FlexCAN Module
Freescale Semiconductor
24-41
PXR40 Microcontroller Reference Manual, Rev. 1
24.4.8
CAN protocol Related Features
24.4.8.1
Remote Frames
A remote frame is a special kind of frame. The user can program a MB to be a Request Remote frame by
writing the MB as Transmit with the RTR bit set to ‘1’. After the Remote Request frame is transmitted
successfully, the MB becomes a Receive Message Buffer, with the same ID as before.
When a Remote Request frame is received by the FlexCAN module, 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 MB frame is
transmitted. Note that if the matching MB has the RTR bit set, then the FlexCAN module transmits 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 an MB, this message buffer
immediately enters the internal arbitration process, but is considered as normal Tx MB, with no higher
priority. The data length of this frame is independent of the DLC field in the remote frame that initiated its
transmission.
24.4.8.2
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
Table 24-18. Recommended FEN and BCC settings
Case
MCR[FEN]
RxFIFO
MCR[BCC]
Rx Individual Mask
Notes
Case 1
FEN = 0
BCC = 0
RXGMASK, RX14MASK, and RX15MASK can safely be used.
This allows backwards compatibility to older devices (e.g.,
devices without the individual masks feature). In this case,
individual masks are not used.
Case 2
FEN = 1
BCC = 0
1st alternative:
Do not use RXGMASK, RX14MASK, and RX15MASK in this
case, leave the masks in their reset state.
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.
Summary of Contents for PXR4030
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