PXN20RM Datasheet, PDF (919/1376 Pages) Freescale Semiconductor, Inc – PXN20 Microcontroller

Controller Area Network (FlexCAN)
The eight elements of the filter table are individually affected by the first eight individual mask registers
(CANx_RXIMR0 – CANx_RXIMR7), allowing very powerful filtering criteria to be defined. The rest of
the RXIMR, starting from RXIM8, continue to affect the regular MBs, starting from MB8. If the BCC bit
is negated, then the FIFO filter table is affected by the legacy mask registers as follows: element 6 is
affected by CANx_RX14MASK, element 7 is affected by CANx_RX15MASK and the other elements (0
to 5) are affected by CANx_RXGMASK.
29.4.7 CAN Protocol Related Features
29.4.7.1 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 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 MB frame is transmitted.
Note that if the matching MB has the RTR bit set, then FlexCAN 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 a 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.
If the Rx FIFO is enabled (bit FEN set in CANx_MCR), FlexCAN does 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 is 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).
29.4.7.2 Overload Frames
FlexCAN transmits overload frames due to detection of these conditions on CAN bus:
• Detection of a dominant bit in the first/second bit of intermission
• Detection of a dominant bit at the 7th bit (last) of end of frame field (Rx frames)
• Detection of a dominant bit at the 8th bit (last) of error frame delimiter or overload frame delimiter
29.4.7.3 Time Stamp
The value of the free-running timer is sampled at the beginning of the identifier field on the CAN bus, and
is stored at the end of move in the TIME STAMP field, providing network behavior with respect to time.
Note that the free-running timer can be reset on a specific frame reception, enabling network time
synchronization. Refer to TSYN description in Section 29.3.4.2, Control Register (CANx_CTRL).
Freescale Semiconductor
PXN20 Microcontroller Reference Manual, Rev. 1
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