AN1709 datasheet(30/38 Pages) STMICROELECTRONICS

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EMC DESIGN GUIDE FOR ST MICROCONTROLLERS

4 EMC GUIDELINES FOR MCU BASED APPLICATIONS

The following guidelines result from experience gained in a wide variety of applications.

4.1 HARDWARE

The major noise receptors and generators are the tracks and wiring on the Printed Circuit

Board (PCB), especially those near the MCU. The first actions to prevent noise problems thus

concern the PCB layout and the design of the power supply.

In general, the smaller the number of components surrounding the MCU, the better the immu-

nity versus noise. A ROMless solution, for instance, is typically more sensitive to and a bigger

generator of noise than an embedded memory circuit.

Optimized PCB layout

Noise is basically received and transmitted through tracks and components which, once ex-

cited, act as antennas. Each loop and track includes parasitic inductance and capacitance

which radiate and absorb energy once submitted to a variation of current, voltage or electro-

magnetic flux.

An MCU chip itself presents high immunity to and low generation of EMI since its dimensions

are small versus the wave lengths of EMI signals (typically mm versus 10's of cm for EMI sig-

nals in the GHz range). So a single chip solution with small loops and short wires reduces

noise problems.

The initial action at the PCB level is to reduce the number of possible antennas. The loops and

wires connected to the MCU such as supply, oscillator and I/O should be considered with a

special attention. The oscillator loop has to be especially small since it operates at high fre-

quency Figure 18.

A reduction of both the inductance and the capacitance of a track is generally difficult. Prac-

tical experience suggests that in most cases the inductance is the first parameter to be mini-

mized.

AN1709 Datasheet(PDF) 30 Page - STMicroelectronics