About Touch screen / Touch panel
:: General Informations collected on the Internet , moved from the bitmap2lcd forum ::
The active layers of a 4-wire type touch screen consist of a partially conductive (resistive) coating applied uniformly to the panel. Conductive bus bars are screened with silver ink across opposing edges of the panel.
The ridged and flexible panels are mounted with the bus bars perpendicular to each other.
Measurements are made by applying a voltage gradient across one of the layers and measuring the voltage on the other layer. This measurement is made twice, once with the
gradient across the ridged layer and the measurement taken from the flexible layer and again with the gradient applied to the flexible layer and the measurement taken from the ridged layer.
The gradient is usually produced by grounding one bus bar and applying +5v to the other bus bar. This will produce a smooth voltage gradient in one axis across the panel.
With a 4-wire touch screen two separate setups and measurements are required, one in the X-axis (left-right) and one in the Y-axis (up-down) to define the touch point. Figure 2
illustrates the setup for making the two measurements. The actual switching of the 4 touch screen connections is done using an array of low on-resistance FET transistors and the
voltage measurements are made with an A/D converter. The host micro controller controls both these elements. When a layer is being used as a sens layer all other connections to
that layer must be left floating.
The resistance of the bus bars and the connection circuitry introduces an error (offset) in the voltage measurements.
These offsets can also drift with changes in temperature, humidity and time. If the touch screen is to be used only with a finger the offsets will constitute a small percentage of the voltage represented by the large size of the finger and can be ignored. If, however, the touch screen is to be used with a stylus for drawing or signature capture then these offsets should be taken into account. This can be done by calibrating the screen periodically or by utilizing an 8-wire touch screen.
8-wire touch screens compensate for drift by adding 4 additional reference lines. This allows the voltage to be measured directly at the touch screen bus bars. Note: you can use an 8-wire touch screen in 4-wire mode by connecting the drive and reference lines together. Use of this type of touch screen won’t eliminate the need for an initial calibration of the touch screen but should eliminate the need for any subsequent calibrations.
The simplest way to connect an 8- wire touch screen and take advantage of its benefits is to use a ratio-metric A/D converter supplying the reference voltages fromthe touch screen directly into the references of the A/D converter.
The 5-wire touch screen differs from the 4-wire type mainly layer, the ridged layer, while the other layer is the sense for making the measurements.
The sequence of events during the measurements is this. The voltage gradient is set up on the x-axis and a voltage measurement is taken from the sens layer.
The voltage gradient is then switched to the y-axis and another voltage measurement is taken from the same sense layer.
Hantronix Source Application Note
TSC2000 / TSC2003 / TSC2005 / TSC2046
MAX11801 / MAX11801 / MAX11803
MAX11810 / MAX11811
MAX1233 / MAX1234
MAX7843 / MAX7846
MXB7843 / MXB7846
AT42QT4120 / AT42QT4160
LM8300 / LM8500
AR 1000 / AR1010 / AR1020