SAM D21 Peripheral
Touch Controller
Noise Immunity
1
© 2013 Atmel
Presentation Outline
•
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
2
•
Video : 10VCI Bench
•
Conclusion
© 2013 Atmel
Impact of noise on a Touch application
3
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Radiated Noise
Definition
Radiated noise correspond to unwanted ’noisy’ RF voltages emitted by external
element from the system.
Fluorescent
lamp
Induction plate
4
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Radiated noise
Impact on touch application
Lamp impact
5
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Conducted noise definition
Impact on touch application
EARTH
No touch
Without noise
With radiated noise
Noise always present in the system
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Touch
Conducted noise
Definition
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 noise
Impact on touch application
Noisy Power supply (3.3V)
Not tuned board
8
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Conducted noise definition
Impact on touch application
EARTH
No touch
Touch
Without noise
With conducted
noise
power supply lines maintain a stable
difference between VDD and GND
9
© 2013 Copyright Atmel Corporation
user’s finger now provides a return
path and effectively couples noise directly
into the capacitive sensor
Noise immunity
International Standard IEC/EN 61000-4-6
•
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.
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PTC noise immunity features
11
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PTC noise immunity features
PTC acquisition/ software features
•
Manage by combination of hardware acquisition features and
software post acquisition treatment .
Input control
Y0
Y1
Y15
Compensation
Circuit
RS
100K
X0
X1
X Line Driver
QTouch Library
Acquisition
Module
•Sensitivity ctrl
•ADC
•Oversampling
IRQ
Result
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•
•
•
•
•
Set
Set
Set
Set
Set
RSEL_VAL
DI
Filter_Level
Auto_OS
Frequency_Mode
A
P
I
X15
•
12
Fast and easy Tuning is possible using dedicated Library parameters
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PTC noise immunity features
Post-processing and pre-processing features
13
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PTC noise immunity features
Serial Resistor
(RSEL_VAL_x)
Input control
Compensation
Circuit
Y0
RS
Y1
Y15
1-100k
Acquisition
Module
•Sensitivity ctrl
•ADC
•Oversampling
X0
X1
X Line Driver
X15
14
•
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.
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PTC noise immunity features
Detect Integration (DI)
•
Principle : Post processing Filter that requires several consecutive measurements to
confirm touch/Release (Similar to debounce system)
• Example DI = 3 :
No touch
reported
•
15
No touch
reported
Touch
reported
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
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No Touch
reported
Illustration : 10CI Bench Video
16
© 2013 Atmel
Illustration : 10CI Bench Video
Bench description
Test
Software
CI Signal
Generator
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Power
amplifier
Coupling /
decoupling
network
10cm
standoff
from earth
plane
Application
to test
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Illustration : 10CI Bench Video
Test 1 : Test Application without noise
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•
Configuration : Filter_Level = 4
•
Result: Good and fast operation
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Illustration : 10CI Bench Video
Test 2 : First frequency of standard CI sweep (150kHz)
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•
Configuration : frequency = 150kHz , Filter_Level = 4
•
Result: Shows false detects and failures.
© 2013 Atmel
Illustration : 10CI Bench Video
Test 3 : First frequency of standard CI sweep (150kHz)
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•
Configuration : Frequency = 150kHz, Rs = 100k, Filter_Level = 32
•
Result: Works very well
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Illustration : 10CI Bench Video
Test 5 : Test frequency = 265kHz
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•
Configuration : frequency = 256kHz, Rs = 100k, Filter_Level = 32
•
Result: Shows false detects, failures
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Illustration : 10CI Bench Video
Test 5 : Test frequency = 265kHz
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•
Configuration : Frequency = 256kHz , Rs = 100k , Filter_Level = 64, Auto_OS = 8
•
Result: Works very well
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Illustration : 10CI Bench Video
Test 6 : Test frequency = 267kHz
(Harmonic Close to Oversampling 266.67kHz)
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•
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 Video
Test 7 : Test frequency = 267kHz
(Harmonic Close to Oversampling 266.67kHz)
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•
Configuration : Frequency = 267kHz , Rs = 100k , Filter_Level = 64, Auto_OS = 8
Frequency hopping enabled
•
Result: Works very well
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Conclusion
25
•
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 .
© 2013 Copyright Atmel Corporation
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