SX8674ICSTRT datasheet(14/64 Pages) SEMTECH

14 / 64 page

ADVANCED COMMUNICATIONS & SENSING

Rev 1 – 25

th July 2011

www.semtech.com

SX8674/SX8675/SX8676

Haptics Enabled Multitouch 4/5-Wire

Resistive Touchscreen Controller with Proximity Sensing

Higher reliability and better endurance are the advantages of 5-wire touchscreens but they do not allow pressure

measurement

Figure 11 – 5-wire Touchscreen

4.2

Coordinates Measurement

4.2.1

4-wire Touchscreen

The electrode plates are internally connected through terminals X+, X- and Y+, Y- to an analog to digital

converter (ADC) and a reference voltage (Vref). The resistance between the terminals X+ and X- is defined by

Rxtot. Rxtot will be split in 2 resistors, R1 and R2, in case the screen is touched. Similarly, the resistance

between the terminals Y+ and Y- is represented by R3 and R4. The connection between the top and bottom

sheet is represented by the touch resistance (RT).

In order to measure the Y coordinate, the top resistive sheet (Y) is biased with a voltage source (Vref). Resistors

R3 and R4 determine a voltage divider proportional to the Y position of the contact point. Since the converter has

a high input impedance, no current flows through R1 (and RT) so that the voltage X+ at the converter input is

given by the voltage divider created by R3 and R4.

The X coordinate is measured in a similar fashion with the bottom resistive sheet (X) biased to create a voltage

divider by R1 and R2, while the voltage on the top sheet is measured through R3.

The resistance RT is the resistance obtained when a pressure is applied on the screen. RT is created by the

contact area of the X and Y resistive sheet and varies with the applied pressure.

Figure 12 – 4-wire Touchscreen Coordinates Measurement

The X and Y positions output by the ADC correspond to the formulas below:

Xpos

4095

R 1

---------------------

⋅

Y pos

4095

R 3

---------------------

⋅

4095 corresponds to the max output value of the ADC (12 bits => 2

12 – 1).

For example, a touch in the center of the screen will output (Xpos, Ypos) = ~(2048, 2048)

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