Cirrus Logic EP93xx - Manual (Page 748)

25-10 DS785UM1
Copyright 2007 Cirrus Logic
Analog Touch Screen Interface
EP93xx User’s Guide
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The algorithm then would continue by discharging and detecting a valid touch. With no X
interrupt pending flag, the algorithm also continues by discharging and detecting a valid
touch, but without interrupt generation. If the difference between the new Y value and the last
Y value is greater than the YMIN value, the algorithm continues by comparing the difference
between the new Y value and the last Y value to the YMAX value in the TSXYMaxMin
register. If the difference is greater than YMAX, it is assumed that this distance is too far for a
touch input to possibly move in the short scan time interval and that the key press is invalid.
However, it is stored as the last recorded Y location in case a truly new location is being
determined (that is, no contact to contact or contact to no contact).
The Y move interrupt pending flag is also set at this point to cause the algorithm to skip over
the comparison to YMIN on consecutive sample sets. This flag will also cause an ARM Core
interrupt as soon as a valid Y sample can be established. The algorithm then starts over
discharging and detecting a valid press. If the difference between the new Y value and the
last stored Y value is less than the YMAX value, the algorithm stores the new Y value in the Y
last register, and the stored X value is moved to the X last register. The interrupt to the ARM
Core is generated, and the algorithm continues by moving to the settle state and detecting a
valid press. The interrupt is cleared when the TSXYResult register is read or by clearing the
TSSetup2.TINT bit.
As can be seen from Figure 25-3 , 4-wire touch screens are operated in much the same way a
8-wire touch screens, except that the reference for the A / D converter is internal instead of a
feedback. The register values TSDetect, TSDischarge, TSXSample, and TSYSample are
derived from the switch positions in the diagram. They are documented in
Table 25-2 .
25.2.2 Five-wire and Seven-wire Operation
Five-wire touch screens require a different connection scheme than 4- or 8-wire touch
screens. The X+ and X- lines become static V+ and V- lines. The Y+ and Y- lines are Z+/- and
Z-/+ and are used to switch between X and Y axis. The reference for the A / D converter is
internal. One of the feedback lines, for example, sY+ is used as a Wiper input to the A / D
converter. The register values TSDetect, TSDischarge, TSXSample, and TSYSample are
derived from the switch positions in the diagram. They are provided in
Table 25-2 .
Seven-wire touch screens (5-wires with feedback) are very similar to a normal 5-wire touch
screen, except that the A / D reference is taken from the external circuit. The X+ and X- lines
are still static V+ and V-lines. The Y+ and Y- lines are still Z+/- and Z-/+ and are used to
switch between X and Y axis. However, the reference for the A / D converter is on the sX+
and sX- lines relabelled as sV+ and sV-. The sY+ feedback line is still used as a Wiper input
to the A / D converter.
The register values TSDischarge, TSXSample, and TSYSample are derived from the switch
positions in the diagram. They are provided in
Table 25-2 .