MPC5553/MPC5554 Microcontroller Reference Manual, Rev. 5
Freescale Semiconductor
22-27
•
If while a MB is locked, two or more RX frames with matching ID are received, then the last
received one is kept within the SMB, while all preceding ones are lost. There is no indication that
the preceding ones were lost in the CAN
x
_ESR.
•
If a locked MB is released, and there exists a matching frame within the SMB, this frame is then
transferred to the matching MB.
•
If the CPU reads a RX MB while it is being transferred into (from) SMB, then the BUSY bit is set
in the CODE field of the control and status word. In order to assure data coherency, the CPU should
wait until this bit is negated before further reading from that MB. Note that in this case such MB
is not locked.
•
If the CPU deactivates a locked RX MB, then its lock status is negated, but no data is transferred
into that MB.
22.4.5
CAN Protocol Related Features
22.4.5.1
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 will be
transmitted. Note that if the matching MB has the RTR bit set, then FlexCAN2 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 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.
22.4.5.2
Overload Frames
FlexCAN2 will transmit overload frames due to detection of following 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
22.4.5.3
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’ in the TIME STAMP field, providing network behavior with respect to
time.
Note that the free running timer can be reset upon a specific frame reception, enabling network time
synchronization. Refer to TSYN description in
Section 22.3.3.2, “Control Register (CANx_CR)
.”
Summary of Contents for MPC5553
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