MC68HC08AZ32A Datasheet, PDF (127/312 Pages) Motorola, Inc – HCMOS Microcontroller Unit

Clock System
The clock source bit (CLKSRC) in the MSCAN08 module control register (CMCR1) (see 11.13.1
MSCAN08 Module Control Register 0) defines whether the MSCAN08 is connected to the output of the
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.
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 11-9):
• 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
The synchronization jump width (SJW) can be programmed in a range of 1 to 4 time quanta by setting the
SJW parameter.
The above parameters can be set by programming the bus timing registers, CBTR0 and CBTR1. See
11.13.3 MSCAN08 Bus Timing Register 0 and 11.13.4 MSCAN08 Bus Timing Register 1.
NOTE
It is the user’s responsibility to make sure that the bit timing settings are in
compliance with the CAN standard,
Table 11-8 gives an overview on the CAN conforming segment settings and the related parameter values.
1. For further explanation of the underlying concepts please refer to ISO/DIS 11 519-1, Section 10.3.
MC68HC08AZ32A Data Sheet, Rev. 2
Freescale Semiconductor
127