Table 43-112. Composition of the arbitration value when Local Priority is enabled
Format
Mailbox Arbitration Value (35 bits)
Standard (IDE =
0)
PRIO (3 bits)
Standard ID (11
bits)
RTR (1 bit)
IDE (1 bit)
- (18 bits)
- (1 bit)
Extended (IDE =
1)
PRIO (3 bits)
Extended
ID[28:18] (11
bits)
SRR (1 bit)
IDE (1 bit)
Extended
ID[17:0] (18 bits)
RTR (1 bit)
As the PRIO field is the most significant part of the arbitration value Mailboxes with low
PRIO values have higher priority than Mailboxes with high PRIO values regardless the
rest of their arbitration values.
Note that the PRIO field is not part of the frame on the CAN bus. Its purpose is only to
affect the internal arbitration process.
43.5.2.3 Arbitration process (continued)
After the arbitration winner is found, its content is copied to a hidden auxiliary MB called
Tx Serial Message Buffer (Tx SMB), which has the same structure as a normal MB but is
not user accessible. This operation is called move-out and after it is done, write access to
the C/S word of the corresponding MB is blocked (if the AEN bit in CAN_MCR register
is asserted). Write access is restored in the following events:
• After the MB is transmitted and the corresponding IFLAG bit is cleared by the CPU
• FlexCAN enters in Freeze mode or Bus Off
• FlexCAN loses the bus arbitration or there is an error during the transmission
At the first opportunity window on the CAN bus, the message on the Tx SMB is
transmitted according to the CAN protocol rules.
Arbitration process can be triggered in the following situations:
• During Rx and Tx frames from CAN CRC field to end of frame.
CAN_CTRL2[TASD] bit value may be changed to optimize the arbitration start
point.
• During CAN BusOff state from TX_ERR_CNT=124 to 128. CAN_CTRL2[TASD]
bit value may be changed to optimize the arbitration start point.
• During C/S write by CPU in BusIdle. First C/S write starts arbitration process and a
second C/S write during this same arbitration restarts the process. If other C/S writes
are performed, Tx arbitration process is pending. If there is no arbitration winner
after the arbitration process has finished, then the TX arbitration machine begins a
new arbitration process. If there is a pending arbitration and BusIdle state starts then
an arbitration process is triggered. In this case the first and second C/S write in
Chapter 43 Flex Controller Area Network (FlexCAN)
KV4x Reference Manual, Rev. 2, 02/2015
Freescale Semiconductor, Inc.
Preliminary
1133
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