6/14

EMC GUIDELINES FOR MICROCONTROLLER-BASED APPLICATIONS

3.2.3 Free Memory

In many cases, the internal program space is not used 100%. This creates a free memory area

where normally, the application program must never take instructions. This area must be used

as a trap which leads to a Reset routine. This is done by filling this area with No-Operation in-

structions (NOPs) followed by a “JUMP to Reset Routine” command.

3.2.4 Software Hardening

There are several other methods for improving EMC performances:

– periodic self-checks of data integrity (checksum...),

– when critical tasks are executed, verify data redundancy and check for runaway condi-

tions,

– create a kind of milestone (i.e. trace point) throughout the program that is verified using a

“status register” that makes sure that step n follows step n-1,

– periodic updating of the control/data registers, which is particularly useful for the I/O reg-

isters which are in the first in line to face EMI.

Each time a runaway condition is detected, the initialization routine must be performed.

3.3 SYSTEM ARCHITECTURE

At the very beginning of a project, certain preliminary decisions must be made to meet EMC

optimization requirements.

3.3.1 PCB Location

The PCB must be kept as far away as possible from the mains supply wiring as well as extra-

high voltage lines or very high current lines. Also, they should not be repeatedly switched on/

off.

In certain cases, “natural” shielding may exist in the application. In this case, it should be used

wisely.

3.3.2 Component Mounting

Surface-mounted components (SMCs) have a higher density than standard through-hole

mounted components, and therefore require shorter traces on the PCB. For microcontrollers,

SMC packages such as small outline (SO) and quad flat (QFP) packages reduce the length of

signal lines and require a smaller power supply loop.