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SAM D21 Peripheral Touch Controller

Noise Immunity

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• Impact of noise on a touch application• Noise sources• Noise immunity standard

• Peripheral Touch Controller noise immunity features• Pre-processing and post-processing features

• Serial Resistor (Rs)• Detect Integration (DI)

• Acquisition Features• Filter_Level• Auto Oversampling (Auto_OS)• Frequency Hopping

• Video : 10VCI Bench

• Conclusion

Presentation Outline

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Impact of noise on a Touch application

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Radiated NoiseDefinition

Radiated noise correspond to unwanted ’noisy’ RF voltages emitted by external element from the system.

Fluorescent lamp

Induction plate

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Radiated noiseImpact on touch application

Lamp impact

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Conducted noise definition Impact on touch application

EARTH

Without noise

With radiated noise

No touch Touch

Noise always present in the system

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Conducted noiseDefinition

Conducted noise correspond to unwanted ’noisy’ RF voltages and currents carried by its external wires and cables. Common mode noise

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Conducted noiseImpact on touch application

Noisy Power supply (3.3V)

Not tuned board

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Conducted noise definition Impact on touch application

EARTH

Without noise

With conducted noise

No touch Touch

power supply lines maintain a stabledifference between VDD and GND

user’s finger now provides a returnpath and effectively couples noise directly

into the capacitive sensor

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• Define Common reference and a set of testing methods• Modulated RF signal stepped over the frequency range from 150kHz to

80MHz. • At each step there is a ’dwell period’ whilst the EUT (Equipment Under

Test) is checked for performance degradation.

• Result classifications :

• Class A – No significant degradation.• Class B – Degradation in operation but the product fully recovers once the stress

is removed without any operator intervention. (no loss of data)• Class C – Operation is affected and operator intervention is required (no loss of

data)• Class D – Unrecoverable loss of function or degradation of performance. Loss of

data may occur.

Noise immunityInternational Standard IEC/EN 61000-4-6

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PTC noise immunity features

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• Manage by combination of hardware acquisition features and software post acquisition treatment .

• Fast and easy Tuning is possible using dedicated Library parameters

PTC noise immunity featuresPTC acquisition/ software features

100K 16

IRQAcquisitionModule• Sensitivity ctrl• ADC• Oversampling

RS

CompensationCircuit

X Line Driver

Input control

Y0

Y1

Y15

Result

X0

X1

X15

QTouch Library

• Set RSEL_VAL• Set DI• Set Filter_Level• Set Auto_OS • Set Frequency_Mode

API

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PTC noise immunity featuresPost-processing and pre-processing features

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• Increasing the impedance of the signal path can significantly improve immunity to conducted noise while maintaining the overall signal integrity.

• Intermediate resistor value can be chosen to achieve the required level of noise suppression while meeting other system design requirements such as power consumption and response time.

PTC noise immunity featuresSerial Resistor (RSEL_VAL_x)

1-100k

AcquisitionModule• Sensitivity ctrl• ADC• Oversampling

RS

CompensationCircuit

X Line Driver

Input control

Y0

Y1

Y15

X0

X1

X15

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• Principle : Post processing Filter that requires several consecutive measurements to confirm touch/Release (Similar to debounce system)

• Example DI = 3 :

• DI Limit range • A DI count of 3 is the minimum practical setting • A DI count of 6 is common • Higher values : better noise immunity, slower response

PTC noise immunity featuresDetect Integration (DI)

No touch reported

No touch reported

Touch reported

No Touch reported

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Illustration : 10CI Bench Video

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Illustration : 10CI Bench VideoBench description

Application to test

Test Software

Coupling / decoupling network

Power amplifier

10cm standoff

from earth plane

CI Signal Generator

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Illustration : 10CI Bench Video Test 1 : Test Application without noise • Configuration : Filter_Level = 4

• Result: Good and fast operation

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• Configuration : frequency = 150kHz , Filter_Level = 4

• Result: Shows false detects and failures.

Illustration : 10CI Bench VideoTest 2 : First frequency of standard CI sweep (150kHz)

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• Configuration : Frequency = 150kHz, Rs = 100k, Filter_Level = 32

• Result: Works very well

Illustration : 10CI Bench VideoTest 3 : First frequency of standard CI sweep (150kHz)

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Illustration : 10CI Bench VideoTest 5 : Test frequency = 265kHz

• Configuration : frequency = 256kHz, Rs = 100k, Filter_Level = 32

• Result: Shows false detects, failures

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Illustration : 10CI Bench VideoTest 5 : Test frequency = 265kHz

• Configuration : Frequency = 256kHz , Rs = 100k , Filter_Level = 64, Auto_OS = 8

• Result: Works very well

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Illustration : 10CI Bench VideoTest 6 : Test frequency = 267kHz (Harmonic Close to Oversampling 266.67kHz)

• Configuration : Frequency = 267kHz , Rs = 100k , Filter_Level = 64, Auto_OS = 8

• Result: Shows flickering on both keys and slider LEDs

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Illustration : 10CI Bench VideoTest 7 : Test frequency = 267kHz (Harmonic Close to Oversampling 266.67kHz)

• Configuration : Frequency = 267kHz , Rs = 100k , Filter_Level = 64, Auto_OS = 8 Frequency hopping enabled

• Result: Works very well

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• When dealing with noise immunity, It is important to understand the environment in which the touch application is designed to operate in, and where appropriately apply suitable techniques to address the effects of unwanted noise disturbances.

• The Peripheral touch controller embed in SAM D21 and associated Software Library provide key features for building suitable touch application with techniques to overcome noise disturbance .

03/04/2013Introducing Atmel SAM D20 Flash MCUs

Conclusion