M68HC08 datasheet(65/188 Pages) FREESCALE

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MC68HC908RF2 — Rev. 4.0

Data Sheet

MOTOROLA

Internal Clock Generator Module (ICG)

Data Sheet — MC68HC908RF2

Section 6. Internal Clock Generator Module (ICG)

6.1 Introduction

The internal clock generator module (ICG) is used to create a stable clock source

for the microcontroller without using any external components. The ICG generates

the oscillator output clock (CGMXCLK), which is used by the COP, LVI, and other

modules. The ICG also generates the clock generator output (CGMOUT), which is

fed to the system integration module (SIM) to create the bus clocks. The bus

frequency will be one-fourth the frequency of CGMXCLK and one-half the

frequency of CGMOUT.

6.2 Features

The ICG has these features:

•

External clock generator, either 1-pin external source or 2-pin crystal

•

Internal clock generator with programmable frequency output in integer

multiples of a nominal frequency (307.2 kHz

±25 percent)

•

Frequency adjust (trim) register to improve variability to

±2 percent

•

Bus clock software selectable from either internal or external clock

•

Clock monitor for both internal and external clocks

6.3 Functional Description

The ICG, shown in Figure 6-1, contains these major submodules:

•

Clock enable circuit

•

Internal clock generator

•

External clock generator

•

Clock monitor circuit

•

Clock selection circuit

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