1. Touchscreens in embedded applications
Shaun Levin
Joe Quesada
Dru Steeby

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

3. Resistive Touchscreens

4. Resistive Touchscreens
Advantages:
Cost Effective and durable, can be used in most environments.
Screen responds with any type of object
fingernail, naked finger, pen, stylus, gloved finger, etc
Disadvantages:
No Multitouch.
Less transparency (~80%) than capacitive screens (~90%).

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.

6. Resistive Touchscreens

7. 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.

8. Resistive Touchscreens 4-wire method:

9. Resistive Touchscreens

10. Resistive Touchscreens 5-wire method:

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

12. 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

15. Capacitive Touchscreen

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
naked finger, special stylus
Requires a dedicated controller
Complicated and expensive

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.

18. Capacitive Touchscreens
Surface Capacitance
content/uploads/2010/03/Capacitive-Touch-Screen.png
Uniform electrostatic field
Sensor reads capacitance at each corner
Touching finger increases capacitance at a point
Low resolution
Example of surface capacitance (mainly used in kiosks)
add multitouch

19. Capacitive Touchscreens
Self Capacitance
Grid of independent electrodes and current sensors
Sensors read capacitance at each row and column
Finger increases capacitance at node
Single finger
High resolution
Mutual Capacitance:
- Rows of driving lines and columns of sensing lines, making a grid
- There exists a mutual capacitance between the sensing lines and the driving lines
- When a finger touches the top, the mutual capacitance between the sensing and driving lines decreases
- Detection algorithms determine where the finger touched based off of the changed mutual capacitance on the sensing lines.
Self Capacitance:
- A grid of electrodes connected with capacitance-sensing circuitry
- When a finger touches the top, the charge held by nearby electrodes changes due to the increased capacitance
- Detection algorithms determine the X and Y lines with the peak changes to determine where the finger touched
Mutual capacitance allows for multi touch, but it becomes incredibly processor-intensive as the size of the screen increases. It is no longer feasible after the screen reaches about 15 inches.

20. Capacitive Touchscreens
Mutual Capacitance
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
Mutual Capacitance:
- Rows of driving lines and columns of sensing lines, making a grid
- There exists a mutual capacitance between the sensing lines and the driving lines
- When a finger touches the top, the mutual capacitance between the sensing and driving lines decreases
- Detection algorithms determine where the finger touched based off of the changed mutual capacitance on the sensing lines.
Self Capacitance:
- A grid of electrodes connected with capacitance-sensing circuitry
- When a finger touches the top, the charge held by nearby electrodes changes due to the increased capacitance
- Detection algorithms determine the X and Y lines with the peak changes to determine where the finger touched
Mutual capacitance allows for multi touch, but it becomes incredibly processor-intensive as the size of the screen increases. It is no longer feasible after the screen reaches about 15 inches.

21. Capacitive Touchscreens
Multitouch
only works on Mutual Capacitance

22. Capacitive Touchscreens
CPU work required

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

24. Capacitive Touchscreen
Example Product: NHD MF-ATXL#- CTP-1
$45 from Mouser or DigiKey
3.5" TFT Display
SPI controller (left)
Capacitive Touch panel
I2C controller (right)
Capable of up to 5 touch points

25. Capacitive Touchscreen
Sample code to read touch data (taken from product datasheet):
byte i, touchdata_buffer[0x20];
i2c_start(); // send a start code
i2c_tx(0x70); // send device address 0x70 in write mode (LSB==0)
i2c_tx(0x00); // tell device that we are reading from internal address 0x00
i2c_stop(); // send a stop code
i2c_tx(0x71); // send device address 0x70 in read mode (LSB==1)
for(i=0x00; i<0x1F; i++) { // read 1 byte each from internal addresses 0x00- 0x1F
touchdata_buffer[i] = i2c_rx(1); // store the one byte into the buffer }
Cite it, make it more clear

26. Surface Acoustic Wave Touchscreen

27. Surface Acoustic Wave Overview
Based on concepts first explained in 1885 and observed in nature
First described by Lord Rayleigh as pressure waves travelling through a solid parallel to its surface
Sometimes called Rayleigh waves
Other forms exist with different names

28. Surface Acoustic Devices
In the simplest form, contain three main components to function
Two transducers to generate and receive surface waves
A piezoelectric medium to conduct the waves
Quartz glass for touch panels
Aluminum nitride or lithium tantalate for sensors, circuit components, or semiconductor applications
Example devices include filters, pressure and temperature sensors, and of course…

29. Touch Screens
Built from two transducer pairs and reflectors mounted directly on the glass
All transducers share the same medium, touch detection is one layer of material
Reflectors spread surface waves in a grid-like manner

30. Touch Detection
Reflector arrays spread and direct the waves over the glass in a grid-like manner
Detection based on changes in wave amplitude when a soft object touches the screen
Bare finger, gloves, soft tip stylus, etc. absorb some wave energy
Hard objects like pens do not register a touch.
Onboard control circuitry detects the amplitude change and computes the coordinates
Detection generally limited to a single point of contact

31. Reflector Function

32. Touch Screen Construction

33. Microcontroller Interface
Communication protocols vary by manufacturer
May not be possible with some models
Low level protocols like I2C and SPI are used, but are rare
Vendors provide a controller module with host connectivity options
Common options include USB and RS-232
PC software provided to use panel with a computer

34. Pricing Most vendors don’t disclose pricing info
Keetouch offers panels of assorted sizes with prices
7” SAW touch panel from Keetouch, including controller module: $56
Sizes from 7” up to 42”, costs up to $410

35. Why Choose Surface Acoustic
Very durable
Up to 50 million touches on a single point
No film or coating that can be worn out with heavy use
Superior image quality
Up to 92% transmittance for some models
Only a single layer of glass, so distortion is minimal
High Resolution and Accuracy
Up to 4096 x 4096 resolution
Touches accurate to within 1% tolerance
Scalable
Made in larger sizes than capacitive or resistive panels
Larger panels requires almost no additional circuitry

36. Drawbacks and Common Uses
Pricing and availability
Most vendors don’t disclose component prices
Resellers often perform custom installations and require a quote
When prices are given, surface acoustic panels are much more expensive than other types
Susceptible to dead spots caused from dirt and other contaminants
Despite these restrictions, these panels are used in high traffic environments where durability is required
Industrial control systems, medical devices, kiosks
Touch screen computer monitors in consumer sector

37. Picking a Touchscreen Type
Short answer: Choose resistive
Long answer:
Resistive
Capacitive
Surface Acoustic Wave
Transparency
~80%
~90%
up to 92%
Multi-Touch No
Yes (mutual)
Resolution
High
Medium
Speed
Fast
Slow
Contact required
Yes
Cost
Low
Very High
Durability
Fair
Good
Very Good
Power
Not Disclosed

38. Sources Resistive overview
Capacitive overview and comparisons of types
Surface acoustic wave overview
Touchscreen comparisons:
Example resistive product
Example capacitive product
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Example surface acoustic wave product