sx8675iwltrt Semtech Corporation, sx8675iwltrt Datasheet - Page 14

• Manufacturer Part Number: sx8675iwltrt
• Description: Haptics Enabled Multitouch 4/5-wire Resistive Touchscreen Controller With Proximity Sensing
• Manufacturer: Semtech Corporation

ADVANCED COMMUNICATIONS & SENSING
Higher reliability and better endurance are the advantages of 5-wire touchscreens but they do not allow pressure
measurement
4.2
4.2.1
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.
The X and Y positions output by the ADC correspond to the formulas below:
4095 corresponds to the max output value of the ADC (12 bits => 2
For example, a touch in the center of the screen will output (Xpos, Ypos) = ~(2048, 2048)
Rev 1 – 25
Coordinates Measurement
4-wire Touchscreen
th
July 2011
Figure 12 – 4-wire Touchscreen Coordinates Measurement
Xpos
=
Figure 11 – 5-wire Touchscreen
4095
-------------------- -
R1
R2
+
Resistive Touchscreen Controller with Proximity Sensing
R2
14
Y pos
=
4095
SX8674/SX8675/SX8676
12
– 1).
-------------------- -
R3 R4
R4
Haptics Enabled Multitouch 4/5-Wire
+
www.semtech.com