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.
