1 configuring the flexcan bit timing, Configuring the flexcan bit timing -29, Figure 21-14 – Freescale Semiconductor MCF5480 User Manual
Page 599: Table 21-16
Functional Overview
MCF548x Reference Manual, Rev. 3
Freescale Semiconductor
21-29
•
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
CANCTRL 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 CANCTRL register (plus 1) to be 2 to 8 time quanta
long.
Eqn. 21-1
Figure 21-14. Segments within the Bit Time
gives an overview of the CAN compliant segment settings and the related parameter values.
NOTE
It is the user’s responsibility to ensure the bit time settings are in compliance
with the CAN standard. For bit time calculations, use an IPT (Information
Processing Time) of 2, which is the value implemented in the FlexCAN
module.
21.4.9.1
Configuring the FlexCAN Bit Timing
The following considerations must be observed when programming bit timing functions:
•
If the programmed PRESDIV value results in a single system clock per one time quantum, then the
PSEG2 field in CANCTRL register should not be programmed to zero.
•
If the programmed PRESDIV value results in a single system clock per one time quantum, then the
information processing time (IPT) equals three time quanta, otherwise it equals two 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.
Table 21-16. Time Segment Syntax
Syntax
Description
SYNC_SEG
System expects transitions to occur on the bus during this period.
Transmit Point
A node in transmit mode transfers a new value to the CAN bus at this point.
Sample Point
A node samples the bus at this point. If the three samples per bit option is
selected, then this point marks the position of the third sample.
Bit Rate
f
Tq
(number of Time Quanta)
-------------------------------------------------------------
=
NRZ Signal
Time Segment 1
(PROP_SEG + PSEG1 + 2)
Time Segment 2
(PSEG2 + 1)
SYNC_SEG
1
4 ... 16
2 ... 8
8 ... 25 Time Quanta = 1 Bit Time
Sample Point
(Single or Triple Sampling)
Transmit Point