68HC908RF datasheet(76/188 Pages) MOTOROLA

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Internal Clock Generator Module (ICG)

Data Sheet

MC68HC908RF2 — Rev. 4.0

Internal Clock Generator Module (ICG)

MOTOROLA

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.

6.4 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

6.4.1 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 shown in Figure 6-7.

This code is for illustrative purposes only and does not represent valid syntax for

any particular assembler.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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