MC68HC908EY16 Datasheet, PDF (77/278 Pages) Motorola, Inc – Microcontrollers

Chapter 8
Internal Clock Generator (ICG) Module
8.1 Introduction
The internal clock generator (ICG) module 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).
8.2 Features
The ICG has these features:
• Selectable external clock generator, either 1-pin external source or 2-pin crystal, multiplexed with
port pins
• Internal clock generator with programmable frequency output in integer multiples of a nominal
frequency (307.2 kHz ± 25 percent)
• Internal oscillator trimmed accuracy of ±3.5 percent
• Bus clock software selectable from either internal or external clock (bus frequency range from 76.8
kHz ± 25 percent to 9.75 MHz ± 25 percent in 76.8-kHz increments)
NOTE
For the MC68HC908EY16, do not exceed the maximum bus frequency of
8 MHz at 5.0 V.
• Timebase clock automatically selected from external clock if external clock is available
• Clock monitor for both internal and external clocks
8.3 Functional Description
The ICG, shown in Figure 8-2, contains these major submodules:
• Clock enable circuit
• Internal clock generator
• External clock generator
• Clock monitor circuit
• Clock selection circuit
MC68HC908EY16 • MC68HC908EY8 Data Sheet, Rev. 10
Freescale Semiconductor
77