MC68HC908RK2 Datasheet, PDF (59/158 Pages) Motorola, Inc – Microcontroller Unit

Chapter 6
Internal Clock Generator Module (ICG)
6.1 Introduction
The internal clock generator (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%)
• Frequency adjust (trim) register to improve variability to ±2%
• 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 6-2, contains these major submodules:
• Clock enable circuit
• Internal clock generator
• External clock generator
• Clock monitor circuit
• Clock selection circuit
6.3.1 Clock Enable Circuit
The clock enable circuit is used to enable the internal clock (ICLK) or external clock (ECLK). The clock
enable circuit generates an ICG stop (ICGSTOP) signal which stops all clocks (ICLK, ECLK, and the
low-frequency base clock, IBASE) low. ICGSTOP is set and the ICG is disabled in stop mode.
The internal clock enable signal (ICGEN) turns on the internal clock generator which generates ICLK.
ICGEN is set (active) whenever the ICGON bit is set and the ICGSTOP signal is clear. When ICGEN is
clear, ICLK and IBASE are both low.
The external clock enable signal (ECGEN) turns on the external clock generator which generates ECLK.
ECGEN is set (active) whenever the ECGON bit is set and the ICGSTOP signal is clear. When ECGEN
is clear, ECLK is low.
MC68HC908RK2 Data Sheet, Rev. 5.1
Freescale Semiconductor
59