M68ICS08GR datasheet(102/408 Pages) FREESCALE

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Clock Generator Module (CGMC)

Technical Data

MC68HC908GR8 — Rev 4.0

102

Clock Generator Module (CGMC)

MOTOROLA

7.4.1 Crystal Oscillator Circuit

The crystal oscillator circuit consists of an inverting amplifier and an

external crystal. The OSC1 pin is the input to the amplifier and the OSC2

pin is the output. The SIMOSCEN signal from the system integration

module (SIM) or the OSCSTOPENB bit in the CONFIG register enable

the crystal oscillator circuit.

The CGMXCLK signal is the output of the crystal oscillator circuit and

runs at a rate equal to the crystal frequency. CGMXCLK is then buffered

to produce CGMRCLK, the PLL reference clock.

CGMXCLK can be used by other modules which require precise timing

for operation. The duty cycle of CGMXCLK is not guaranteed to be 50%

and depends on external factors, including the crystal and related

external components. An externally generated clock also can feed the

OSC1 pin of the crystal oscillator circuit. Connect the external clock to

the OSC1 pin and let the OSC2 pin float.

7.4.2 Phase-Locked Loop Circuit (PLL)

The PLL is a frequency generator that can operate in either acquisition

mode or tracking mode, depending on the accuracy of the output

frequency. The PLL can change between acquisition and tracking

modes either automatically or manually.

7.4.3 PLL Circuits

The PLL consists of these circuits:

•

Voltage-controlled oscillator (VCO)

•

Reference divider

•

Frequency prescaler

•

Modulo VCO frequency divider

•

Phase detector

•

Loop filter

•

Lock detector

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