ST STM32G0 1 Series Reference Manual page 1203

RM0444
For messages to be transmitted, Tx FIFO or Tx queue can be initialized or updated.
Automated transmission on reception of remote frames is not supported.
CAN FD operation
There are two variants in the FDCAN protocol:
1. Long Frame Mode (LFM), where the data field of a CAN frame may be longer that eight
bytes
2. Fast Frame Mode (FFM), where control field, data field, and CRC field of a CAN frame
are transmitted with a higher bit rate compared to the beginning and to the end of the
frame
Fast Frame Mode can be used in combination with Long Frame Mode.
The previously reserved bit in CAN frames with 11-bit identifiers and the first previously
reserved bit in CAN frames with 29-bit identifiers are decoded as FDF bit: FDF recessive
signifies a CAN FD frame, while 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 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 FDCAN receives a frame with FDF recessive and res recessive, it
signals 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 FDCAN treats a recessive
res bit as a form error and responds 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, which 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.
FD controller area network (FDCAN)
RM0444 Rev 5 1203/1390
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