M68ICS08GR datasheet(105/408 Pages) FREESCALE

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Clock Generator Module (CGMC)

Functional Description

MC68HC908GR8 — Rev 4.0

Technical Data

MOTOROLA

Clock Generator Module (CGMC)

105

to use as the source for the base clock. (See Base Clock Selector

Circuit.) If the VCO is selected as the source for the base clock and the

LOCK bit is clear, the PLL has suffered a severe noise hit and the

software must take appropriate action, depending on the application.

(See Interrupts for information and precautions on using interrupts.)

The following conditions apply when the PLL is in automatic bandwidth

control mode:

•

The ACQ bit (see PLL Bandwidth Control Register) is a read-only

indicator of the mode of the filter. (See Acquisition and Tracking

Modes.)

•

The ACQ bit is set when the VCO frequency is within a certain

tolerance and is cleared when the VCO frequency is out of a

certain tolerance. (See Acquisition/Lock Time Specifications for

more information.)

•

The LOCK bit is a read-only indicator of the locked state of the PLL.

•

The LOCK bit is set when the VCO frequency is within a certain

tolerance and is cleared when the VCO frequency is out of a

certain tolerance. (See Acquisition/Lock Time Specifications for

more information.)

•

CPU interrupts can occur if enabled (PLLIE = 1) when the PLL’s

lock condition changes, toggling the LOCK bit. (See PLL Control

The PLL also may operate in manual mode (AUTO = 0). Manual mode is

used by systems that do not require an indicator of the lock condition for

proper operation. Such systems typically operate well below fBUSMAX.

The following conditions apply when in manual mode:

•

ACQ is a writable control bit that controls the mode of the filter.

Before turning on the PLL in manual mode, the ACQ bit must be

clear.

•

Before entering tracking mode (ACQ = 1), software must wait a

given time, tACQ (see Acquisition/Lock Time Specifications), after

turning on the PLL by setting PLLON in the PLL control register

(PCTL).

Freescale Semiconductor, Inc.

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