Robust Touch sensing with mTouch™ CVD · mTouch CVD Solutions Pass: IEC61000 Emission and...

© 2013 Microchip Technology Incorporated. Slide 1

Robust Touch sensing

with mTouch™ CVD


Robust Touch Sensing

  What is capacitive sensing?   Capacitive Sensing is a technology,

based on capacitive coupling, that takes human body capacitance as input. (wikipedia)

Cs

CP

PCB

Front Panel

Sensor

Ground Trace

CF CP = Parasitic capacitance CF = Finger capacitance Cs = total Sensor capacitance = CP+CF



Bezel

Light Guide

PCB

Cover

Case Study: 15-button HMI (Human Machine Interface)



  Touch technology is not new   First patents filed during the 1970s   Most of them are oscillator based   Can be implemented with a simple 555 timer



  IEC 61000-4-6 : Immunity to Conducted Disturbances


mTouch™ CVD Technology

(Capacitive Voltage Divider)



Implementation

PIC® Microcontroller TRISx Register

0 0PORTx Register

0 0

SENSOR 1 CSENSOR1

SENSOR 2 CSENSOR2

Rx0/AN0

Rx1/AN1

  System is idle

ADC + _

CHOLD VCHOLD


  Prepare to read Sensor 2   Output on first I/O driven HIGH

  Can use any A/D pin   Can use internal Vref

Implementation


1 1PORTx Register

0 1

SENSOR 1 CSENSOR1

SENSOR 2 CSENSOR2

Rx0/AN0

Rx1/AN1

VDD

ADC + _

CHOLD VCHOLD

OUTPUT = 1

OUTPUT = 0


  ADC MUX configured to select HIGH driven I/O to charge CHOLD (VCHOLD = VDD)

Implementation


1 1PORTx Register

0 1

SENSOR 1 CSENSOR1

SENSOR 2 CSENSOR2

Rx0/AN0

Rx1/AN1

OUTPUT = 1

OUTPUT = 0

VDD

ADC + _

CHOLD VCHOLD


  Second I/O configured as OUTPUT and driven LOW to discharge CSENSOR2

Implementation


0 0

PORTx Register

0 1

SENSOR 1 CSENSOR1

Rx0/AN0 OUTPUT = 1

OUTPUT = 0

VDD

ADC + _

CHOLD VCHOLD

SENSOR 2 CSENSOR2

Rx1/AN1


  Second I/O configured as INPUT

Implementation


1 0

PORTx Register

0 1

SENSOR 1 CSENSOR1

Rx0/AN0 OUTPUT = 1

INPUT

VDD

ADC + _

CHOLD VCHOLD

SENSOR 2 CSENSOR2

Rx1/AN1


  ADC MUX configured to select second I/O

Implementation

PIC® Microcontroller

SENSOR 1 CSENSOR1

Rx0/AN0 OUTPUT = 1

INPUT

VDD

ADC + _

CHOLD VCHOLD

SENSOR 2 CSENSOR2

Rx1/AN1

TRISx Register

1 0

PORTx Register

0 1


  VCHOLD averages across CHOLD and CSENSOR2

Implementation


SENSOR 1 CSENSOR1

Rx0/AN0 OUTPUT = 1

INPUT

VDD

ADC + _

CHOLD VCHOLD

SENSOR 2 CSENSOR2

Rx1/AN1

TRISx Register

1 0

PORTx Register

0 1


  Analog to Digital Conversion performed on averaged voltage

Implementation


SENSOR 1 CSENSOR1

Rx0/AN0 OUTPUT = 1

INPUT

VDD

ADC + _

CHOLD VCHOLD

SENSOR 2 CSENSOR2

Rx1/AN1

TRISx Register

1 0

PORTx Register

0 1

VAVERAGE

RESULT

SOFTWARE


  A sensor press increases parallel capacitance reducing the voltage

Implementation

SENSOR

CSENSOR CFINGER CHOLD

to ADC

TIME

VAVERAGE PRESS CHOLD

VADC = (CHOLD + CSENSOR + CFINGER)


CVD Framework Robust Touch Sensing



  Free CVD Framework available online   Includes many software filter algorithms

  Jitter   Decimator (slew rate limiter)   Oversampling   Key debounce   And more…

  Full source code included



  mTouch CVD Solutions Pass:   IEC61000 Emission and

Susceptibility tests   IEC61000-4-6 (Conducted noise)

  Appliance, Consumer   Automotive EFT and BCI tests   High Signal-to-Noise Ratio (SNR)

  Stability over Voltage and Temperature


How to start with mTouch CVD




  4 Motherboards   PIC16F1937 8-bit MCU   PIC18F46J50 8-bit MCU   PIC24FJ64GB106 16-bit MCU   PIC32MX795F512H 32-bit MCU

  4 Sensor Daughter Boards   2-Channel Slider   4-Channel Slider   8 Keys Direct Sense   12-Key Matrix

  PICkit™ Serial Analyzer   mTouch Graphical User Interface

DM183026-2


Gestic Demo


More info can be found at: www.microchip.com/mTouch/

Questions…?


Robust Touch sensing with mTouch™ CVD · mTouch CVD Solutions Pass: IEC61000 Emission and...

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