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Chapter 25 FlexCAN
MPC5606BK Microcontroller Reference Manual, Rev. 2
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Freescale Semiconductor
•
There is an error during the transmission
•
The module is put into Freeze Mode
If none of these conditions is reached, the MB is transmitted correctly, the interrupt flag is set in the IFLAG
register, and an interrupt to the CPU is generated (if enabled). The abort request is automatically cleared
when the interrupt flag is set. On the other hand, if one of the above conditions is reached, the frame is not
transmitted, therefore the abort code is written into the Code field, the interrupt flag is set in the IFLAG,
and an interrupt is (optionally) generated to the CPU.
If the CPU writes the abort code before the transmission begins internally, then the write operation is not
blocked, therefore the MB is updated and no interrupt flag is set. In this way the CPU just needs to read
the abort code to make sure the active MB was deactivated. Although the AEN bit is asserted and the CPU
wrote the abort code, in this case the MB is deactivated and not aborted, because the transmission did not
start yet. One MB is only aborted when the abort request is captured and kept pending until one of the
previous conditions are satisfied.
The abort procedure can be summarized as follows:
1. CPU writes 1001 into the code field of the C/S word.
2. CPU reads the CODE field and compares it to the value that was written.
3. If the CODE field that was read is different from the value that was written, the CPU must read the
corresponding IFLAG to check if the frame was transmitted or it is being currently transmitted. If
the corresponding IFLAG is set, the frame was transmitted. If the corresponding IFLAG is reset,
the CPU must wait for it to be set, and then the CPU must read the CODE field to check if the MB
was aborted (CODE=1001) or it was transmitted (CODE=1000).
25.5.7.2
Message buffer deactivation
Deactivation is mechanism provided to maintain data coherence when the CPU writes to the Control and
Status word of active MBs out of Freeze Mode. Any CPU write access to the Control and Status word of
an MB causes that MB to be excluded from the transmit or receive processes during the current matching
or arbitration round. The deactivation is temporary, affecting only for the current match/arbitration round.
The purpose of deactivation is data coherency. The match/arbitration process scans the MBs to decide
which MB to transmit or receive. If the CPU updates the MB in the middle of a match or arbitration
process, the data of that MB may no longer be coherent, therefore deactivation of that MB is done.
Even with the coherence mechanism described above, writing to the Control and Status word of active
MBs when not in Freeze Mode may produce undesirable results. Examples are:
•
Matching and arbitration are one-pass processes. If MBs are deactivated after they are scanned, no
reevaluation is done to determine a new match/winner. If an Rx MB with a matching ID is
deactivated during the matching process after it was scanned, then this MB is marked as invalid to
receive the frame, and FlexCAN will keep looking for another matching MB within the ones it has
not scanned yet. If it cannot find one, then the message will be lost. Suppose, for example, that two
MBs have a matching ID to a received frame, and the user deactivated the first matching MB after
FlexCAN has scanned the second. The received frame will be lost even if the second matching MB
was free to receive.
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