MC68HC908AT32 Datasheet, PDF (262/378 Pages) Freescale Semiconductor, Inc – Microcontrollers

MSCAN Controller
CGMXCLK
OSC
CGM
MSCAN08
÷2
PLL
CLKSRC
÷2
CGMOUT
(TO SIM)
BCS
÷2
(2 * BUS FREQ.)
PRESCALER
(1 .. 64)
MSCANCLK
Figure 23-7. Clocking Scheme
The MSCAN08 clock is used to generate the atomic unit of time handled by the MSCAN08: the time
quantum. A bit time is subdivided into three segments defined here. For further explanation of the
underlying concepts, refer to ISO/DIS 11519-1, Section 10.3.
• 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.
The synchronization jump width (SJW) can be programmed in a range of 1 to 4 time quanta by setting the
SJW parameter.
The parameters can be set by programming the bus timing registers, CBTR0–CBTR1 (see 23.13.3
MSCAN08 Bus Timing Register 0 and 23.13.4 MSCAN08 Bus Timing Register 1).
The user is responsible for making sure that the bit time settings comply with the CAN standard (see
Figure 23-8). Table 23-2 gives an overview on the CAN conforming segment settings and the related
parameter values.
MC68HC908AT32 Data Sheet, Rev. 3.1
262
Freescale Semiconductor