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Chapter 25 FlexCAN
MPC5606BK Microcontroller Reference Manual, Rev. 2
586
Freescale Semiconductor
Figure 25-16. CAN Engine Clocking Scheme
The crystal oscillator clock should be selected whenever a tight tolerance (up to 0.1%) is required in the
CAN bus timing. The crystal oscillator clock has better jitter performance than PLL generated clocks.
The FlexCAN module supports a variety of means to set up bit timing parameters that are required by the
CAN protocol. The Control Register has various fields used to control bit timing parameters: PRESDIV,
PROPSEG, PSEG1, PSEG2, and RJW. See
Section 25.4.4.2, Control (CTRL) register.
The PRESDIV field controls a prescaler that generates the Serial Clock (Sclock), whose period defines the
time quantum used to compose the CAN waveform. A time quantum is the atomic unit of time handled by
the CAN engine.
Eqn. 25-1
A bit time is subdivided into three segments
1
):
•
SYNC_SEG: This segment has a fixed length of one time quantum. Signal edges are expected to
happen within this section.
•
Time Segment 1: This segment includes the Propagation Segment and the Phase Segment 1 of the
CAN standard. It can be programmed by setting the PROPSEG and the PSEG1 fields of CTRL so
that their sum (plus 2) is in the range of 4 to 16 time quanta.
•
Time Segment 2: This segment represents the Phase Segment 2 of the CAN standard. It can be
programmed by setting the PSEG2 field of CTRL (plus 1) to be 2 to 8 time quanta long.
Eqn. 25-2
1.
For further explanation of the underlying concepts please refer to ISO/DIS 11519
–
1, Section 10.3. Reference also the
Bosch CAN 2.0A/B protocol specification dated September 1991 for bit timing.
Peripheral Clock (PLL)
Oscillator Clock (Xtal)
CLK_SRC
Prescaler
(1 .. 256)
Sclock
CPI Clock
f
Tq
f
CANCLK
Prescaler
V
alue
Þ
--------------------------------------------------------
=
Bit
Rate
f
Tq
number
of
Time
Quanta
Þ
Þ
Þ
-----------------------------------------------------------------------------------------
=
Þ
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