AN1709 datasheet(33/38 Pages) STMICROELECTRONICS

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EMC DESIGN GUIDE FOR ST MICROCONTROLLERS

Figure 20. Power Supply Lay-out Examples

I/O configuration

An open (floating) pin is a potential hazard to the circuit.

I/O pins which are not used in the application should be preferably configured in output low

state. This will also minimize the current consumption.

A major source of emission in Microcontroller based applications can be due to high speed

digital I/O and communication interfaces such as SPI, I2C clocks, USB or PWM... The Rise/

Fall times are critical. Typical designs add RC low pass filters.

Shielding

Shielding can help in reducing noise sensitivity and emission, but its success depends directly

on the material chosen as shield (high permeability, low resistivity) and on its connection to a

stable voltage source including a decoupling capacitance via a low serial impedance (low in-

ductance, low resistance).

If the generator of major disturbances is near to the MCU board and can be identified as a

strong dV/dt generator (i.e. a transformer or Klystron), the noise is carried mainly by the elec-

trostatic field. The critical coupling between the noise generator and the control board is ca-

pacitive. A highly conductive shield (i.e. copper) creating a Faraday cage around the control

board may strongly increase the immunity.

AN1709 Datasheet(PDF) 33 Page - STMicroelectronics