Touchscreens in embedded applications Shaun Levin Joe Quesada
Dru Steeby
Slide 2
Agenda Explain Resistive Touchscreens Explain Capacitive
Touchscreens Surface Acoustic Wave Touchscreens How to Pick a
Touchscreen
Slide 3
Resistive Touchscreens
Slide 4
Advantages: Cost Effective and durable, can be used in most
environments. Screen responds with any type of object o fingernail,
naked finger, pen, stylus, gloved finger, etc Disadvantages: No
Multitouch. Less transparency (~80%) than capacitive screens
(~90%).
Slide 5
Resistive Touchscreens How do they work? General Idea: Two
planes separated by a "microdot" surface. This makes it so that the
planes are separated when not being touched, but when they are
touched the planes make contact. This sits on top of a separate LCD
screen.
Resistive Touchscreens But how does that determine the
coordinates? There are several different methods to determine where
on the screen your finger has touched. 4-wire and 5-wire are among
them, with each getting more complicated and more expensive as you
add more wires.
Resistive Touchscreens Interfacing with a microcontroller:
Arduino Uno makes this really easy!
Slide 12
https://www.sparkfun.com/products/8977
https://www.sparkfun.com/products/9170 Nintendo DS touchscreen $10
at sparkfun.com 2.2 by 2.75 inches Nintendo DS touchscreen Breakout
board $4 at sparkfun.com used to easily connect the pins to
wires
Slide 13
Slide 14
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Capacitive Touchscreen
Slide 16
Capacitive Touchscreens Advantages: Very transparent display
(up to 90%) Can support multi-touch High accuracy touch resolution
No finger pressure needed Disadvantages: Can only sense conductive
material o naked finger, special stylus Requires a dedicated
controller Complicated and expensive
Slide 17
Capacitive Touchscreens How do they work? General Idea: One
side of an insulator is coated with conductive material and
energized, creating a uniform electrostatic field. When a finger or
other conductor touches the other side of the insulator, the
capacitance at that point increases. A controller reads the
capacitance to find the location of the finger.
http://www.maxwellrosspierson.com/wp-content/uploads/2009/03/capacitor.gif
Slide 18
Capacitive Touchscreens http://www.telecomcircle.com/wp-
content/uploads/2010/03/Capacitive-Touch-Screen.png Surface
Capacitance Uniform electrostatic field Sensor reads capacitance at
each corner Touching finger increases capacitance at a point Low
resolution
Slide 19
Capacitive Touchscreens Self Capacitance
http://static.ddmcdn.com/gif/iphone-rev-3.jpg Grid of independent
electrodes and current sensors Sensors read capacitance at each row
and column Finger increases capacitance at node Single finger High
resolution
Slide 20
Capacitive Touchscreens Mutual Capacitance
http://static.ddmcdn.com/gif/iphone-rev-2.jpg Grid of intersecting
sensing and driving lines where each intersection has natural
mutual capacitance Sensor reads mutual capacitance at each
intersection Finger touch reduces mutual capacitance at
intersection Multi-touch - up to 15 points Large grids (>15")
become too CPU expensive
Slide 21
Capacitive Touchscreens Multitouch only works on Mutual
Capacitance
http://electronicdesign.com/content/content/64835/64835-fig2.jpg
Slide 22
Capacitive Touchscreens CPU work required
http://static.ddmcdn.com/gif/iphone-rev-4.jpg
http://static.ddmcdn.com/gif/iphone-rev-5.jpg
Slide 23
Capacitive Touchscreens Interfacing with a microcontroller:
Either have to buy the touchscreen and controller separately
(difficult to match) or buy them bundled together Can also buy LCD,
touchscreen, and controllers all bundled together Controllers use
standard protocols: I2C, SPI