ST AN901 Application note
AN901
APPLICATION NOTE
EMC GUIDELINES FOR
MICROCONTROLLER-BASED APPLICATIONS
by Microcontroller Division Applications
INTRODUCTION
Electromagnetic compatibility (EMC) must be taken into account at the very beginning of a
project as the cost of correcting an EMC problem encountered at the start of production can be
far greater that the cost of a detailed EMC study during the development phase of an application.
The use of microco ntroller- based syste ms is increasin gly wide -spread, especia lly in s uch
areas as consumer, industrial and automotive applications, where the drive for cost reduction
is the common trend. This emphasis on cost reduction and the increasing complexity of such
systems requires the manufacturers of semiconductor componen ts to develop highly integrated, single chip, high operating frequency microcontrollers using the highest density technology possible. Unfo rtunatel y, for semico nductor struc tures, the highe r the density and the
faster the operation, intrinsically the higher the level of electrical noise generated, and the increased sensitivity to spikes induced from external noise. Therefore, the PCB layout, the soft-
ware and the system must now apply EMC “hardening” techniques in their design.
This note a ims to provi de gu ideline s for d esign ers of m icroco ntro ller-b ased applic ations so
that the optimum level of EMC performances can be achieved.
For general information about EMC performances, please refer to application note AN898.
AN901/1100 1/14
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EMC GUIDELINES FOR MICROCONTROLLER-BASED APPLICATIONS
1 DEFINITION OF TERMS
Electromagnetic compatibility (EMC) is the capacity of a piece of equipment to work properly
in its normal environment, and not create electrical di sturbances that would interfere w ith other
equipment.
Electromagnetic susceptibility (EMS) is the level of resistance to electrical disturbances such
as electromagnetic fields and conducted electrical noise.
Electromagnetic interference (EMI) is the level of conducted/radiated electrical noise created
by the equipment.
There exists several standards addressing EMS or EMI issues, and for every type of application area. These standards apply to finished equipment. Up to now, there is no official
standard applicable to sub-systems or electronic components. Nevertheless, EMC tests must
be performed on the sub-systems in order to evaluate and optimize applications for EMC performances.
1.1 EMC STANDARDS
Table 1. Electromagnetic Emissions
STANDARD
EN50081-1 Generic emissions standards - Residential
EN50081-2 Generic emissions standards - Industrial
EN55011 CISPR 11 For industrial, scientific and medical equipment
EN55013 CISPR 13 For broadcast receivers
EN 55014 CISPR 14 For household appliances/tools
EN 55022 CISPR 22 For data processing equipment
INTERNATIONAL STANDARD
SAE 1752/3 American Measurements Procedure for suscept ibility
EQUIVALENT
DESCRIPTION
Table 2. Electromagnetic Susceptibility
STANDARD
EN50082-1 Generic immunity standards - Residential
EN50082-2 Generic immunity standards - Industrial
EN50140
EN50141
EN50142
EN????
(TBD)
INTERNATIONAL STANDARD
IEC 1000-4-3
(old nb: IEC 801-3)
IEC 1000-4-6
(old nb: IEC 801-6)
IEC 1000-4-5
(old nb: IEC 801-5)
IEC 1000-4-4
(old nb: IEC 801-4)
EQUIVALENT
DESCRIPTION
RFI (radiated test)
(80 MHz - 1 GHz at 1 to 10 V/m)
Induced RF fields (conducted test)
(150 kHz - 80 MHz at 1 to 10V (80% AM, 1 kHz))
Surge
EFT / Burst
(250V - 2kV I/O lines; 0.5 - 4kV AC/DC mains)
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EMC GUIDELINES FOR MICROCONTRO LLER-BASED APPLICATIONS
2 SCOPE
Specific EMC requirements apply to each part of a m icrocontroller-based application according to EMI references.
2.1 NOISE SOURCES
Electros tati c di sc harg es, m ai ns, sw it chi ng of hi gh c urren ts an d v olta ge s or ra dio f req uen cy
(RF) generators are just some of the causes of electromagnetic interference, or noise, in microcontroller 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, conduc tion, and inductive/capacitive coupling. Obviously, EMI must reach the c onductors 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 intrinsicall y sequential and must rely on
valid data. Once a non-EMC-protected pr ogr am is disturbed, it cannot resume normal operation.
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 STRO BE.
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EMC GUIDELINES FOR MICROCONTROLLER-BASED APPLICATIONS
3 EMC COMP LIANCE
Once the areas involved are identified, EMC performances are improved by decreasing noise
source emissions, increasing EMI immunity in susceptible areas and weakening the capacity
of noise carriers.
3.1 PRINTED CIRCUIT BOARD
For technical reasons, it is best to use a multi-layer printed circuit board (PCB) with a separate
layer dedicated to the ground and another one to the V
coupling, as well as a good shielding effect. For many applications, economical requirements
prohibit the use of this type of board. In this cas e, the most impo rtant feature i s to ensure a
good structure for the ground and power supply.
3.1.1 Component Position
A preliminary layout of the PCB must separate the different circuits according to their EMI contribution in or der to redu ce cross -couplin g on the PC B, i.e. noi sy, high- curre nt circuits , lowvoltage circuits, and digital components.
supply, which results in a good de-
DD
3.1.2 Ground and Power Supply (V
, VDD)
SS
The GROUND should be distributed individually to every block (noisy, low level sensit ive, di gital,...) with a s ingle point f or gat hering al l ground return s. L oops m ust be avoided or h ave a
minimum surface. The power supply should be implemented close to the ground line to m inimize the surface of the supply loop. This is due to the fact that the supply loop acts as an antenna, and is therefore the main emitter and receiver of EMI.
All comp onent- fre e sur fac es of t he PC B m ust be fille d w ith a dditi onal gro und ing t o create a
kind of shielding (especially when using single-layer PCBs).
3.1.3 Decoupling
The standard decoupler for microcontrollers is a 100-µF pool capacitor, and in parallel, a
0.1-µF high frequency capacitor (typical values). Aluminium electrolytic capacitors should be
avoided due to their poor performance at high frequencies. These capacitors must physically
be as close as possible to the V
SS/VDD
pins of the component in order to reduce the surface of
the actual loop.
As a general rule, decoupling all sensitive or noisy signals improves EMC performances.
There are 2 types of decouplers:
– Capacitors close to components. Inductive characteristics, which apply to all capacitors be-
yond a certain frequency, must be taken into account. If possible, parallel capacitor s with decreasing values (0.1, 0.01,... µF) should be used.
– Inductors. Although often ignored, ferrite beads, for example, are excellent inductors due to
their good dissipation of EMI energy and there is no loss of DC voltage (which not the case
when simple resistors are used).
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EMC GUIDELINES FOR MICROCONTRO LLER-BASED APPLICATIONS
3.1.4 Oscillator
Almost all microcontrollers have an oscillator coupled to an ext ernal crysta l or ceram ic reso nator. On the PCB, the copper traces to pins EXTAL/XTAL/V
(for external capacitors) must
SS
be kept as short as possible. These capacitors are included in certai n resonators which further
shorten traces.
Since the RC option is potentially sensitive to spik es which can shorten clock per iods, the resonator option is preferable.
3.1.5 Other Signals
When designing an application, the fol lowing areas should be closely s tudied to improve EMC
performances:
– noisy signals (clock...),
– sensitive signals (high impedance...).
In addition to:
– signals for which a temporary disturbance affects the running process permanently (the
case of interrupts and handshaking strobe signals, and not the case for LED commands).
A surround ing g round tr ace fo r th ese signa l inc reases EMC perform ances, as we ll as a
shorter length and the absence nearby of noisy and sensitive traces (crosstalk effect).
For digital signals, the best possible electrical margin must be reached for the 2 logical states
and slow Schmitt triggers are recommended for eliminating parasitic states.
3.2 PROGRAMMING EMC-HARDENED SOFTWARE
3.2.1 Parallel Processes
With a programmable syste m, an obvious po ssible EMS we akness arises from an unique
process that relies on valid memorized data. At first, the unique process must be split into as
many parallel and independent processes as possible. This is particularly important for security functions such as the watchdog, refresh routi ne and the initialization routine. Additionally,
such a split is useful for locating weaknesses during EMC debugging.
3.2.2 Watchdog
The watchdog is a circuit whi ch mus t be updated within a max imum time slot. The bes t s ystems maintain the watchdog independent of the CPU (not built with a soft routine). For example, STMicroelectronics ST62 microcontrollers have a watchdog integrated in the component, and is able to run independently of the CPU.
The watchdog update routine must be treated as a critical process to reduce chances that the
watchdog is updated when the process is no longer in normal operation.
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