Preface
MPC5565 Microcontroller Reference Manual, Rev. 1.0
21-30
Freescale Semiconductor
The PRESDIV field controls a prescaler that generates the serial clock (SCK), whose period defines the
‘time quantum’ used to compose the CAN waveform. A time quantum is the atomic unit of time handled
by FlexCAN.
A bit time is subdivided into three segments
1
):
•
SYNCSEG: 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.
•
Time segment 2: This segment represents the phase segment 2 of the CAN standard. It can be
programmed by setting the PSEG2 field of the CTRL register (plus 1) to be 2 to 8 time quanta long.
Figure 21-14. Segments within the Bit Time
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.
f
Tq
f
CANCLK
Prescaler Value
------------------------------------------
=
Bit rate
f
Tq
Number of Time Quanta
(
)
---------------------------------------------------------------------
=
SYNCSEG
Time segment 1
Time segment 2
1
4 ... 16
2 ... 8
8 ... 25 time quanta = 1 bit time
NRZ signal
Sample point
(single or triple sampling)
(P PSEG1 + 2)
(PSEG2 + 1)
Transmit Point
Содержание MPC5565
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