PXD10RM Datasheet, PDF (705/1332 Pages) Freescale Semiconductor, Inc – PXD10 Microcontroller

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:
• CPU writes 1001 into the code field of the C/S word
• CPU reads the CODE field and compares it to the value that was written
• If the CODE field that was read is different from the value that was written, the CPU must read the
corresponding interrupt flag to check if the frame was transmitted or it is being currently
transmitted. If the corresponding interrupt flag bit is set, the frame was transmitted. If the
corresponding interrupt flag bit 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).
18.4.6.2 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
re-evaluation 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 can not 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”.
• If a Tx MB containing the lowest ID is deactivated after FlexCAN has scanned it, then FlexCAN
will look for another winner within the MBs that it has not scanned yet. Therefore, it may transmit
an MB with ID that may not be the lowest at the time because a lower ID might be present in one
of the MBs that it had already scanned before the deactivation.
• There is a point in time until which the deactivation of a Tx MB causes it not to be transmitted (end
of move-out). After this point, it is transmitted but no interrupt is issued and the Code field is not
Freescale Semiconductor
PXD10 Microcontroller Reference Manual, Rev. 1
Preliminary—Subject to Change Without Notice
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