mc68hc08ln56 Freescale Semiconductor, Inc, mc68hc08ln56 Datasheet - Page 39

• Manufacturer Part Number: mc68hc08ln56
• Description: M68hc08 Microcontrollers
• Manufacturer: Freescale Semiconductor, Inc

Chapter 4
Clock Generator Module (CGMB)
4.1 Introduction
This section describes the clock generator module (CGM, Version B). The CGM generates the crystal
clock signal, CGMXCLK, which operates at the frequency of the crystal. The CGM 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. This is the clock from which the SIM derives the system
clocks, including the bus clock, which is at a frequency of CGMOUT/2. 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.
4.2 Features
Features of the CGMB include:
4.3 Functional Description
The CGMB consists of three major submodules:
Figure 4-1
Freescale Semiconductor
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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 Reference Divider for Even Greater 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
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 CGMOUT.
shows the structure of the CGM.
MC68HC08LN56 • MC68HC708LN56 General Release Specification, Rev. 2.1
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