Freescale’s Scalable Controller Area Network (S12MSCANV3)
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. 8-2
f
CANCLK
= -----------------------------------------------------
Tq
(Prescaler value)
A bit time is subdivided into three segments as described in the Bosch CAN specification. (see Figure 8-
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. 8-3
f
Tq
Bit Rate= --------------------------------------------------------------------------------
(number of Time Quanta)
NRZ Signal
Time Segment 1
Time Segment 2
(PHASE_SEG2)
SYNC_SEG
1
(PROP_SEG + PHASE_SEG1)
4 ... 16
2 ... 8
8 ... 25 Time Quanta
= 1 Bit Time
Transmit Point
Sample Point
(single or triple sampling)
Figure 8-44. Segments within the Bit Time
S12P-Family Reference Manual, Rev. 1.13
294
Freescale Semiconductor
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