electromagnetic noise reduction techniques part 2

8/9/2019 Electromagnetic Noise Reduction Techniques Part 2

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1 2009, Renesas Technology America, Inc., All Rights Reserved

Purpose:

This course discusses techniques that can be applied to reduce problems inembedded control systems caused by electromagnetic noise

Objectives: Gain a basic knowledge of the types of noise that affect embedded control

systems

Learn approaches and design methods for protecting systems against problemscaused by external sources of noise

Get basic insights for handling noise problems during the system design anddevelopment cycle

Content: 18 pages 3 questions

Learning Time: 25 minutes

Course Introduction


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Two types of noise:

Electromagnetic Compatibility (EMC) issues encompass both types

Noise reduction approaches: Techniques for reducing EMI (Electromagnetic Interference)

Cutting the noise emitted by a specific system, circuit or devicethat causes other devices/circuits to operate incorrectly

Techniques for decreasing EMS (Electromagnetic Susceptibility) minimizing the effect that external noise has on the operation ofa system, circuit or device

Noise reduction: a goal common to both microcontroller (MCU) designers and

the system engineers who apply those devices

Noise = Unwanted electrical signals that produce undesirable effectsin the circuits of control systems in which they occur.

Noise = Unwanted electrical signals that produce undesirable effectsin the circuits of control systems in which they occur.

Noise Can Cause Big Problems


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EMS Protection Is Essential

External sources of noise could cause a microcontroller to run out ofcontrol

Results can range from inconvenient momentary disruptions of embeddedfunctionality to failures of microcontroller based equipment that cause injuries

or fatalities

MCU

ACpower line

Noise is induced in power lineby nearby lightning storm

Noise is induced in power lineby sparks from electric motor

MCU-based equipment

Spark noise

Noise can disruptmicrocontroller

Staticnoise

Whether a systemruns from the powerline or a battery, it is

essential that the MCU

include EMS measures thatprotect it against external noise.

!


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EMS Protection Measures - 1

Reduce the lengthof the systemwiring

Shorten allinterconnect wires as

much as possible

Connection inside the QFP is about90% shorter than connection in the DIP

Select a smaller package type toachieve shorter interconnect

wiring The length of the connection between

the Vcc & GND terminals of the packageand the Vcc & GND terminals of the chipis much shorter in a small outline

package such as a QFP or SOP than itis in a DIP package and that shorterlength makes bypass capacitors moreeffective in shunting external noise toground


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Mount and connect bypasscapacitors in the bestpossible locations

Put bypass capacitors

close to the device andat positions where theVcc and GND (Vss) wiringtraces are isometric andas short as possible

Good

Better

GoodVcc

GND

DIP

Vcc GND

DIP

QFP

VccGND

Use multiple bypass capacitors ofdifferent types and make Vcc andGND lines run parallel to each other

A combination of a ceramic capacitor and atantalum type provides good filtering; seriesinductances further improve EMS protection

IC

C1 C2

TantalumCeramic

Vcc

GND

L1

(Devices with multiple Vcc & GNDterminals facilitate noise reductionmeasures that are exceptionallyeffective)


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PROPERTIES

On passing, 'Finish' button: Goes to Next Slide

On failing, 'Finish' button: Goes to Next SlideAllow user to leave quiz: After user has completed quiz

User may view slides after quiz: At any time

User may attempt quiz: Unlimited times


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Adopt a serial bus connection

Reduce the wiring length,which is proportional to thenumber of data lines, byreplacing a parallel bus with

a serial bus

Use a single-chipmicrocontroller

MCUs with on-chip memory caneliminate external RAM and ROMand associated wiring

Memory ICs

MCUMCU

On-chipflash

ParallelData Bus

Peripheral ICs

MCU

Peripheral ICs

SerialData Bus

MCU


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Noise

Resistor

MCU

Noise

Resistor

MCU

Mount current-limitingresistors close to the MCU

Reduce noise problems byplacing the resistors within 2cmor less from the microcontroller

Use serial damping resistors

When signal lines are long, mount serialdamping resistors close to the MCU

Their value should be about 100 Ohmsto match the characteristic impedanceof the wiring on the printed circuit board

MCU

PeripheralIC

DampingResistors

(~100)


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RESETCircuit

RESET

GND

Low-pass Filter

MCU

Noise

Put a low-pass filter on RESET line

Adding an R-C filter to the signal line to the device or circuit that resets theMCU will shunt to ground external noise that might otherwise result in partialinitializations that cause the microcontroller to run out of control

RESET

GND

MCU

Noise

RESETCircuit

Make wiring pattern of clock oscillator compact and close to MCU

Especially, locate the ground connection for the oscillator circuit very near tothe MCUs GND (Vss) terminal and make a direct connection to it

MCU

XIN

XOUT

GND

Noise MCU

XIN

XOUT

GND

MCU

XIN

XOUT

GND

Separate GNDtrace connectsdirectly to Vss pin


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WriteCurrent

12.5V

EPROM

Vpp

Write Operation


Reduce vulnerability of EPROMprogramming pin (Vpp)

When you mount a previously writtenEPROM on a circuit board, connectone end of a series resistor to Vpp and

the other end of it to either 5V or GND

EPROM

Vpp OR

5VAfter Writing

~2k to27k

Use a serial resistor to reduce noise picked up by wiring toexternal analog sensor

Insert a resistor (100 to 1k) in series with the MCUs A/D input pin, close tothe MCU, and if possible, connect a capacitor to ground to form a low-passfilter

Analog Sensor(Thermistor, etc.)

Noise

PCB

MCU

A/DIN

GNDLow-pass filterfor noise reduction,placed close to MCU

Overload may be appliedat A/D input terminal whentransducer is placed farfrom the MCU

[Sensor is typically located remotefrom the PCB and needs long wiresto make electrical connection]

100-1k


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Be particularly careful when laying outthe oscillator circuit and the area near it

Here are some key printed circuit board layoutsuggestions regardingthe MCU clockcircuit(s) Do not intersect signallines and clock lines

Port Out

XIN

XOUT

MCU

A

B

Keep lines that carry large currents,and those that switch at highfrequencies, away from clock lines

Hidden inductance

N/A (GND)

XIN

XOUT

GND MCU

Develop wider ground area to separatedual clock signals and preventsignal interference between them

D

XCIN

XCOUT

XIN

XOUT

N/A (GND)

MCU

GND

If possible, do notuse the terminalsnear the clock pins;connect themto GND

Port Out

Port Out

XIN

XOUT

MCU Recommended

C

GND

XIN

XOUT

MCU

N/A (GND)


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Use the best methods forimplementing power and groundwiring

Develop a solid GND pattern on one side of

a 2-layer circuit board

Develop solid GND and Vcc patterns on theinner layers of a 4-layer PCB

Beneficial results of these approachesinclude the following:

- Wiring impedance from chip through bypasscapacitors is minimized

- Noise from each signal line through ground or Vccdissipates readily- Even if strong noise enters power line, the electric

potential is maintained with hardly any fluctuation;by contrast, a narrow GND line causes a voltagechange

Noise

Sheet GND pattern

MCU

MCU

Noise

Logic

IC

MemoryIC

Sheet GND pattern

MCU MemoryIC

Noise

Narrow GND lineChange in electric potential

Logic

IC


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Apply PCB layout methods for power and ground patternsthat reduce system susceptibility to noise

If you cant use a multi-layer PCB and solid Vcc and GND patterns, use loop or net-likepatterns to make connections

Vcc GND

Avoid

MCU

Avoid

MCU

Avoid Use

MCU

Avoid

Vcc GND

Use

Vcc GND

If loop or net-like patterns arent feasible, arrange the Vcc and GND wiring in paralleland make wiring lengths isometric


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Choose a microcontroller

that offers good noise

immunity

To provide good protection

against noise, the MCUs in theM16C family have suitable

internal noise filters in all the

necessary places

a pin layout that minimizes noise

problems by various measures,

placing a GND pin between the

oscillator pins,

easing the design of Vcc and GND

wiring on the PCB, and aiding the

optimum placement of bypasscapacitors

AVSS

VREF

AVCC

BYTE

CNVSS

XCIN

XCOUT

/RESET

XOUT

VSS

XIN

VCC

/NMI

VCC

VSS

ResetIC

GND VCC

Oscillation capacitors(symetrically placed)

Bypasscapacitor(~0.01F)

Tantalum capacitor

Capacitor for RESET(1000pF)

Oscillation capacitors(symetrically placed)

Sheet pattern of GND

Sheet pattern of VCC

Bypass capacitor(~0.01F)

M16CMicrocontroller

Bypass capacitor(~0.01F)


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Attempts to eliminate all possible

EMI/EMS problems typically lead to

unnecessarily high costs

The optimum design approach is to take

pinpoint measures in key areas that require

solutions

Basic Design Insights on Noise

It is very important to

implement noise measures

at initial stage of design work

If a problem is discovered in the later stages

of the system development process,the noise measures required to solve the

problem will end up being far more costly than

expected

Noise problems sometimes

require a fundamental solution,

such as a re-design

Unless noise measures are taken at the initial

design stage, a malfunction that occurs later

might be extremely difficult to eliminate using

superficial correction methods; a re-design

may be necessary to correct the problem

Investigations of noise problems

take time and are costly

Because its difficult to simulate a noise-

oriented malfunction, a lot of work will benecessary to identify the root cause of the

problem

Noise


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PROPERTIES

On passing, 'Finish' button: Goes to Next Slide

On failing, 'Finish' button: Goes to Next SlideAllow user to leave quiz: After user has completed quiz

User may view slides after quiz: At any time

User may attempt quiz: Unlimited times


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Course Summary

Need for EMS protection

EMS protection measures

Design insights

electromagnetic noise reduction techniques part 2

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