Freescale Semiconductor, Inc.
MSCAN Controller
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.
For microcontrollers without the CGM module, CGMCANCLK is driven
from the crystal oscillator (EXTALi).
A programmable prescaler is used to generate out of msCANCLK the
time quanta (Tq) clock. A time quantum is the atomic unit of time handled
by the msCAN12.
f
CGMCANCLK
Presc ⋅ value
f
= ------------------------------------------
Tq
(1)
A bit time is subdivided into three segments :
• 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.
f
Tq
BitRate = ------------------------------------------------------------------------
number ⋅ of ⋅ TimeQuanta
The synchronisation jump width can be programmed in a range of 1 to 4
time quanta by setting the SJW parameter.
Above parameters can be set by programming the bus timing registers
(CBTR0–1, see msCAN12 Bus Timing Register 0 (CBTR0) and
msCAN12 Bus Timing Register 1 (CBTR1).).
NOTE: It is the user’s responsibility to make sure that his bit time settings are in
compliance with the CAN standard. Table 17-3 gives an overview on the
CAN conforming segment settings and the related parameter values.
1. For further explanation of the under-lying concepts please refer to ISO/DIS 11519-1, Section
10.3.
Advance Information
296
68HC(9)12D60 — Rev 4.0
MOTOROLA
MSCAN Controller
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