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arbitration ends too early before the first bit of Intermission field, then there is a chance
that the CPU reconfigures some Tx MBs and the winner MB is no longer the best
candidate to be transmitted.
TASD is useful to optimize the transmission performance by defining the arbitration start
point, as shown in the next figure, based on factors such as:
• Peripheral-to-oscillator clock ratio
• CAN bit timing variables that determine the CAN bit rate
• Number of message buffers (MBs) in use by the matching and arbitration processes
CRC
EOF
Intermission
Arbitration
start point
A
C
K
S
O
F
Arbitration
Process
TASD countdown
Arbitration window (25 to 31 CAN bits)
9 last CAN bits
Nominal bit rate
Data bit rate when CAN FD is enabled
Figure 49-12. Optimal Tx Arbitration start point
The duration of an arbitration process, in terms of CAN bits, is directly proportional to
the number of available MBs and to the CAN bit rate, and inversely proportional to the
peripheral clock frequency.
The optimal arbitration timing is that in which the last MB is scanned just before the first
bit of the Intermission field of a CAN frame. For instance, if there are few MBs and the
peripheral/oscillator clock ratio is high and the CAN baud rate is low, then the arbitration
can be placed closer to the frame's end, adding more delay to its start point, and vice-
versa.
If TASD is set to 0 then the arbitration start is not delayed and more time is reserved for
arbitration. On the other hand, if TASD is close to 24 then the CPU can configure a Tx
MB later and less time is reserved for arbitration. If too little time is reserved for
arbitration the FlexCAN may be not be able to find a winner MB in time to be
transmitted with the best chance to win the bus arbitration against external nodes on the
CAN bus.
The optimal TASD value can be calculated as follows:
Functional description
MWCT101xS Series Reference Manual, Rev. 3, 07/2019
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NXP Semiconductors
Содержание MWCT101 S Series
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