MC68HC908GT16 Datasheet, PDF (125/412 Pages) Motorola, Inc – Microcontrollers

Internal Clock Generator (ICG) Module
Usage Notes
7.5 Usage Notes
The ICG has several features which can provide protection to the
microcontroller if properly used. Other features can greatly simplify
usage of the ICG if certain techniques are employed. This section
describes several possible ways to use the ICG and its features. These
techniques are not the only ways to use the ICG and may not be
optimum for all environments. In any case, these techniques should be
used only as a template, and the user should modify them according to
the application’s requirements.
These notes include:
• Switching clock sources
• Enabling the clock monitor
• Using clock monitor interrupts
• Quantization error in digitally controlled oscillator (DCO) output
• Switching internal clock frequencies
• Nominal frequency settling time
• Improving frequency settling time
• Trimming frequency
7.5.1 Switching Clock Sources
Switching from one clock source to another requires both clock sources
to be enabled and stable. A simple flow requires:
• Enable desired clock source
• Wait for it to become stable
• Switch clocks
• Disable previous clock source
The key point to remember in this flow is that the clock source cannot be
switched (CS cannot be written) unless the desired clock is on and
stable. A short assembly code example of how to employ this flow is
MC68HC908GT16 • MC68HC908GT8 — Rev. 2
MOTOROLA
Internal Clock Generator (ICG) Module
Technical Data
125