Datasheet

MC68HC908KX8 • MC68HC908KX2 • MC68HC08KX8 Data Sheet, Rev. 2.1
Freescale Semiconductor 67
Chapter 7
Internal Clock Generator Module (ICG)
7.1 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 computer operating properly (COP), low-voltage inhibit (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. Finally, the ICG generates the timebase clock (TBMCLK), which
is used in the timebase module (TBM).
7.2 Features
Features of the ICG include:
• Selectable external clock generator, either one pin external source or two pin crystal, multiplexed
with port pins
• Internal clock generator with programmable frequency output in integer multiples of a nominal
frequency (307.2 kHz ± 25%)
• Frequency adjust (trim) register to improve variability to ± 2%
• Bus clock software selectable from either internal or external clock (bus frequency range from 76.8
kHz ± 25% to 9.75 MHz ± 25% in 76.8 kHz increments — note that for the MC68HC908KX8,
MC68HC908KX2, and MC68HC08KX8, do not exceed the maximum bus frequency of 8 MHz at
5.0 V and 4 MHz at 3.0 V
• Timebase clock automatically selected externally, if external clock is available
• Clock monitor for both internal and external clocks
7.3 Functional Description
As shown in Figure 7-1, the ICG contains these major submodules:
• Clock enable circuit
• Internal clock generator
• External clock generator
• Clock monitor circuit
• Clock selection circuit