SPMC68CK338CPV14 datasheet(18/133 Pages) FREESCALE

18 / 133 page

MOTOROLA

MC68CK338

MC68CK338TS/D

Figure 7 System Clock Filter Network

When the clock synthesizer is used, SYNCR determines the operating frequency of the MCU. The fol-

lowing equation relates the MCU operating frequency to the clock synthesizer reference frequency (fref)

and the W, X, and Y fields in the SYNCR:

The W bit controls a prescaler tap in the feedback divider. Setting W increases VCO speed by a factor

of four. The Y field determines the count modulus for a modulo 64 downcounter, causing it to divide by

a value of Y+1. When W or Y changes, VCO frequency (fVCO) changes, and the VCO must relock.

The X bit controls a divide-by-two circuit that is not in the synthesizer feedback loop. When X=0 (reset

state), the divider is enabled, and the system clock is one-fourth the VCO frequency. Setting X=1

disables the divider, doubling the clock speed without changing the VCO frequency. There is no relock

delay when clock speed is changed by the X bit.

Internal VCO frequency is determined by the following equations:

For the MCU to operate correctly, system clock and VCO frequencies selected by the W, X, and Y bits

must be within the limits specified for the MCU. Do not use a combination of bit values that selects either

an operating frequency or a VCO frequency greater than the maximum specified values.

3.3.3 Clock Synthesizer Control

The clock synthesizer control circuits determine system clock frequency and clock operation under spe-

cial circumstances, such as following loss of synthesizer reference or during low-power operation. Clock

source is determined by the logic state of the MODCLK pin during reset.

NOTES:

1. Ensure that initialization software does not change the value of this bit (it should always be zero).

SYNCR — Clock Synthesizer Control Register

$YFFA04

EDIV

STCPU

RSVD1

SLOCK

RSVD1

STSIM

STEXT

RESET:

32 XFC CONN

* MAINTAIN LOW LEAKAGE ON THE XFC NODE.

0.01

µF

0.1

µF

XFC*

0.1

µF

VDDSYN

VSSI

sys

ref Y1

() 2

()

VCO

sys if X = 0

VCO

sys if X = 1

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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