
CHAPTER 19 CAN CONTROLLER
Preliminary User’s Manual U16541EJ1V0UM
676
Priority
Conditions
Description
1 (highest)
11 MSB identifier value rule
[ID28 to ID18]
The first 11 bits of the identifier (i.e. ID28 to ID18) are the first criteria
used to judge which message frame has to be sent first.
As a result, message frames with the lowest value represented by the
11 most significant bits of the identifier have to be sent first. 11-bit
standard identifiers have a higher priority than message frames with
29-bit extended identifiers if the value of the 11-bit standard identifier is
equal to or smaller than the 11 most significant bits (11 MSB) of the
29-bit extended identifier.
2
Frame type rule
The priority 1 rule does not provide an unambiguous result when the
11 most significant bits of the identifier are equal. Then the frame type
represented by the RTR bit of 11-bit standard identifier message
frames and the SRR bit of the 29-bit extended identifier message
frames is the next criteria to judge which message frame has to be
sent first. In this case data frames with 11-bit standard identifiers (i.e.
RTR bit cleared (0)) have a higher priority than remote frames with a
standard identifier and message frames with an extended identifier.
3
Identifier type rule
If even the priority 2 rule cannot deliver an unambiguous result, the
identifier type represented by the IDE bit is the next criteria in the
decision process. In this case a standard identifier message frame (i.e.
IDE bit cleared (0)) has a higher priority than a message frame with an
extended identifier.
4
18 LSB identifier value rule
[ID17 to ID0]
The next criteria to find the message that has to be sent first are the 18
least significant bits of the extended identifier. The message frame with
the lowest value represented by those bits is sent first.
The priority 4 rule applies in cases where two transmission-pending
extended identifier message frames have equal values in the 11 most
significant bits of the identifier and have the same frame type (the RTR
bit value is the same).
5 (lowest)
Message buffer number
The last criteria to find the message that has to be sent first is the
message buffer number. The priority 5 criteria applies when 2 or more
message buffers try to send message frames with exactly the same
identifier. In this case the message from the message buffer with the
lowest message buffer number is sent first.
Remark
When the CAN module operates in “normal operating mode with ABT” only one message buffer from
the ABT message buffer group has the TRQ bit set to 1 at a time. This buffer competes with transmit
message buffers, which do not belong to the ABT message buffers.
Within the ABT message buffers, a fixed order determines which message buffer is sent next.
Upon successful transmission of a message frame, the TRQ flag in the corresponding transmit message buffer is
automatically cleared (0) and the corresponding “transmit successful” interrupt status bit CINTS0 in the C0INTS
register is set (1). In addition, an interrupt request signal will be released, when it has been enabled by setting the
CIE0 enable bit in the C0IE register (1).