mc9s12xd256 Freescale Semiconductor, Inc, mc9s12xd256 Datasheet - Page 465

• Manufacturer Part Number: mc9s12xd256
• Description: Hcs12x Microcontrollers
• Manufacturer: Freescale Semiconductor, Inc

If the bus clock is generated from a PLL, it is recommended to select the oscillator clock rather than the
bus clock due to jitter considerations, especially at the faster CAN bus rates.
For microcontrollers without a clock and reset generator (CRG), CANCLK is driven from the crystal
oscillator (oscillator clock).
A programmable prescaler generates the time quanta (Tq) clock from CANCLK. A time quantum is the
atomic unit of time handled by the MSCAN.
A bit time is subdivided into three segments as described in the Bosch CAN specification. (see
Figure
Freescale Semiconductor
•
•
•
10-44):
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 PROP_SEG and the PHASE_SEG1 of the CAN
standard. It can be programmed by setting the parameter TSEG1 to consist of 4 to 16 time quanta.
Time Segment 2: This segment represents the PHASE_SEG2 of the CAN standard. It can be
programmed by setting the TSEG2 parameter to be 2 to 8 time quanta long.
Transmit Point
SYNC_SEG
1
Bit Rate
NRZ Signal
Figure 10-44. Segments within the Bit Time
MC9S12XDP512 Data Sheet, Rev. 2.17
(PROP_SEG + PHASE_SEG1)
Tq
=
=
8 ... 25 Time Quanta
-------------------------------------------------------------------------------- -
Time Segment 1
number of Time Quanta
= 1 Bit Time
----------------------------------------------------- -
Prescaler value
4 ... 16
Chapter 10 Freescale’s Scalable Controller Area Network (S12MSCANV3)
f
CANCLK
f
Tq
Sample Point
(single or triple sampling)
Time Segment 2
(PHASE_SEG2)
2 ... 8
Eqn. 10-2
Eqn. 10-3
465