MPC5566 Microcontroller Reference Manual, Rev. 2
22-30 Freescale Semiconductor
22.4.5.3 Time Stamp
The value of the free running timer is sampled at the beginning of the identifier field on the CAN bus, and
is stored at the end of ‘move in’ in the TIME STAMP field, providing network behavior with respect to
time.
The free running timer can be reset upon a specific frame reception, enabling network time
synchronization. See TSYN description in Section 22.3.3.2, “Control Register (CANx_CR).”
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
(see Figure 22-14 and Table 22-17):
• 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.
1. For further explanation of the underlying concepts see ISO/DIS 11519–1, Section 10.3. See 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()
---------------------------------------------------------------------=
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