CRTOUCH Datasheet, PDF (8/68 Pages) Freescale Semiconductor, Inc – Capacitive and Resistive Touch Sensing Application Specific IC.

Functional Description
When a voltage is applied at the terminals of one layer, the voltage is linearly distributed across the conductive material. For a
four wire screen, the coordinates of the point of contact can be determined by applying voltage to one layer and reading the
voltage on the other. For a five wire screen, one layer is used only for reading the voltage, while the other has voltages applied
in different combinations for determining the value of each axis. The following table shows how the signals are activated for X
and Y measurements in each type of screen.
Measurement
X
V_dd
Y
z
Table 2. Signal states for a four wire screen
X+
X–
Y+
V_ss
Z
Z
V_dd
Y-
Z
V_ss
Table 3. Signal states for a five wire screen
Measurement
UR
UL
LR
LL
X
V_dd
V_ss
Y
V_dd
V_dd
V_dd
V_ss
V_ss
V_ss
Considering these scanning sequences, the screen’s origin (coordinate 0,0) is located in the intersection of the X– and Y–
terminals for a four wires screen, and at the LL terminal for a five wire screen. It is also important to consider that the connection
between the conductive material of the screen and the terminals of the CRTouch controller represent a series resistance with the
screen. Because of this, the coordinates obtained at the edges of the active region of the screen may be a number of counts above
0 and a number of counts below the maximum value. This variation is given by the construction of the touchscreen, the
impedance of its connection lines, the size of the active area, and so on.
Another electrical phenomenon that needs to be considered is the capacitances created in the screen. Because the active area is
composed of two plates of conductive material, this creates a set of capacitances between the two layers. When combined with
the resistors created in the screen, these form an RC circuit with an associated stabilization time constant. If the voltage of one
layer is read before it stabilizes, then a deviation from the real coordinate is produced. In some devices ESD diodes may be
connected to the resistive panel for improved electrostatic immunity. In this case, low capacitance ESD diodes must be used to
avoid increasing the resulting capacitance.
2.1.2 CRTouch scanning process
The CRTouch has three scanning modes for the resistive screen. One is active when only X and Y coordinate values are desired,
the second activates when the pressure measurement is enabled, and the last one when any of the two touch gestures are enabled.
Before each sample is taken, the CRTouch waits for a configurable amount of time for the signals to stabilize. The following
figure shows the differences in the scanning process for each scanning mode.
CRTouch Data Sheet, Rev. 3
8
Freescale Semiconductor