MC68HC908GR4MP datasheet(71/286 Pages) FREESCALE

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Functional Description

MC68HC908GR8 • MC68HC908GR4 Data Sheet, Rev. 7

Freescale Semiconductor

7.3.1 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.

7.3.2 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.

7.3.3 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

The operating range of the VCO is programmable for a wide range of frequencies and for maximum

immunity to external noise, including supply and CGM/XFC noise. The VCO frequency is bound to a

range from roughly one-half to twice the center-of-range frequency, fVRS. Modulating the voltage on the

CGM/XFC pin changes the frequency within this range. By design, fVRS is equal to the nominal

center-of-range frequency, fNOM, (38.4 kHz) times a linear factor, L, and a power-of-two factor, E, or

× 2E)f

NOM.

CGMRCLK is the PLL reference clock, a buffered version of CGMXCLK. CGMRCLK runs at a frequency,

fRCLK, and is fed to the PLL through a programmable modulo reference divider, which divides fRCLK by a

factor, R. The divider’s output is the final reference clock, CGMRDV, running at a frequency,

fRDV =fRCLK/R. With an external crystal

(30 kHz–100 kHz), always set R = 1 for specified performance. With an external high-frequency clock

source, use R to divide the external frequency to between 30 kHz and 100 kHz.

The VCO’s output clock, CGMVCLK, running at a frequency, fVCLK, is fed back through a programmable

prescale divider and a programmable modulo divider. The prescaler divides the VCO clock by a

power-of-two factor P and the modulo divider reduces the VCO clock by a factor, N. The dividers’

output is the VCO feedback clock, CGMVDV, running at a frequency, fVDV =fVCLK/(N × 2

P). (See

7.3.6 Programming the PLL for more information.)

MC68HC908GR4MP Datasheet(PDF) 71 Page - Freescale Semiconductor, Inc

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