FlexCAN ModuleMPC5644A Microcontroller Reference Manual, Rev. 6Freescale Semiconductor 1423buffers with PRIO = 000 have higher priority. If two or more message buffers have the same priority, theregular ID will determine the priority of transmission. If two or more message buffers have the samepriority (three extra bits) and the same regular ID, the lowest message buffer will be transmitted first.Once the highest priority message buffer is selected, it is transferred to a temporary storage space calledSerial Message Buffer (SMB), which has the same structure as a normal message buffer but is not useraccessible. This operation is called “move-out” and after it is done, write access to the correspondingmessage buffer is blocked (if MCR[AEN] is asserted). The write access is released in the following events:• After the message buffer is transmitted• FlexCAN enters in HALT or BUS OFF• FlexCAN loses the bus arbitration or there is an error during the transmissionAt the first opportunity window on the CAN bus, the message on the SMB is transmitted according to theCAN protocol rules. FlexCAN transmits up to eight data bytes, even if the DLC (Data Length Code) valueis bigger.32.5.4 Receive processTo be able to receive CAN frames into the mailbox message buffers, the CPU must prepare one or moreMessage Buffers for reception by executing the following steps:1. If the message buffer has a pending transmission, write an ABORT code (‘1001’) to the Code fieldof the Control and Status word to request an abortion of the transmission, then read back the Codefield and the IFRL or IFRH register to check if the transmission was aborted (see Section 32.5.6.1,Transmission abort mechanism). If backwards compatibility is desired (AEN in MCR negated),just write ‘1000’ to the Code field to inactivate the message buffer, but then the pending frame maybe transmitted without notification (see Section 32.5.6.2, Message buffer deactivation). If themessage buffer already programmed as a receiver, just write ‘0000’ to the Code field of the Controland Status word to keep the message buffer inactive.2. Write the ID word3. Write ‘0100’ to the Code field of the Control and Status word to activate the message bufferOnce the message buffer is activated in the third step, it will be able to receive frames that match theprogrammed ID. At the end of a successful reception, the message buffer is updated by the MBM asfollows:1. The value of the Free Running Timer is written into the Time Stamp field2. The received ID, Data (8 bytes at most) and Length fields are stored3. The Code field in the Control and Status word is updated (see Table 32-5 and Table 32-6 inSection 32.4.3, Message buffer structure)4. A status flag is set in the Interrupt Flag Register and an interrupt is generated if allowed by thecorresponding Interrupt Mask Register bitUpon receiving the MB interrupt, the CPU should service the received frame using the followingprocedure:1. Read the Control and Status word (mandatory – activates an internal lock for this buffer)
