1’ and CCCR.BRSE = ‘0’, only bit FDF of a Tx Buffer element is evaluated. With CCCR.FDOE = ‘1’ and CCCR.BRSE = ‘1’, transmission of CAN FD frames with bit rate switching is enabled. All Tx Buffer elements with bits FDF and BRS set are transmitted in CAN FD format with bit rate switching.

A mode change during CAN operation is only recommended under the following conditions:

• The failure rate in the CAN FD data phase is significantly higher than in the CAN FD arbitration phase. In this case disable the CAN FD bit rate switching option for transmissions.
• During system startup all nodes are transmitting Classic CAN messages until it is verified that they are able to communicate in CAN FD format. If this is true, all nodes switch to CAN FD operation.
• Wake-up messages in CAN Partial Networking have to be transmitted in Classic CAN format.
• End-of-line programming in case not all nodes are CAN FD capable. Non CAN FD nodes are held in silent mode until programming has completed. Then all nodes switch back to Classic CAN communication.

In the CAN FD format, the coding of the DLC differs from the standard CAN format. The DLC codes 0 to 8 have the same coding as in standard CAN, the codes 9 to 15, which in standard CAN all code a data field of 8 bytes, are coded according to the table below.

In CAN FD frames, the bit timing will be switched inside the frame, after the BRS (Bit Rate Switch) bit, if this bit is recessive. Before the BRS bit, in the CAN FD arbitration phase, the nominal CAN bit timing is used as defined by the Nominal Bit Timing & Prescaler Register NBTP. In the following CAN FD data phase, the fast CAN bit timing is used as defined by the Data Bit Timing & Prescaler Register DBTP. The bit timing is switched back from the fast timing at the CRC delimiter or when an error is detected, whichever occurs first.

The maximum configurable bit rate in the CAN FD data phase depends on the CAN clock frequency (GCLK_CAN). Example: with a CAN clock frequency of 20MHz and the shortest configurable bit time of 4 t_q, the bit rate in the data phase is 5 Mbit/s.

In both data frame formats, CAN FD long and CAN FD fast, the value of the bit ESI (Error Status Indicator) is determined by the transmitter’s error state at the start of the transmission. If the transmitter is error passive, ESI is transmitted recessive, else it is transmitted dominant.