50.5.2.2.2 Local Priority enabled
If Local Priority is desired CAN_MCR[LPRIOEN] must be asserted. In this case the
Mailbox PRIO field is included at the very left of the arbitration value (see the following
table).
Table 50-11. 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.
50.5.2.3 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.
Chapter 50 CAN (FlexCAN)
Kinetis KE1xF Sub-Family Reference Manual, Rev. 4, 06/2019
NXP Semiconductors
1437
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