MC68908GZ8MFJE Datasheet, PDF (134/314 Pages) Freescale Semiconductor, Inc – Microcontrollers

MSCAN08 Controller (MSCAN08)
CGMXCLK
OSC
CGM
MSCAN08
÷2
PLL
CLKSRC
÷2
CGMOUT
(TO SIM)
BCS
÷2
(2 * BUS FREQUENCY)
PRESCALER
(1 ... 64)
MSCANCLK
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.
f_Tq =
f_MSCANCLK
Presc value
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.
f_Tq
Bit rate =
No. of time quanta
1. For further explanation of the underlying concepts please refer to ISO/DIS 11 519-1, Section 10.3.
MC68HC908GZ16 • MC68HC908GZ8 Data Sheet, Rev. 4
134
Freescale Semiconductor