AccuTouch Product Guide - Elo TouchSystems

The logical sequence of operation of an AccuTouch controller, used in
combination with the AccuTouch touchscreen, is as follows:
1 When the controller is waiting for a touch, the resistive layer of the
touchscreen (the coating on the glass) is biased at +5 Vdc through all four
drive lines, and the cover sheet is grounded through high resistance. When
the touchscreen is not being touched, the voltage on the cover sheet remains
at zero. The voltage level of the cover sheet is continuously converted by the
analog to digital converter (ADC) and monitored by the microprocessor on
the controller. When the touchscreen is touched, the microprocessor detects
the rise in the voltage of the cover sheet and begins coordinate conversion.
2 The microprocessor places the X drive voltage on the touchscreen by
applying +5 Vdc to Pins H and X and grounding Pins Y and L.
3 An analog voltage proportional to the X (horizontal) position of the touch
appears on the cover sheet at Pin S of the touchscreen connector. This voltage
is then digitized by the ADC and subjected to an averaging algorithm, then
stored for transmission to the host.
The averaging algorithm reduces noise resulting from contact bounce during
the making and breaking of contact with the touchscreen. Successive X
coordinates are tested to determine that their values differ by no more than a
certain range. If one or more samples fall outside this range, the coordinates
are discarded and the process is restarted. This is continued until several
successive X coordinates fall within the range. The average of these values is
used as the X coordinate.
4 Next, the microprocessor places the Y drive voltage on the touchscreen by
applying +5 Vdc to Pins H and Y and grounding Pins X and L.
5 An analog voltage proportional to the Y (vertical) position of the touch now
appears on the cover sheet at Pin S of the touchscreen connector. This signal
is converted and processed as described above for the X position.
6 Successive coordinate pairs are sampled to eliminate the effects of noise. If a
sample does not fall within an internal range, all X and Y coordinates are
discarded and the X sequence is restarted at step 2.
7 Once acceptable coordinates have been obtained, an average coordinate is
determined and communicated to the host processor.
The X and Y values are similar to Cartesian coordinates, with X increasing
from left to right and Y increasing from bottom to top. Z increases with touch
pressure. These absolute coordinates are arbitrary and unscaled, and will vary
slightly from unit to unit. Any coordinate normalization or scaling must be
done at the driver level, and calibration to the display image must be
performed for each unit. Because of the stability of the AccuTouch system,
recalibration is not necessary unless the position of the image changes.
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