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EMC GUIDELINES FOR MICROCONTROLLER-BASED APPLICATIONS

2 SCOPE

Specific EMC requirements apply to each part of a microcontroller-based application ac-

cording to EMI references.

2.1 NOISE SOURCES

Electrostatic discharges, mains, switching of high currents and voltages or radio frequency

(RF) generators are just some of the causes of electromagnetic interference, or noise, in mi-

crocontroller environments.

Within the microcontroller itself, the main contributors to noise are:

– oscillator: continuous RF source,

– system clock circuits: RF divider followed by large amplifiers which drive long lines inside

the component,

– output transitions: the relative weight depends on the frequency of the transitions and their

duration; i.e. the shorter the transitions, the richer the frequency spectrum,

– data/address buses: for some microcontrollers, a part of the memory space is external,

which implies continuous transitions on several lines.

2.2 NOISE CARRIERS

EMI can be transferred by electromagnetic waves, conduction, and inductive/capacitive cou-

pling. Obviously, EMI must reach the conductors in order to disturb the components. This

means that the loops, long length and large surface of the conductors are vulnerable to EMI,

making the PCB the principal subject of EMC improvements.

2.3 AFFECTED AREAS

In a microcontroller-based system, the core process is intrinsically sequential and must rely on

valid data. Once a non-EMC-protected program is disturbed, it cannot resume normal opera-

tion.

From the electrical point of view, the following areas are vulnerable:

– system-clock integrity

– memory cells: memory blocks, in addition to registers and memory cells supporting the

state machine of the processor,

– important signals, i.e. RESET, INTERRUPT, HANDSHAKING STROBE.