MC68HC908GZ8 Datasheet, PDF (176/344 Pages) Motorola, Inc – Microcontrollers

Freescale Semiconductor, Inc.
MSCAN08 Controller (MSCAN08)
NOTE:
The clock source bit (CLKSRC) in the MSCAN08 module control register (CMCR1)
(see 16.13.1 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.
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 16-8):
• 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
NOTE:
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 16.13.3 MSCAN08 Bus Timing Register 0 and 16.13.4
MSCAN08 Bus Timing Register 1.
It is the user’s responsibility to make sure that the bit timing settings are in
compliance with the CAN standard,
Table 16-8 gives an overview on the CAN conforming segment settings and the
related parameter values.
Data Sheet
176
1. For further explanation of the underlying concepts please refer to ISO/DIS 11 519-1,
Section 10.3.
MSCAN08 Controller (MSCAN08)
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MC68HC908GZ8
MOTOROLA