Datasheet

MSCAN08 Controller (MSCAN08)
MC68HC908GZ16 • MC68HC908GZ8 Data Sheet, Rev. 4
134 Freescale Semiconductor
Figure 12-8. Clocking Scheme
NOTE
If the system clock is generated from a PLL, it is recommended to select the
crystal clock source rather than the system clock source due to jitter
considerations, especially at faster CAN bus rates.
A programmable prescaler is used to generate out of the MSCAN08 clock the time quanta (Tq) clock. A
time quantum is the atomic unit of time handled by the MSCAN08.
A bit time is subdivided into three segments
(1)
(see Figure 12-9):
• 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 PHASE_SEG2 of the CAN standard. It can be
programmed by setting the TSEG2 parameter to be 2 to 8 time quanta long.
1. For further explanation of the underlying concepts please refer to ISO/DIS 11 519-1, Section 10.3.
PLL
÷ 2
MSCAN08
PRESCALER
(1 ... 64)
OSC
CGMXCLK
÷ 2
CGMOUT
(TO SIM)
CGM
÷ 2
CLKSRC
MSCANCLK
(2 * BUS FREQUENCY)
BCS
f
Tq
=
f
MSCANCLK
Presc value
Bit rate =
No. of time quanta
f
Tq