PXD20RM Datasheet, PDF (804/1628 Pages) Freescale Semiconductor, Inc – PXD20 Microcontroller

20.5.8.3 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.
Note that the Free Running Timer can be reset upon a specific frame reception, enabling network time
synchronization. Refer to TSYN description in Section 20.4.4.2, Control Register (CTRL).
20.5.8.4 Protocol timing
Figure 20-16 shows the structure of the clock generation circuitry that feeds the CAN Protocol Interface
(CPI) sub-module. The clock source bit (CLK_SRC) in the CTRL Register defines whether the internal
clock is connected to the output of a crystal oscillator (Oscillator Clock) or to the Peripheral Clock
(generally from a PLL). In order to guarantee reliable operation, the clock source should be selected while
the module is in Disable Mode (bit MDIS set in the Module Configuration Register).
Peripheral Clock (PLL)
CPI Clock
Prescaler
(1 .. 256)
Sclock
Oscillator Clock (Xtal)
CLK_SRC
Figure 20-16. CAN engine clocking scheme
The crystal oscillator clock should be selected whenever a tight tolerance (up to 0.1%) is required in the
CAN bus timing. The crystal oscillator clock has better jitter performance than PLL generated clocks.
NOTE
This clock selection feature may not be available in all MCUs. A particular
MCU may not have a PLL, in which case it would have only the oscillator
clock, or it may use only the PLL clock feeding the FlexCAN module. In
these cases, the CLK_SRC bit in the CTRL Register has no effect on the
module operation.
The FlexCAN module supports a variety of means to setup bit timing parameters that are required by the
CAN protocol. The Control Register has various fields used to control bit timing parameters: PRESDIV,
PROPSEG, PSEG1, PSEG2 and RJW. See Section 20.4.4.2, Control Register (CTRL).
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--^C---^A----^N----^C----^L---^K------------------
Prescaler Þ Value
20-38
PXD20 Microcontroller Reference Manual, Rev. 1
Preliminary—Subject to Change Without Notice
Freescale Semiconductor