MC908JL3ECPE Datasheet, PDF (67/180 Pages) Freescale Semiconductor, Inc – Microcontrollers

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_B
• 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:
• C_EXT
• R_EXT
The RC connection is shown in Figure 6-2.
MC68HC908JL3E Family Data Sheet, Rev. 4
Freescale Semiconductor
67