M68HC08 datasheet(71/172 Pages) FREESCALE

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MC68HC908JL3E Family Data Sheet, Rev. 3

Freescale Semiconductor

Chapter 6

Oscillator (OSC)

6.1 Introduction

The oscillator module provides the reference clock for the MCU system and bus. Two types of oscillator

modules are available:

•

MC68HC908JL3E/JK3E/JK1E — built-in oscillator module (X-tal) that requires an external crystal

or ceramic-resonator. This option also allows an external clock that can be driven directly into

OSC1.

•

MC68HRC908JL3E/JK3E/JK1E — built-in oscillator module (RC) that requires an external RC

connection only.

6.2 X-tal Oscillator (MC68HC908JL3E/JK3E/JK1E)

The X-tal oscillator circuit is designed for use with an external crystal or ceramic resonator to provide

accurate clock source.

In its typical configuration, the X-tal oscillator is connected in a Pierce oscillator configuration, as shown

in Figure 6-1. This figure shows only the logical representation of the internal components and may not

represent actual circuitry. The oscillator configuration uses five components:

•Crystal, X1

•

Fixed capacitor, C1

•

Tuning capacitor, C2 (can also be a fixed capacitor)

•

Feedback resistor, R

•

Series resistor, R

S (optional)

The series resistor (R

S) is included in the diagram to follow strict Pierce oscillator guidelines and may not

be required for all ranges of operation, especially with high frequency crystals. Refer to the crystal

manufacturer’s data for more information.

6.3 RC Oscillator (MC68HRC908JL3E/JK3E/JK1E)

The RC oscillator circuit is designed for use with external R and C to provide a clock source with tolerance

less than 10%.

In its typical configuration, the RC oscillator requires two external components, one R and one C.

Component values should have a tolerance of 1% or less, to obtain a clock source with less than 10%

tolerance. The oscillator configuration uses two components:

•CEXT

•REXT

The RC connection is shown in Figure 6-2.

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