Datasheet

MC68HC908GP20 — Rev 2.1 Advance Information
Freescale Semiconductor 109
7.3 Features
Features of the CGMC include:
• Phase-locked loop with output frequency in integer multiples of an
integer dividend of the crystal reference
• Low-frequency crystal operation with low-power operation and
high-output frequency resolution
• Programmable prescaler for power-of-two increases in frequency
• Programmable hardware voltage-controlled oscillator (VCO) for
low-jitter operation
• Automatic bandwidth control mode for low-jitter operation
• Automatic frequency lock detector
• CPU interrupt on entry or exit from locked condition
• Configuration register bit to allow oscillator operation during stop
mode
7.4 Functional Description
The CGMC consists of three major submodules:
• Crystal oscillator circuit — The crystal oscillator circuit generates
the constant crystal frequency clock, CGMXCLK.
• Phase-locked loop (PLL) — The PLL generates the
programmable VCO frequency clock, CGMVCLK.
• Base clock selector circuit — This software-controlled circuit
selects either CGMXCLK divided by two or the VCO clock,
CGMVCLK, divided by two as the base clock, CGMOUT. The SIM
derives the system clocks from either CGMOUT or CGMXCLK.
Figure 7-1 shows the structure of the CGMC.