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While the message is being moved-in from the SMB to the MB, the BUSY bit on the code field is set. 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 cleared.
If the BUSY bit is set or if the MB is empty, then reading the control and status word does not lock the MB.
Deactivation takes precedence over locking. If the CPU deactivates a locked
Rx MB, then its lock status is negated, and the MB is marked as invalid for
the current matching round. Any pending message on the SMB is not
transferred to the MB anymore.
23.3.16 CAN Protocol Related Frames
23.3.16.1 Remote Frames
The remote frame is a message frame transmitted to request a data frame. The FlexCAN can be configured
to transmit a data frame automatically in response to a remote frame, or to transmit a remote frame and
then wait for the responding data frame to be received.
When transmitting a remote frame, the user initializes a message buffer as a transmit message buffer with
the RTR bit set. After this remote frame is transmitted successfully, the transmit message buffer
automatically becomes a receive message buffer, with the same ID as the remote frame that was
transmitted.
When a remote frame is received by the FlexCAN, the remote frame ID is compared to the IDs of all
transmit message buffers programmed with a CODE of 1010. If there is an exact matching ID, the data
frame in that message buffer is transmitted. If the RTR bit in the matching transmit message buffer is set,
the FlexCAN transmits a remote frame as a response.
A received remote frame is not stored in a receive message buffer. It is only used to trigger the automatic
transmission of a frame in response. The mask registers are not used in remote frame ID matching. All ID
bits (except RTR) of the incoming received frame must match for the remote frame to trigger a response
transmission. The matching message buffer immediately enters the internal arbitration process, but is
considered as a normal Tx MB, with no higher priority. The data length of this frame is independent of the
data length code (DLC) field in the remote frame that initiated its transmission.
23.3.16.2 Overload Frames
Overload frame transmissions are not initiated by the FlexCAN unless certain conditions are detected on
the CAN bus. These conditions include detection of a dominant bit in the following:
•
First or second bit of intermission
•
Seventh (last) bit of the end-of-frame (EOF) field in receive frames
•
Eighth (last) bit of the error frame delimiter or overload frame delimiter
Freescale Semiconductor
NOTE
MCF5329 Reference Manual, Rev 3
FlexCAN
23-25
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