Datasheet

880
SAM9G15 [DATASHEET]
11052D–ATARM–31-Oct-12
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.
In an ideal world (linear, with no loss through switches), the horizontal position is equal to:
VY
M
/ VDD or VY
P
/ VDD.
The implementation with on-chip power switches is shown in Figure 41-6. The voltage measurement at the output of the
switch compensates for the switches loss.
It is possible to correct for switch loss by performing the operation:
[VY
P
- VX
M
] / [VX
P
- VX
M
].
This requires additional measurements, as shown in Figure 41-6.
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