MC68HC908GT16_07 Datasheet, PDF (77/292 Pages) Freescale Semiconductor, Inc – Microcontrollers

Chapter 7
Internal Clock Generator (ICG) Module)
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 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) and the computer operating properly (COP) clock (COPCLK) which is used by the COP
module.
7.2 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)
• Frequency adjust (trim) register to improve variability to ±4 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
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 from external if external clock is available
• Clock monitor for both internal and external clocks
7.3 Functional Description
The ICG, shown in Figure 7-2, contains these major submodules:
• Clock enable circuit
• Internal clock generator
• External clock generator
• Clock monitor circuit
• Clock selection circuit
MC68HC908GT16 • MC68HC908GT8 • MC68HC08GT16 Data Sheet, Rev. 5.0
Freescale Semiconductor
77