MPC5553/MPC5554 Microcontroller Reference Manual, Rev. 5
22-28 Freescale Semiconductor
22.4.5.4 Protocol Timing
The clock source to the CAN protocol interface (CPI) can be either the system clock or a direct feed from
the oscillator pin EXTAL. The clock source is selected by the CLK_SRC bit in the CAN_CR. The clock
is fed to the prescaler to generate the serial clock (SCK).
The FlexCAN2 module supports a variety of means to setup bit timing parameters that are required by the
CAN protocol. The CANx_CR has various fields used to control bit timing parameters: PRESDIV,
PROPSEG, PSEG1, PSEG2 and RJW. See Section 22.3.3.2, “Control Register (CANx_CR).”
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
(reference Figure 22-13 and Table 22-16):
• 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 22-13. 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)
(PROPSEG + PSEG1 + 2) (PSEG2 + 1)
Transmit Point
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