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MPC563XM Reference Manual, Rev. 1
Freescale Semiconductor
1307
Preliminary—Subject to Change Without Notice
Figure 28-16. 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.
NOTE
This clock selection feature may not be available in all MCUs. A particular
MCU may not have a PLL, in which case it would have only the oscillator
clock, or it may use only the PLL clock feeding the FlexCAN module. In
these cases, the CLK_SRC bit in the CTRL Register has no effect on the
module operation.
The FlexCAN module supports a variety of means to setup 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 28.4.4.2, “Control Register (CTRL).”
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.
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 the CTRL
Register so that their sum (plus 2) is in the range of 4 to 16 time quanta
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
(
)
--------------------------------------------------------
=
Содержание MPC5632M
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