UR7HCTS2-S840 Datasheet, PDF (6/16 Pages) Semtech Corporation – High-Performance, Low Power Touch Screen Controller/Digitizer

TOUCH SCREEN SENSOR INTERFACE (CON’T)
Touch Measurement
Position information is only collected if it has been determined that the touch pressure is sufficient for an accurate
reading. To obtain position information from the Resistive Touch Screen Sensor, the ScreenCoder^TM SPI/Serial uses
four internal drivers (two pins for each driver) and four sense lines. During sampling, the drivers of the X-axis are
activated by setting one X-drive high and the other X-drive low. The drivers for Y-axis are set floating. This action
produces a voltage gradient across the touch screen's surface in the X direction. The internal A/D measures both
the voltage across the activated X-plane and the voltage potential between the planes. Next, the drivers for the Y-
axis are activated while the drivers for X-axis are set floating. Again, the internal A/D measures both voltage across
the activated plane and potential between the planes. The X and Y absolute position is obtained from these four
A/D measurements. The IC keeps measuring the touch pressure even while the positioning data is acquired. If
insufficient touch pressure is detected during sampling of the position, the current data set will be rejected.
Touch algorithm
Due to vibration and electrical noise, the raw data can not be used directly. Internal data processing delivers
consistent position accuracy and quick touch response.
Resolution
The ScreenCoder^TM SPI/Serial uses a built-in A/D converter to get the touch position. The maximum absolute
position resolution is 1000 points per direction. Resolution varies somewhat for different touch screen sensors, due
to voltage droop on connecting wires and sensor’s mechanical construction.
Touch screen from different makers
Operating parameters for the touch screens from different manufacturers vary depending on the size of the sensor,
material it is made from, and mechanical construction. The ScreenCoder^TM SPI/Serial takes these properties into
account, and adjusts automatically for plate-to-plate capacitance, intrinsic plate resistances, etc.
Touch-screen's coordinates
The zero point for the touchscreen is at the left upper corner. See the diagram (0,0)
to the right.
X
Buttons
The ScreenCoder^TM SPI/Serial implements left and right buttons. The buttons
information is combined with external mouse's buttons information when it is
sent to the system. Typically, the buttons are not used, they are provided as a
development aid.
Y
Copyright Semtech 2000-2001
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DOC7-TS2-S840-DS-102
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