MC68HC908AS60ACFN FREESCALE [Freescale Semiconductor, Inc], MC68HC908AS60ACFN Datasheet - Page 279

• Manufacturer Part Number: MC68HC908AS60ACFN
• Description: M68HC08 Microcontrollers
• Manufacturer: FREESCALE [Freescale Semiconductor, Inc]

The clock source bit (CLKSRC) in the MSCAN08 module control register (CMCR1) (see
MSCAN08 Module Control Register
crystal oscillator or to the PLL output.
The clock source has to be chosen such that the tight oscillator tolerance requirements (up to 0.4%) of
the CAN protocol are met.
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. For further explanation of the underlying concepts please refer to ISO/DIS 11 519-1, Section 10.3.
Freescale Semiconductor
•
•
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.
CGM
MSCAN08
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.
MC68HC908AZ60A • MC68HC908AS60A • MC68HC908AS60E Data Sheet, Rev. 6
OSC
÷ 2
CGMXCLK
Figure 23-7. Clocking Scheme
0) defines whether the MSCAN08 is connected to the output of the
PLL
CLKSRC
f
Tq
=
(1)
(see
Presc value
f
MSCANCLK
NOTE
Figure
÷ 2
÷ 2
23-8).
PRESCALER
(1 .. 64)
BCS
(2 * BUS FREQ.)
MSCANCLK
23.13.1
CGMOUT
(TO SIM)
Clock System
279