MPC5604B/C Microcontroller Reference Manual, Rev. 8
458 Freescale Semiconductor
• 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
updated. In order to avoid this situation, the abort procedures described in Section 22.4.7.1,
“Transmission abort mechanism should be used.
22.4.7.3 Message buffer lock mechanism
Besides MB deactivation, FlexCAN has another data coherence mechanism for the receive process. When
the CPU reads the Control and Status word of an “active not empty” Rx MB, FlexCAN assumes that the
CPU wants to read the whole MB in an atomic operation, and thus it sets an internal lock flag for that MB.
The lock is released when the CPU reads the Free Running Timer (global unlock operation), or when it
reads the Control and Status word of another MB. The MB locking is done to prevent a new frame to be
written into the MB while the CPU is reading it.
NOTE
The locking mechanism only applies to Rx MBs which have a code different
than INACTIVE (‘0000’) or EMPTY
1
(‘0100’). Also, Tx MBs cannot be
locked.
Suppose, for example, that the FIFO is disabled and the second and the fifth MBs of the array are
programmed with the same ID, and FlexCAN has already received and stored messages into these two
MBs. Suppose now that the CPU decides to read MB number 5 and at the same time another message with
the same ID is arriving. When the CPU reads the Control and Status word of MB number 5, this MB is
locked. The new message arrives and the matching algorithm finds out that there are no “free to receive”
MBs, so it decides to override MB number 5. However, this MB is locked, so the new message cannot be
written there. It will remain in the SMB waiting for the MB to be unlocked, and only then will be written
to the MB. If the MB is not unlocked in time and yet another new message with the same ID arrives, then
the new message overwrites the one on the SMB and there will be no indication of lost messages either in
the Code field of the MB or in the Error and Status Register.
NOTE
The FlexCAN module has 2 SMBs thus if a message with another ID arrives
it is not lost. So overall the probability to lose message is very low unless a
series of messages with the same ID arrives, which is not common in
FlexCAN.
While the message is being moved-in from the SMB to the MB, the BUSY bit on the Code field is asserted.
If the CPU reads the Control and Status word and finds out that the BUSY bit is set, it should defer
accessing the MB until the BUSY bit is negated.
1. In previous FlexCAN versions, reading the C/S word locked the MB even if it was EMPTY. This behavior will be honored when
the BCC bit is negated.
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