HC705RC17GRSSLASHD datasheet(85/126 Pages) NXP

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Phase-Locked Loop (PLL) Synthesizer

Phase-Locked Loop Control Register

MC68HC705RC17 — Rev. 2.0

General Release Specification

Phase-Locked Loop (PLL) Synthesizer

10.3 Phase-Locked Loop Control Register

This read/write register contains the control bits that select the PLL

frequency and enable/disable the synthesizer.

BCS — Bus Clock Select

When this bit is set, the output of the PLL is used to generate the

internal processor clock. When clear, the internal bus clock is driven

by the crystal (OSC1

÷2). Once BCS has been changed, it may take up

to 1.5 OSC1 cycles + 1.5 PLLOUT cycles to make the transition.

During the transition, the clock select output will be held low and all

CPU and timer activity will cease until the transition is complete.

Before setting BCS, allow at least a time of tPLLS after PLLON is set.

This bit should not be set unless the PLLON bit is already set on a

previous instruction. Reset clears this bit.

BWC — Bandwidth Control

This bit selects high bandwidth control when set and low bandwidth

control when clear. The low bandwidth driver is always enabled, so

this bit determines whether the high bandwidth driver is on or off.

When the PLL is turned on, the BWC bit should be set to 1 for a time

90% tPLLS to allow the PLL time to acquire a frequency close to the

desired frequency. The BWC bit should then be cleared and software

should delay for a time, 10% tPLLS, to allow the PLL time to make the

final adjustments. The PLL clock cannot be used (BCS bit set).

Although it is NOT prohibited in hardware, the BCS bit should not be

set unless the BWC bit is cleared and the proper delay times have

been followed. The PLL will generate a lower jitter clock when the

BWC bit is cleared. Reset clears this bit.

Address:

$0007

Bit 7

654321

Bit 0

Read:

BCS

BWC

PLLON

VCOTST

PS1

PS0

Write:

Reset:

00001101

Figure 10-2. Phase-Locked Loop Control Register (PLLCR)

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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