ST AN2860 APPLICATION NOTE

AN2860

Application note

EMC guidelines

for STM8 microcontrollers

Introduction

To meet the demand for higher performance, complexity and cost reduction, the
semiconductor industry develops microcontrollers with both high density design technology
and higher clock frequencies. This has intrinsically increased the potential noise emission
and noise sensitivity.

Therefore, application developers must now apply electromagnetic compatibility (EMC)
“hardening” techniques in the design of hardware, PCB layout, firmware and at system level.

This application note is intended for application designers who need to reach the optimum
level of EMC performance. It gives an overview of microcontroller EMC features and
compliance standards.

This document focuses on STM8 features, behaviors and on the differences between the
STM8 family members. For general information on EMC for ST microcontrollers, refer to the
“EMC design guide for ST microcontrollers” application note (AN1709).

General recommendations for STM8 devices are summarized in the “Getting started”
application notes. These documents also provide basic EMC recommendations.

Related documents

AN1015: Software techniques for improving microcontroller EMC performance

AN1709: EMC design guide for ST microcontrollers

AN2752: Getting started with the STM8S and STM8A

AN2867: Oscillator design guide for ST microcontrollers

AN3029: Getting started with the STM8L

AN3181: Guidelines for obtaining IEC60335 Class B certification on STM8 family

RM0013: STM8L101xx microcontroller family reference manual

RM0016: STM8S and STM8A microcontroller family reference manual

RM0031: STM8L15x microcontroller family reference manual

August 2011 Doc ID 15267 Rev 4 1/16

www.st.com

Contents AN2860

Contents

1 STM8 and EMC robustness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2 Application design robustness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.1 Power supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3 PCB layout recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.1 Two-layer board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.2 Single layer board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4 EMC characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

5 Software recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

5.1 Critical bytes and opcode technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

5.2 Reset sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

6 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

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AN2860 List of tables

List of tables

Table 1. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Doc ID 15267 Rev 4 3/16

List of figures AN2860

List of figures

Figure 1. STM8S power connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Figure 2. STM8L power connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Figure 3. Top view of a 64-pin package layout. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Figure 4. Bottom view of a 64-pin package layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Figure 5. Top view of a 32-pin package layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

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AN2860 STM8 and EMC robustness

1 STM8 and EMC robustness

Most STM8 product family applications require a high level of EMC robustness. The
considered level of robustness is closely linked to application purposes. The different levels
of robustness are defined by international EMC class standards together with criteria for
testing and measurement techniques used to classify these levels.

One of the pivotal IEC (international electrotechnical commission) standards is the IEC
61000 standard which defines the following robustness classes:

– Class A: No failure detected

– Class B: Failure detected but self-recovery after disturbance

– Class C: An external user action needed to recover normal functionality

– Class D: Normal functionality cannot be recovered

Note: IEC is a world wide recognized authority on international standards for a vast range of safety

issues concerning electrical, electronic and related technologies. It is important to note that
there are several other recognized bodies concerning electronic safety standards besides
IEC, such as VDE in Germany, IET in the United Kingdom and the IEEE in the United
States.

Class B has a different meaning from that used in the IEC 61000 standard as it means in
this case that the application is classified from a safety point of view. See the IEC 60335
standard in the AN3181: Guidelines for obtaining IEC60335 Class B certification on STM8
family.

EMC robustness is a complex result of several factors and it depends on the component
design. However, a very good EMC robustness of any component may be degraded by its
incorrect position into the PCB design or by an incorrect hardware project. These factors are
discussed in more details in the next chapters.

Another important factor is the software solution which can significantly improve the
robustness of the application by a proper software reaction when any disturbance is
detected, which is fundamental, especially for Class-B compliant applications.

In most cases, it is possible to recover the application internally even if the user is not aware
of the internal issues. It is also possible to predict an imminent error state by using some
efficient techniques capable of testing permanently if both the MCU and the application work
properly.

Note: These techniques are further described in the following documents: application notes

AN1015 (Software techniques for improving microcontroller EMC performance) and
AN3181 (Guidelines for obtaining IEC60335 Class B certification on STM8 family)

Doc ID 15267 Rev 4 5/16