Datasheet

933
SAM9G45 [DATASHEET]
Atmel-6438O-ATARM-SAM9G45-Datasheet_08-Dec-15
The field TSSHTIM defines also the time the power switches of the Touchscreen are closed when the TSADCC
performs a conversion for the Touchscreen.
39.7 Touchscreen
39.7.1 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. Between the two, there is a layer that acts as an
insulator, but also enables contact when you press the screen. This is illustrated in Figure 39-2.
The TSADC controller has the ability to perform without external components:
 Position Measurement
 Pressure Measurement
 Pen Detection
Figure 39-2. Touchscreen Position Measurement
39.7.2 Position Measurement Method
As shown in Figure 39-2, 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.
In an ideal world (linear, with no loss through switches), the horizontal position is equal to:
VY
M
/ VDD or VY
P
/ VDD.
The proposed implementation with on-chip power switches is shown in Figure 39-3. The voltage measurement at
the output of the switch compensates for the switches loss.
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