FlexCAN ModuleMCF5253 Reference Manual, Rev. 125-26 Freescale Semiconductor6. If the user reads the control/status word of a receive message buffer while a frame is beingtransferred from a serial message buffer, the BUSY code will be indicated. The user should waituntil this code is cleared before continuing to read from the message buffer to ensure datacoherency. In this situation, the read of the control/status word will not lock the message buffer.Polling the control/status word of a receive message buffer can lock it, preventing a message from beingtransferred into that buffer. If the control/status word of a receive message buffer is read, it should then befollowed by a read of the control/status word of another buffer, or by reading the free-running timer, toensure that the locked buffer is unlocked.NOTEDeactivation takes precedence over locking. If the CPU deactivates a lockedRx MB, then its lock status is negated, and the MB is marked as invalid forthe current matching round. Any pending message on the SMB will not betransferred to the MB anymore.25.6.6 CAN Protocol Related Frames25.6.6.1 Remote FramesThe remote frame is a message frame that is transmitted to request a data frame. The FlexCAN can beconfigured to transmit a data frame automatically in response to a remote frame, or to transmit a remoteframe 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 withthe RTR bit set. Once this remote frame is transmitted successfully, the transmit message bufferautomatically becomes a receive message buffer, with the same ID as the remote frame that wastransmitted.When a remote frame is received by the FlexCAN, the remote frame ID is compared to the IDs of alltransmit message buffers programmed with a CODE of 1010. If there is an exact matching ID, the dataframe in that message buffer is transmitted. If the RTR bit in the matching transmit message buffer is set,the FlexCAN will transmit 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 automatictransmission of a frame in response. The mask registers are not used in remote frame ID matching. All IDbits (except RTR) of the incoming received frame must match for the remote frame to trigger a responsetransmission. The matching message buffer immediately enters the internal arbitration process, but isconsidered as a normal Tx MB, with no higher priority. The data length of this frame is independent of thedata length code (DLC) field in the remote frame that initiated its transmission.25.6.6.2 Overload FramesOverload frame transmissions are not initiated by the FlexCAN unless certain conditions are detected onthe 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
