MPC5566 Microcontroller Reference Manual, Rev. 2Freescale Semiconductor 22-2522.4.2 Transmit ProcessThe CPU prepares a message buffer for transmission by executing the following steps:• Write the CODE field of the control and status word to keep the TX MB inactive (code = 1000).• Write the ID word.• Write the DATA bytes.• Write the LENGTH, SRR, IDE, RTR, and CODE fields of the control and status word to activatethe TX MB.The first and last steps are mandatory.22.4.2.1 Arbitration ProcessThis process selects the next MB to transmit. All MBs programmed as transmit buffers are scanned to findthe lowest ID1 or the lowest MB number, depending on the LBUF bit in the CANx_CR. The selected MBis transferred to an internal serial message buffer (SMB), which is not software accessible, and thentransmitted.The arbitration process is triggered by the following events:• During the CRC field of the CAN frame• During the error delimiter field of the CAN frame• During Intermission, if the winner MB defined in a previous arbitration was deactivated, or if therewas no MB to transmit, but the CPU wrote to the C/S word of any MB after the previous arbitrationfinished.• When MBM is in idle or bus off state and the CPU writes to the C/S word of any MB• Upon leaving freeze modeWhen arbitration is complete, and an MB is selected to transmit, the frame is first transferred to the SMB.This is called ‘move out.’ After move out, the CAN transmit machine starts to transmit the frame accordingto the CAN protocol rules. FlexCAN2 transmits up to eight data bytes, even if the value of the data lengthcode (DLC) is greater.At the end of a successful transmission, the value of the free running timer at the beginning of the identifierfield is written into the TIME STAMP field in the MB, the CODE field in the control and status word ofthe MB is updated, a status flag is set in CANx_IFRL or CANx_IFRH, and an MB interrupt is generatedif allowed by the corresponding interrupt mask register bit.1. Actually, if LBUF is negated, the arbitration considers not only the ID, but also the RTR and IDE bits placed inside the ID at thesame positions they are transmitted in the CAN frame.
