Chapter 11 Freescale’s Scalable Controller Area Network (S12MSCANV3)
S12ZVHY/S12ZVHL Family Reference Manual Rev. 1.05
Freescale Semiconductor
465
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
•
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
Þ
--------------------------------------------------------
=
Bit
Rate
f
Tq
number
of
Time
Quanta
Þ
Þ
Þ
---------------------------------------------------------------------------------------------
=
Þ
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
