MC908QT4MPE datasheet(89/184 Pages) FREESCALE

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MC68HC908QY/QT Family Data Sheet, Rev. 5

Freescale Semiconductor

Chapter 11

Oscillator Module (OSC)

11.1 Introduction

The oscillator module is used to provide a stable clock source for the microcontroller system and bus. The

oscillator module generates two output clocks, BUSCLKX2 and BUSCLKX4. The BUSCLKX4 clock is

used by the system integration module (SIM) and the computer operating properly module (COP). The

BUSCLKX2 clock is divided by two in the SIM to be used as the bus clock for the microcontroller.

Therefore the bus frequency will be one fourth of the BUSCLKX4 frequency.

11.2 Features

The oscillator has these four clock source options available:

Internal oscillator: An internally generated, fixed frequency clock, trimmable to ±5%.This is the

default option out of reset.

External oscillator: An external clock that can be driven directly into OSC1.

External RC: A built-in oscillator module (RC oscillator) that requires an external R connection only.

The capacitor is internal to the chip.

External crystal: A built-in oscillator module (XTAL oscillator) that requires an external crystal or

ceramic-resonator.

11.3 Functional Description

The oscillator contains these major subsystems:

•

Internal oscillator circuit

•

Internal or external clock switch control

•

External clock circuit

•

External crystal circuit

•

External RC clock circuit

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