Information

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.
f
Tq
=
f
CANCLK
(Prescaler Value)
A bit time is subdivided into three segments
2
(see Figure 45-67 and Table 45-79):
• 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 CTRL1 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 CTRL1 Register (plus 1) to
be 2 to 8 time quanta long
Bit Rate
=
f
Tq
(number of Time Quanta)
S Y N C _ S E G
Time Segment 2
2 ... 8
8 ... 25 Time Quanta
= 1 Bit Time
Sample Point
(single or triple sampling)
(PSEG2 + 1)
Transmit Point
Time Segment 1
1 4 ... 16
NRZ Signal
(PROP_SEG + PSEG1 + 2)
Figure 45-67. Segments within the bit time
2. For further explanation of the underlying concepts, see ISO/DIS 11519–1, Section 10.3. See also the CAN 2.0A/B
protocol specification for bit timing.
Chapter 45 CAN (FlexCAN)
K20 Sub-Family Reference Manual, Rev. 1.1, Dec 2012
Freescale Semiconductor, Inc.
Preliminary
1169
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