MC908MR16CFUE Datasheet, PDF(63/282 Page) Freescale Semiconductor, Inc – On-chip programming firmware for use with host personal computer, Clock generator module (CGM)

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MC908MR16CFUE Datasheet, PDF (63/282 Pages) Freescale Semiconductor, Inc – On-chip programming firmware for use with host personal computer, Clock generator module (CGM)

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Functional Description

two to correct the duty cycle. Therefore, the bus clock frequency, which is one-half of the base clock

frequency, is one-fourth the frequency of the selected clock (CGMXCLK or CGMVCLK).

The BCS bit in the PLL control register (PCTL) selects which clock drives CGMOUT. The VCO clock

cannot be selected as the base clock source if the PLL is not turned on. The PLL cannot be turned off if

the VCO clock is selected. The PLL cannot be turned on or off simultaneously with the selection or

deselection of the VCO clock. The VCO clock also cannot be selected as the base clock source if the

factor L is programmed to a 0. This value would set up a condition inconsistent with the operation of the

PLL, so that the PLL would be disabled and the crystal clock would be forced as the source of the base

clock.

4.3.4 CGM External Connections

In its typical configuration, the CGM requires seven external components. Five of these are for the crystal

oscillator and two are for the PLL.

The crystal oscillator is normally connected in a Pierce oscillator configuration, as shown in Figure 4-3.

Figure 4-3 shows only the logical representation of the internal components and may not represent actual

circuitry.

SIMOSCEN

CGMXCLK

OSC1

RB

X1

C1

OSC2

RS*

C2

VSS

CGMXFC

VDDA

CF

VDD

CBYP

*RS can be 0 (shorted) when used with

higher frequency crystals. Refer to

manufacturerâs data.

Figure 4-3. CGM External Connections

The oscillator configuration uses five components:

1. Crystal, X1

2. Fixed capacitor, C1

3. Tuning capacitor, C2 (can also be a fixed capacitor)

4. Feedback resistor, RB

5. Series resistor, RS (optional)

The series resistor (RS) is included in the diagram to follow strict Pierce oscillator guidelines and may not

be required for all ranges of operation, especially with high-frequency crystals. Refer to the crystal

manufacturerâs data for more information.

MC68HC908MR32 â¢ MC68HC908MR16 Data Sheet, Rev. 6.1

Freescale Semiconductor

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