NXP Semiconductors MPC5644A, Reference Manual, Page: 1430 / 1735

FlexCAN Module
MPC5644A Microcontroller Reference Manual, Rev. 6
1430 Freescale Semiconductor
32.5.8 CAN protocol related features
32.5.8.1 Remote frames
Remote frame is a special kind of frame. The user can program a message buffer to be a Request Remote
Frame by writing the message buffer as Transmit with the RTR bit set to ‘1’. After the Remote Request
frame is transmitted successfully, the message buffer becomes a Receive Message Buffer, with the same
ID as before.
When a Remote Request frame is received by FlexCAN, its ID is compared to the IDs of the transmit
message buffers with the Code field ‘1010’. If there is a matching ID, then this message buffer frame will
be transmitted. Note that if the matching message buffer has the RTR bit set, then FlexCAN will transmit
a Remote Frame as a response.
A received Remote Request Frame is not stored in a receive buffer. It is only used to trigger a transmission
of a frame in response. The mask registers are not used in remote frame matching, and all ID bits (except
RTR) of the incoming received frame should match.
In the case that a Remote Request Frame was received and matched a message buffer, this message buffer
immediately enters the internal arbitration process, but is considered as normal Tx message buffer, with
no higher priority. The data length of this frame is independent of the DLC field in the remote frame that
initiated its transmission.
If the Rx FIFO is enabled (bit FEN set in MCR), FlexCAN will not generate an automatic response for
Remote Request Frames that match the FIFO filtering criteria. If the remote frame matches one of the
target IDs, it will be stored in the FIFO and presented to the CPU. Note that for filtering formats A and B,
it is possible to select whether remote frames are accepted or not. For format C, remote frames are always
accepted (if they match the ID).
32.5.8.2 Overload frames
FlexCAN does transmit overload frames due to detection of following conditions on CAN bus:
• Detection of a dominant bit in the first/second bit of Intermission
Case 3 FEN = 1 BCC = 0 2nd alternative:
Do not configure any MB as Rx (i.e., let all MBs as either Tx or
inactive).
In this case, RXGMASK, RX14MASK, and RX15MASK can be
used to affect ID Tables without affecting the filtering process
for Rx MBs.
Case 4 Don’t care BCC = 1 If MCR[BCC] = 1, then the RXIMRs are enabled. Thus,
RXGMASK, RX14MASK, and RX15MASK are not used.
Particularly, when MCR[FEN] = 0, RxFIFO is disabled;
RXGMASK, RX14MASK, and RX15MASK do not affect
filtering. Individual masks are used.
Table 32-18. Recommended FEN and BCC settings
Case
MCR[FEN]
RxFIFO
MCR[BCC]
Rx Individual Mask
Notes