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 microcontroller-based systems is increasingly wide-spread, especially in such 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 components to develop highly integrated, single chip, high operating frequency microcontrollers using the highest density technology possible. Unfortunately, for semiconductor structures, the higher 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 software and the system must now apply EMC “hardening” techniques in their design.

This note aims to provide guidelines for designers of microcontroller-based applications 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

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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 disturbances that would interfere with 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	EQUIVALENT	DESCRIPTION
		INTERNATIONAL 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
		SAE 1752/3	American Measurements Procedure for susceptibility
	Table 2. Electromagnetic Susceptibility

	STANDARD	EQUIVALENT	DESCRIPTION
		INTERNATIONAL STANDARD
	EN50082-1		Generic immunity standards - Residential
	EN50082-2		Generic immunity standards - Industrial
	EN50140	IEC 1000-4-3	RFI (radiated test)
		(old nb: IEC 801-3)	(80 MHz - 1 GHz at 1 to 10 V/m)
	EN50141	IEC 1000-4-6	Induced RF fields (conducted test)
		(old nb: IEC 801-6)	(150 kHz - 80 MHz at 1 to 10V (80% AM, 1 kHz))
	EN50142	IEC 1000-4-5	Surge
		(old nb: IEC 801-5)
	EN????	IEC 1000-4-4	EFT / Burst
	(TBD)	(old nb: IEC 801-4)	(250V - 2kV I/O lines; 0.5 - 4kV AC/DC mains)

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EMC GUIDELINES FOR MICROCONTROLLER-BASED APPLICATIONS

2 SCOPE

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

2.2NOISE CARRIERS

EMI can be transferred by electromagnetic waves, conduction, and inductive/capacitive coupling. 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 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 STROBE.

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EMC GUIDELINES FOR MICROCONTROLLER-BASED APPLICATIONS

3 EMC COMPLIANCE

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 VDD supply, which results in a good decoupling, as well as a good shielding effect. For many applications, economical requirements prohibit the use of this type of board. In this case, the most important feature is 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 order to reduce cross-coupling on the PCB, i.e. noisy, high-current circuits, lowvoltage circuits, and digital components.

3.1.2 Ground and Power Supply (VSS, VDD)

The GROUND should be distributed individually to every block (noisy, low level sensitive, digital,...) with a single point for gathering all ground returns. Loops must be avoided or have a minimum surface. The power supply should be implemented close to the ground line to minimize 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 component-free surfaces of the PCB must be filled with additional grounding to 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 VSS/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 beyond a certain frequency, must be taken into account. If possible, parallel capacitors 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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3.1.4 Oscillator

Almost all microcontrollers have an oscillator coupled to an external crystal or ceramic resonator. On the PCB, the copper traces to pins EXTAL/XTAL/VSS (for external capacitors) must be kept as short as possible. These capacitors are included in certain resonators which further shorten traces.

Since the RC option is potentially sensitive to spikes which can shorten clock periods, the resonator option is preferable.

3.1.5 Other Signals

When designing an application, the following areas should be closely studied 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 surrounding ground trace for these signal increases EMC performances, as well 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 system, an obvious possible EMS weakness 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 routine 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 which must be updated within a maximum time slot. The best systems 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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