单片型微控制器设计的电磁兼容性

中文字数:4987, 英文字数:3031

单片型微控制器设计的电磁兼容性

概述

 这篇应用笔记讨论的是怎样设计一个考虑电磁兼容性问题的单片机应用系统。今天，几乎所有的消费电子，汽车，以及工业应用产品内都包含一个微控制器，大多数情况下，它应该是一个低成本的单片型微控制器。单片型MCU是非常理想的，因为它的灵活性，并且所有的功能都集成在一片硅上。典型的MCU都有它们自己的微处理器，ROM和RAM存储器，输入输出口，并且还可以制定一些功能如模数、数模转换器、LCD驱动电路、屏幕显示、DTMF发生器、交流电动机驱动和电路非易失性数据存储器等。
 随着MCU的功能变得越来越复杂和市场价格的进一步降低，生产厂家必须不断的降低他们的生产成本。通过改变MCU上的晶体管的几何结构可以达到上述目的，这样同样使MCU产品具有更高的操作频率。
 晶体管门电路减小的同时，其导通时间也减小。通过傅立叶分析，快速的边沿触发信号将产生谐波。这些信号，如果被放大，将产生干扰问题。在一个相似的脉络中，如果一个设备有快速的信号转换时间，它将影响到输入信号，进而可以因为干扰信号的存在而导致错误的动作。现在，大多数的MCU的处理 是2~40MHZ，它们内部的器件的开关速度从几NS（微秒）NS以下，使它们存在潜在的电磁兼容性问题。

Designing for Electromagnetic
Compatibility with Single-Chip Microcontrollers
INTRODUCTION
 
 This application note discusses how to design a single-chip microcontroller application considering electromagnetic compatibility (EMC). Today almost every consumer, automotive, and industrial application has a microcontroller (MCU) inside. More often than not, it will be a low-cost, single-chip MCU. Single-chip MCUs are ideal because of the flexibility and functionality incorporated on one piece of silicon. Typical MCUs have their own CPU, RAM, ROM, and input/output (I/O) ports and can hav customized functions such as analog/digital modules, LCD drivers, on-screen display for television applications, dual-tone multifrequency (DTMF) generators for telephones, AC motor drive circuits, and EEPROM for non-volatile data storage.
 As MCU functionality increases becoming more complex and with market costs being driven lower, MCU producers must reduce their manufacturing costs continually. Reducing the geometries of the on-chip transistors and gates achieves this, and also helps produce MCUs capable of functioning at higher operating frequencies.
 As a transistor’s gate size is reduced, the transition time decreases, and, according to Fourier Analysis, fast edges on signals produce harmonic signals. These signals, if amplified, can cause emission problems. In a similar vein, if the devices have faster transition times, they can react to faster incoming signals, which can result in a gate being switched because of a high frequency noise spike and a false signal. Most modern MCUs operate with speeds ranging from 2 MHz to 40 MHz, with internal devices having switching speeds from a few nanoseconds to below a nanosecond, making them potential EMC problems.