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Chapter 18
18.10 Message Transmission
18.10.1 Message transmission
Figure 18-30
610
A message buffer with its TRQ bit set to 1 participates in the search for the
most high-prioritized message when the following conditions are fulfilled. This
behavior is valid for all operational modes.
• Used as a message buffer
(MA0 bit of CnMCONFm register set to 1.)
• Set as a transmit message buffer
(MT[2:0] bits of CnMCONFm register set to 000
• Ready for transmission
(RDY bit of CnMCTRLm register set to 1.)
The CAN system is a multi-master communication system. In a system like
this, the priority of message transmission is determined based on message
identifiers (IDs). To facilitate transmission processing by software when there
are several messages awaiting transmission, the CAN module uses hardware
to check the ID of the message with the highest priority and automatically
identifies that message. This eliminates the need for software-based priority
control.
Transmission priority is controlled by the identifier (ID).
Message No.
Message waiting to be transmitted
0
1
ID = 120H
2
ID = 229H
3
4
5
ID = 223H
6
ID = 023H
7
8
ID = 123H
9
Message processing example
After the transmit message search, the transmit message with the highest
priority of the transmit message buffers that have a pending transmission
request (message buffers with the TRQ bit set to 1 in advance) is transmitted.
If a new transmission request is set, the transmit message buffer with the new
transmission request is compared with the transmit message buffer with a
pending transmission request. If the new transmission request has a higher
priority, it is transmitted, unless transmission of a message with a low priority
has already started. If transmission of a message with a low priority has
already started, however, the new transmission request is transmitted later. To
solve this priority inversion effect, the software can perform a transmission
abort request for the lower priority message. The highest priority is determined
according to the following rules.
User's Manual U18743EE1V2UM00
CAN Controller (CAN)
.)
B
The CAN module transmits messages in the following sequence.
1. Message 6
2. Message 1
3. Message 8
4. Message 5
5. Message 2
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