SX8652ICSTRT Semtech, SX8652ICSTRT Datasheet - Page 12

• Manufacturer Part Number: SX8652ICSTRT
• Description: 4/5-wire Resistive Touch Screen Controller (SPI)
• Manufacturer: Semtech

4. 4-wire Touch Screen Detailed Description
4.1. Touch Screen Operation
A 4-wire resistive touch screen consists of two resistive sheets separated by an insulator (Figure 4.2).
When a pressure is applied on the top sheet with a stylus for example, a connection with the lower sheet is made.
The contact point split the Rxtot bottom resistance in the vertical axis into two resistances
R1 and R2. In the same way, the Rytot resistance in the horizontal axis of the top sheet is
divided into two resistances R3 and R4.
The touchscreen controller imposes a voltage level on X or Y electrodes allowing the
detection of the contact position.
4.2. Coordinates Measurement
During the touch, the top and bottom touchscreen layers
are connected. The resistance between the two sheets
is R
from
measurement. Figure 9 shows the measurement
schematics.
Since the ADC had a high input impedance, no current
flows through R
biased with a voltage created by the R1, R2 voltage
divider.
The conversion with the 12 bit ADC gives the X location.
The Y coordinate is measured in a similar fashion with the measurement setup given in Table 7.
Revision V1.7/October 2010
©2010 Semtech Corp.
Xpos
ADVANCED COMMUNICATIONS & SENSING
Ypos
T
. A current coming from the reference voltage goes
X+
=
=
4095
4095
to
X-
T
------------------- -
R1
X-
and R3. The positive ADC input is
------------------- -
R3
Y+
R2
to
+
R4
+
R2
R4
perform
Y-
X+
the
X
Figure 8. 4-wire Touch Screen
coordinate
Top conductive sheet before
15kV ESD Low Power 4-Wire / 5-Wire Resistive
Page 12
the stylus contact
Touchscreen Controller with SPI Interface
Figure 9. Abscissa (X) coordinates measurement
Vref
+
-
Top conductive sheet after
X+
X-
R1
R2
the stylus contact
R
T
Y+
Y-
R3
R4
Rytot
Rxtot
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DATASHEET
Contact point
with the top
conductive
SX8652
+
-
sheet
ADC
=
=
R3
R1
Xpos
+
+
R2
R4