Datasheet

After passing the acceptance filtering, received messages including Message ID and DLC are stored into
a dedicated Rx Buffer or into Rx FIFO0 or Rx FIFO1.
For messages to be transmitted dedicated Tx Buffers and/or a Tx FIFO or a Tx Queue can be initialized
or updated. Automated transmission on reception of remote frames is not implemented.
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 Integrating (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.
SAM D5x/E5x Family Data Sheet
CAN - Control Area Network
© 2019 Microchip Technology Inc.
Datasheet
DS60001507E-page 1208