April 2007

Rev 1

1/14

AN2441

Application note

Low cost effective oscillator for STR71x MCUs

Introduction

The STR71x 32-bit MCU family from STMicroelectronics runs with an external oscillator

which is connected to the CK pin.

A straightforward solution for the external oscillator is to purchase an oscillator chip which

fits the application requirements. However, this solution is normally costly. This application

note gives the user a low cost oscillator solution with discrete components and based on a

resonator or quartz.

Resonators have shorter start-up time compared to crystals but they have less frequency

accuracy. Depending on the application requirements, users can choose between short

start-up time and frequency accuracy. In this application note, a quartz crystal is used in the

oscillator circuitry.

There are two primary quartz-controlled oscillators circuitries. Such circuitries are generally

described by the type of crystal unit used:

The advantages of the second solution are low cost and low power consumption.

The aim of this application note is to provide a methodology to design an oscillator solution

based on a quartz crystal operating in parallel resonance mode for the STR71x.

This document is split into two main sections. The first provides a brief description of both

oscillator theory and the quartz characteristics while the second proposes an oscillator

design and details the related components selection.

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