AT32UC3L064_1 Datasheet, PDF (614/825 Pages) ATMEL Corporation – AVR32 32-bit Microcontroller

AT32UC3L
26.7
Resisitve Touch Screen
The ADCIFB embeds an integrated resistive touch screen sequencer that can be used to calcu-
late contact coordinates on a resistive film touch screen. When instructed to start, the integrated
touch screen sequencer automatically applies a sequence of voltage patterns to the touch
screen films and the Analog-to-Digital Conversion cell is used to measure the effects. The result-
ing measurements can be used to calculate the horizontal and vertical contact coordinates.
The touch screen film is connected to the ADCIFB using the AD and DP pins. See Section
26.7.3 for details.
Touch Screen ADC Mode is enabled by writing a one to the TSAMOD field in the Mode Register
(MR). In this mode, channels TSPO+0 though TSPO+3 are automatically enabled where TSPO
refers to the Touch Screen Pin Offset field in the Mode Register. For each conversion sequence,
all enabled channles before TSPO+0 and after TSPO+3 are converted as ordinary ADC chan-
nels, producing 1 conversion result each. When the sequencer enters the TSPO+0 channel the
touch screen sequencer will take over control and convert the next 4 channles as described in
Section 26.7.4.
26.7.1
Resistive Touch Screen Principles
A resistive touch screen is based on two resistive films, each one 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 makes a connection when pressure is applied to the
screen. This is illustrated in Figure 26-2 on page 614.
Figure 26-2. Touch Screen Position Measurement
Pen
Contact
X_P
Y_M
Y_P
X_M
VDD
X_P
VDD
Y_P
Y_P
X_P
Volt
X_M
GND
Vertical Position Detection
Volt
Y_M
GND
Horizontal Position Detection
26.7.2
Position Measurement Method
As shown in Figure 26-2 on page 614, to detect the position of a contact, voltage is first applied
to X_P (top) and X_m (bottom) leaving Y_p and Y_m tristated. Due to the linear resistance of the film,
there is a voltage gradient from top to bottom on the first film. When a contact is performed on
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