MC68HC908RK2 Datasheet, PDF (122/232 Pages) Motorola, Inc – Microcontroller Unit

Internal Clock Generator Module (ICG)
The switch automatically selects ICLK during reset. When the clock
monitor is on (CMON is set) and it determines one of the clock sources
is inactive (as indicated by the IOFF or EOFF signals), the circuit is
forced to select the active clock. There are no clocks for the inactive side
of the synchronizer to properly operate, so that side is forced deselected.
However, the active side will not be selected until one to two clock cycles
after the IOFF or EOFF signal transitions.
8.5 Usage Notes
The ICG has several features which can provide protection to the
microcontroller if properly used. There are other features which can
greatly simplify usage of the ICG if certain techniques are employed.
This section will describe 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 DCO output
• Switching internal clock frequencies
• Nominal frequency settling time
• Improving frequency settling time
• Trimming frequency
Advance Information
122
Internal Clock Generator Module (ICG)
MC68HC908RK2 — Rev. 4.0
MOTOROLA