Microchip mTouch

Acquisition Processing/Filtering Decoding

Getting Started – Using the mTouch™ Technology Framework and mTouch Library
1437 CTI Slide 4
© 2010 Microchip Technology Incorporated. All Rights Reserved.

CVD – Capacitive Voltage Divider CTMU – Charge Time Measurement Unit CSM – Capacitive Sensing Module

Choosing a Microchip part for a Capacitive T Touch h Application A li ti Software Processing
CF
Cs
© 2010 Microchip Technology Incorporated. All Rights Reserved. 1437 CTI Slide 10
Equivalent Circuit
Cs
Sensor Capacitance = Cs
Cs
CF
Sensor Capacitance (CT) = CS + CF
ε 0εr A C= d

Plastic


Other

© 2010 Microchip Technology Incorporated. All Rights Reserved.
Slide
17
Materials Chart
% Change to Finger, 0.5x0.5" pad - Differing Materials at Distance
CS + CF = CTOT

Watch for a change in CTOT
PCB
Sensor
© 2010 Microchip Technology Incorporated. All Rights Reserved. 1437 CTI Slide 12
Sensor Overview

A conductive pad that your finger couples to Shape is typically arbitrary A, d, and εr dominate 0rA performance C
Burke Davison
Applications Engineer
Bruce Bohn
Applications Engineer
© 2010 Microchip Technology Incorporated. All Rights Reserved.
1437 CTI
Slide
2
Class Objectives
C
0rA
d
A
d
CF
© 2010 Microchip Technology Incorporated. All Rights Reserved.
1437 CTI
Slide
8
Quick Physics Review

Two-plate model applies Plate 1 = Sensor pad Plate 2 = Finger/conductive or ionizable substance
1437 CTI
Introduction to Microchip’s mTouch™ Capacitive Sensing
© 2010 Microchip Technology Incorporated. All Rights Reserved. 1437 CTI Slide 1
Presenters
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Slide
20
What to Measure
Biblioteka Baidu

1. Voltage Based (ΔV over fixed time t) 2. Frequency Based (Δf over fixed fi d period i d T)
ΔV ΔV Δf
t
© 2010 Microchip Technology Incorporated. All Rights Reserved.

Too Small
© 2010 Microchip Technology Incorporated. All Rights Reserved.
Good
1437 CTI
Excessively Large
Slide 14
Sensor Design - Area

Finger-sized sensors are not always best! Proximity
C
0rA
d
A
d
CF Cs
© 2010 Microchip Technology Incorporated. All Rights Reserved.
1437 CTI
Slide
9
How Does it Work?

Introduction of finger produces a parallel capacitance
1437 CTI Slide 19


© 2010 Microchip Technology Incorporated. All Rights Reserved.
Microchip Sensing Methods
© 2010 Microchip Technology Incorporated. All Rights Reserved.

Fine sensors (require thin covering)
© 2010 Microchip Technology Incorporated. All Rights Reserved.
1437 CTI
Slide
15
Covering Materials (Distance)

Complete product does not stop at sensor Includes enclosure Often covering material is plastic
1. Attain a basic understanding of the basic principles of a capacitive sensor 2. Understand the functional aspects of each of the 3 Microchip mTouch™ technology Sensing Methods 3. Investigate each of the 3 main categories of capacitive touch software algorithms 4. Explore the mTouch technology framework and library; look at the steps to implement a cap touch solution
14.0% 12.0% Percent Chan nge (%) 10.0% 8.0% 6.0% 4.0% 2.0% 0.0% 0.0 2.0 4.0 6.0 d (mm) Acrylic
© 2010 Microchip Technology Incorporated. All Rights Reserved.
C
A
d dv
i C
dt
© 2010 Microchip Technology Incorporated. All Rights Reserved.
1437 CTI
Slide
7
Quick Physics Review

Two-plate model applies Plate 1 = Sensor pad Plate 2 = Finger/conductive or ionizable substance
© 2010 Microchip Technology Incorporated. All Rights Reserved. 1437 CTI Slide 3
Agenda

Basic Physics of a Cap Touch Sensor 3 Microchip Sensing Methods


d
© 2010 Microchip Technology Incorporated. All Rights Reserved.
1437 CTI
Slide
13
Sensor Design - Area

Capacitive Sensors can be very simple As a general rule, consider making the sensor simply square and the size of a fingertip
Glass
y = 0.1615x -0.9834
eA C d
Plastics
8.0
10.0
12.0
14.0
Polycarbonate
1437 CTI
ABS
Glass
Slide 18
Physics Summary

Finger adds parallel capacitance to ground Size sensors appropriately Thinner covers are better A higher Dielectric Constant is better
1437 CTI Slide 16


© 2010 Microchip Technology Incorporated. All Rights Reserved.
The Dielectric Constant - εr

PCB Material

FR-4 CEM1 - 3
4.20 to 4.70 3.8 to 4.5 8.38 to 8.30 4.97 to 4.84 3.78 3.48 to 3.30 3.67 to 3.52 3.02 to 2.96 2.26 78.20 24.0 4.15 1.55
1437 CTI

Glass

Iron-sealing glass Soda-borosilicate Fused quartz Acetate Epoxy resin Polycarbonate Polyethylene Liquid water Ketchup / Mustard Ice Snow

Field effect Simple principles i = instantaneous current
C==capacitance capacitance C = change in voltage εdv =ε x ε ; ε is a constant, εr is 0 r 0 dt the = change in time dielectric constant of the material between the plates A = area of the plates d = distance between the plates
d Cs
Printed Circuit Board (FR4 PCB)
Cs = Sensor Base Capacitance
© 2010 Microchip Technology Incorporated. All Rights Reserved. 1437 CTI Slide 6
Intro to Cap Sensing
Physics of a Capacitive Touch Sensor
© 2010 Microchip Technology Incorporated. All Rights Reserved. 1437 CTI Slide 5
Touch Sensor Construction
A
Protective overlay Copper pad
© 2010 Microchip Technology Incorporated. All Rights Reserved.
1437 CTI
Slide
11
Sensor Capacitance

Changes as CS is modified – environment Changes as finger touches – user