MC68HC908AP16CFB datasheet(113/440 Pages) MOTOROLA

113 / 440 page

Clock Generator Module (CGM)

Functional Description

MC68HC908AP Family — Rev. 2.5

Data Sheet

MOTOROLA

113

8.3.7 Special Programming Exceptions

The programming method described in 8.3.6 Programming the PLL

does not account for three possible exceptions. A value of 0 for R, N, or

L is meaningless when used in the equations given. To account for these

exceptions:

•

A 0 value for R or N is interpreted exactly the same as a value of 1.

•

A 0 value for L disables the PLL and prevents its selection as the

source for the base clock.

(See 8.3.8 Base Clock Selector Circuit.)

8.3.8 Base Clock Selector Circuit

This circuit is used to select either the oscillator clock, CGMXCLK, or the

divided VCO clock, CGMPCLK, as the source of the base clock,

CGMOUT. The two input clocks go through a transition control circuit

that waits up to three CGMXCLK cycles and three CGMPCLK cycles to

change from one clock source to the other. During this time, CGMOUT

is held in stasis. The output of the transition control circuit is then divided

by two to correct the duty cycle. Therefore, the bus clock frequency,

which is one-half of the base clock frequency, is one-fourth the

frequency of the selected clock (CGMXCLK or CGMPCLK).

The BCS bit in the PLL control register (PCTL) selects which clock drives

CGMOUT. The divided VCO clock cannot be selected as the base clock

source if the PLL is not turned on. The PLL cannot be turned off if the

divided VCO clock is selected. The PLL cannot be turned on or off

simultaneously with the selection or deselection of the divided VCO

clock. The divided VCO clock also cannot be selected as the base clock

source if the factor L is programmed to a 0. This value would set up a

condition inconsistent with the operation of the PLL, so that the PLL

would be disabled and the oscillator clock would be forced as the source

of the base clock.

Freescale Semiconductor, Inc.

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Go to: www.freescale.com

MC68HC908AP16CFB Datasheet(PDF) 113 Page - Motorola, Inc