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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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EMS Protection Measures - 2
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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EMS Protection Measures - 3
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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EMS Protection Measures - 4
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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EMS Protection Measures - 5
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
EMS Protection Measures - 6
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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EMS Protection Measures - 7
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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EMS Protection Measures - 8
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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EMS Protection Measures - 9
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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EMS Protection Measures - 10
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