MC68HC908GR4MP datasheet(69/286 Pages) FREESCALE

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MC68HC908GR8 • MC68HC908GR4 Data Sheet, Rev. 7

Freescale Semiconductor

Chapter 7

Clock Generator Module (CGMC)

7.1 Introduction

This section describes the clock generator module. The CGMC generates the crystal clock signal,

CGMXCLK, which operates at the frequency of the crystal. The CGMC also generates the base clock

signal, CGMOUT, which is based on either the crystal clock divided by two or the phase-locked loop (PLL)

clock, CGMVCLK, divided by two. In user mode, CGMOUT is the clock from which the SIM derives the

system clocks, including the bus clock, which is at a frequency of CGMOUT/2. In monitor mode, PTC3

determines the bus clock. The PLL is a fully functional frequency generator designed for use with crystals

or ceramic resonators. The PLL can generate an 8-MHz bus frequency using a 32-kHz crystal.

7.2 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

7.3 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.

MC68HC908GR4MP Datasheet(PDF) 69 Page - Freescale Semiconductor, Inc

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