AN2441 Datasheet, PDF (1/14 Pages) STMicroelectronics – Low cost effective oscillator for STR71x MCUs

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:
■ Series resonant oscillator (when a series resonant quartz crystal is used)
■ parallel resonant oscillator (when a parallel resonant quartz crystal is 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.
April 2007
Rev 1
1/14
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