Datasheet

922
SAM9G35 [DATASHEET]
11053E–ATARM–10-Mar-2014
41.7 Touchscreen
41.7.1 Touchscreen Mode
The TSMODE parameter of “ADC Touchscreen Mode Register” is used to enable/disable the Touchscreen functionality,
to select the type of screen (4-wire or 5-wire) and, in the case of a 4-wire screen, to activate (or not) the pressure
measurement.
In 4-wire mode, channel 0, 1, 2 and 3 must not be used for classic ADC conversions. Likewise, in 5-wire mode, channel
0, 1, 2, 3, and 4 must not be used for classic ADC conversions.
41.7.2 4-wire Resistive Touchscreen Principles
A resistive touchscreen is based on two resistive films, each one being fitted with a pair of electrodes, placed at the top
and bottom on one film, and on the right and left on the other. In between, there is a layer acting as an insulator, but also
enables contact when you press the screen. This is illustrated in Figure 41-5.
The TSADC controller has the ability to perform without external components:
 Position Measurement
 Pressure Measurement
 Pen Detection
Figure 41-5. Touchscreen Position Measurement
41.7.3 4-wire Position Measurement Method
As shown in Figure 41-5, to detect the position of a contact, a supply is first applied from top to bottom. Due to the linear
resistance of the film, there is a voltage gradient from top to bottom. When a contact is performed on the screen, the
voltage propagates at the point the two surfaces come into contact with the second film. If the input impedance on the
right and left electrodes sense is high enough, the film does not affect this voltage, despite its resistive nature.
For the horizontal direction, the same method is used, but by applying supply from left to right. The range depends on the
supply voltage and on the loss in the switches that connect to the top and bottom electrodes.
X
M
X
P
Y
M
Y
P
X
P
X
M
Y
P
VDD
GND
Volt
Horizontal Position Detection
Y
P
Y
M
X
P
VDD
GND
Volt
Vertical Position Detection
Pen
Contact