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45.4.2 Arbitration process
The arbitration process scans the Mailboxes searching the Tx one that holds the message
to be sent in the next opportunity. This Mailbox is called the arbitration winner.
The scan starts from the lowest number Mailbox and runs toward the higher ones.
The arbitration process is triggered in the following events:
• From the CRC field of the CAN frame. The start point depends on the
CTRL2[TASD] field value.
• During the Error Delimiter field of a CAN frame.
• During the Overload Delimiter field of a CAN frame.
• When the winner is inactivated and the CAN bus has still not reached the first bit of
the Intermission field.
• When there is CPU write to the C/S word of a winner MB and the CAN bus has still
not reached the first bit of the Intermission field.
• When CHI is in Idle state and the CPU writes to the C/S word of any MB.
• When FlexCAN exits Bus Off state.
• Upon leaving Freeze mode or Low Power mode.
If the arbitration process does not manage to evaluate all Mailboxes before the CAN bus
has reached the first bit of the Intermission field the temporary arbitration winner is
invalidated and the FlexCAN will not compete for the CAN bus in the next opportunity.
The arbitration process selects the winner among the active Tx Mailboxes at the end of
the scan according to both CTRL1[LBUF] and MCR[LPRIO_EN] bits settings.
45.4.2.1 Lowest-number Mailbox first
If CTRL1[LBUF] bit is asserted the first (lowest number) active Tx Mailbox found is the
arbitration winner. MCR[LPRIO_EN] bit has no effect when CTRL1[LBUF] is asserted.
Functional description
K20 Sub-Family Reference Manual, Rev. 1.1, Dec 2012
1150
Preliminary
Freescale Semiconductor, Inc.
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