Freescale Semiconductor MC68HC908MR16 Crystal Oscillator Circuit, Phase-Locked Loop Circuit PLL

Functional Description

4.3.1 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) enables 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 percent 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.

4.3.2 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.

4.3.2.1 PLL Circuits

The PLL consists of these circuits:

•Voltage-controlled oscillator (VCO)

•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 CGMXFC 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 CGMXFC pin changes the frequency within this range. By design, fVRS is equal to the nominal center-of-range frequency, fNOM, (4.9152 MHz) times a linear factor, L or (L) fNOM.

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 buffer. The buffer output is the final reference clock, CGMRDV, running at a frequency, fRDV = fRCLK.

The VCO’s output clock, CGMVCLK, running at a frequency, fVCLK, is fed back through a programmable modulo divider. The modulo divider reduces the VCO clock by a factor, N. The divider’s output is the VCO

feedback clock, CGMVDV, running at a frequency, fVDV = fVCLK/N. (See 4.3.2.4 Programming the PLL for more information.)

The phase detector then compares the VCO feedback clock, CGMVDV, with the final reference clock, CGMRDV. A correction pulse is generated based on the phase difference between the two signals. The loop filter then slightly alters the dc voltage on the external capacitor connected to CGMXFC based on the width and direction of the correction pulse. The filter can make fast or slow corrections depending on its mode, described in 4.3.2.2 Acquisition and Tracking Modes. The value of the external capacitor and the reference frequency determines the speed of the corrections and the stability of the PLL.

MC68HC908MR32 • MC68HC908MR16 Data Sheet, Rev. 6.1

Freescale Semiconductor 59