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45.4.2.2 Highest-priority Mailbox first
If CTRL1[LBUF] bit is negated, then the arbitration process searches the active Tx
Mailbox with the highest priority, which means that this Mailbox’s frame would have a
higher probability to win the arbitration on CAN bus when multiple external nodes
compete for the bus at the same time.
The sequence of bits considered for this arbitration is called the arbitration value of the
Mailbox. The highest-priority Tx Mailbox is the one that has the lowest arbitration value
among all Tx Mailboxes.
If two or more Mailboxes have equivalent arbitration values, the Mailbox with the lowest
number is the arbitration winner.
The composition of the arbitration value depends on MCR[LPRIO_EN] bit setting.
45.4.2.2.1 Local Priority disabled
If MCR[LPRIO_EN] bit is negated the arbitration value is built in the exact sequence of
bits as they would be transmitted in a CAN frame (see the following table) in such a way
that the Local Priority is disabled.
Table 45-75. Composition of the arbitration value when Local Priority is disabled
Format Mailbox Arbitration Value (32 bits)
Standard (IDE = 0) Standard ID (11
bits)
RTR (1 bit) IDE (1 bit) - (18 bits) - (1 bit)
Extended (IDE = 1) Extended ID[28:18]
(11 bits)
SRR (1 bit) IDE (1 bit) Extended ID[17:0]
(18 bits)
RTR (1 bit)
45.4.2.2.2 Local Priority enabled
If Local Priority is desired MCR[LPRIO_EN] 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 45-76. 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)
Chapter 45 CAN (FlexCAN)
K20 Sub-Family Reference Manual, Rev. 1.1, Dec 2012
Freescale Semiconductor, Inc.
Preliminary
1151
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