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 right 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:
TASD = 25 -
x [1 + (PSEG1 + 1) + (PSEG2 + 1) + (P 1) ] x (P 1)
f SYS
fCANCLK x [ MAXMB + 3 - (RFEN x 8) - (RFEN x RFFN x 2) ] x 2
where:
• MAXMB is the value in CAN_CTRL1[MAXMB] field
• f
CANCLK
is the oscillator clock, in Hz
• f
SYS
is the peripheral clock, in Hz
• RFEN is the value in CAN_CTRL1[RFEN] bit
• RFFN is the value in CAN_CTRL2[RFFN] field
• PSEG1 is the value in CAN_CTRL1[PSEG1] field
• PSEG2 is the value in CAN_CTRL1[PSEG2] field
• PROPSEG is the value in CAN_CTRL1[PROPSEG] field
• PRESDIV is the value in CAN_CTRL1[PRESDIV] field
See also
50.5.9 Clock domains and restrictions
The FlexCAN module has two clock domains asynchronous to each other:
Chapter 50 CAN (FlexCAN)
Kinetis KE1xF Sub-Family Reference Manual, Rev. 4, 06/2019
NXP Semiconductors
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