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Touchscreens in embedded applications Shaun Levin Joe Quesada Dru Steeby

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Agenda Explain Resistive Touchscreens Explain Capacitive Touchscreens Surface Acoustic Wave Touchscreens How to Pick a Touchscreen

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Resistive Touchscreens

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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%).

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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.

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Resistive Touchscreens http://www.telecomcircle.com/2010/03/touchscreen/

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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.

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Resistive Touchscreens 4-wire method: http://mehtadhaval.blogspot.com/2011/04/touch-screen-interfacing-with.html

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Resistive Touchscreens

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Resistive Touchscreens 5-wire method:

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Resistive Touchscreens Interfacing with a microcontroller: Arduino Uno makes this really easy!

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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

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Capacitive Touchscreen

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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

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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

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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

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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

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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

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Capacitive Touchscreens Multitouch only works on Mutual Capacitance http://electronicdesign.com/content/content/64835/64835-fig2.jpg

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Capacitive Touchscreens CPU work required http://static.ddmcdn.com/gif/iphone-rev-4.jpg http://static.ddmcdn.com/gif/iphone-rev-5.jpg

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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

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