mc9s12q128 Freescale Semiconductor, Inc, mc9s12q128 Datasheet - Page 332

• Manufacturer Part Number: mc9s12q128
• Description: Hcs12 Microcontrollers
• Manufacturer: Freescale Semiconductor, Inc

Chapter 10 Freescale’s Scalable Controller Area Network (S12MSCANV2)
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
43):
332
•
•
•
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 10-43. Segments within the Bit Time
(PROP_SEG + PHASE_SEG1)
Tq
=
=
8 ... 25 Time Quanta
-------------------------------------------------------------------------------- -
Time Segment 1
number of Time Quanta
----------------------------------------------------- -
= 1 Bit Time
4 ... 16
Prescaler value
MC9S12Q128
Rev 1.09
f
CANCLK
f
Tq
Sample Point
(single or triple sampling)
Time Segment 2
(PHASE_SEG2)
2 ... 8
Freescale Semiconductor
Figure 10-
Eqn. 10-2
Eqn. 10-3