Freescale’s Scalable Controller Area Network (S12MSCANV3)
MC9S12XHY-Family Reference Manual Rev. 1.01
Freescale Semiconductor
385
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.
Eqn. 11-2
A bit time is subdivided into three segments as described in the Bosch CAN 2.0A/B specification. (see
Figure 11-44
):
•
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.
Eqn. 11-3
Figure 11-44. Segments within the Bit Time
Tq
f
CANCLK
Prescaler value
(
)
------------------------------------------------------
=
Bit Rate
f
Tq
number of Time Quanta
(
)
---------------------------------------------------------------------------------
=
SYNC_SEG
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)
(PR PHASE_SEG1)
(PHASE_SEG2)
Transmit Point
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