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14.2.6 Message handling
In the FCAN system the assignment of message buffers to the CAN modules is not defined by hard-
ware. Each message buffer in the message buffer section can be assigned to any CAN module by soft-
ware. The message buffers have individual configuration registers to assign the CAN module and to
specify the message buffer type.
Basically, a message buffer can be selected as a transmit message buffer or as a receive message
buffer. For receive message buffers there are further differentiations according to the mask links.
(1)
Message transmission
According to the CAN protocol the highest prior message must always gain the CAN bus access
against lower prior messages sent by other nodes at the same time (due to arbitration mechanism
of CAN protocol) and against messages waiting to be transmitted in the same node (i.e. inner pri-
ority inversion).
The FCAN system scans the message buffer section at the beginning of each message transmit to
analyse that no other message with a higher priority is waiting to be transmitted on the same CAN
bus. The FCAN system avoids inner priority inversion automatically.
Example:
5 transmit messages are waiting to be sent at the same time in the example shown in Table 14-10,
"Example for Automatic Transmission Priority Detection," on page 444. Although the priority of the
transmit messages are not sorted according any scheme, the sequence of transmits on the CAN
bus is:
<1> message buffer number 15
<2> message buffer number 1
<3> message buffer number 22
<4> message buffer number 14
<5> message buffer number 2
Chapter 14 FCAN Interface Function
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