FUNCTIONAL DEVICE OPERATION
ADC SUBSYSTEM
TOUCH SCREEN INTERFACE
The touch screen interface provides all circuitry required for the readout of a 4-wire resistive touch screen. The touch screen
X plate is connected to TSX1 and TSX2 while the Y plate is connected to TSY1 and TSY2. A local supply TSREF will serve as
a reference. Several readout possibilities are offered.
In order to use the ADC inputs and properly convert and readout the values, the bit ADSEL should be set to a 1. This is valid
for touch screen readings as well as for general purpose reading on the same inputs.
The touch screen operating modes are configured via the TSMOD[2:0] bits show in the following table.
Table 95. Touch Screen Operating Mode
TSMOD2
TSMOD1
TSMOD0
Mode
Inactive
Description
Inputs TSX1, TSX2, TSY1, TSY2 can be used as general purpose ADC inputs
x
0
0
0
1
Interrupt detection is active. Generates an interrupt TSI when plates make
contact. TSI is dual edge sensitive and 30 ms debounced
0
Interrupt
Reserved
Touch Screen
Reserved
Reserved for a different interrupt mode
1
0
0
1
1
x
ADC will control a sequential reading of 2 times a XY coordinate pair and 2 times
a contact resistance
Reserved for a different reading mode
1
1
x
In inactive mode, the inputs TSX1, TSX2, TSY1, and TSY2 can be used as general purpose inputs. They are respectively
mapped on ADC channels 4, 5, 6, and 7.
In interrupt mode, a voltage is applied to the X-plate (TSX2) via a weak current source to VCORE, while the Y-plate is
connected to ground (TSY1). When the two plates make contact both will be at a low potential. This will generate a pen interrupt
to the processor. This detection does not make use of the ADC core or the TSREF regulator, so both can remain disabled.
In touch screen mode, the XY coordinate pairs and the contact resistance are read.
The X-coordinate is determined by applying TSREF over the TSX1 and TSX2 pins while performing a high-impedance reading
on the Y-plate through TSY1. The Y coordinate is determined by applying TSREF between TSY1 and TSY2, while reading the
TSX1 pin.
The contact resistance is measured by applying a known current into the TSY1 terminal of the touch screen and through the
terminal TSX2, which is grounded. The voltage difference between the two remaining terminals TSY2 and TSX1 is measured by
the ADC, and equals the voltage across the contact resistance. Measuring the contact resistance helps in determining if the touch
screen is touched with a finger or stylus.
To perform touch screen readings, the processor will have to select the touch screen mode, program the delay between the
conversions via the ATO and ATOX settings, trigger the ADC via one of the trigger sources, wait for an interrupt indicating the
conversion is done, and then read out the data. In order to reduce the interrupt rate and to allow for easier noise rejection, the
touch screen readings are repeated in the readout sequence.
Table 96. Touch Screen Reading Sequence
Readout Address (73)
ADC Conversion
Signals sampled
0
1
2
3
4
5
6
7
X position
000
001
010
011
100
101
110
111
X position
Dummy
Y position
Y position
Dummy
Contact resistance
Contact resistance
Notes
73. Address as indicated by ADA1[2:0] and ADA2[2:0]
The dummy conversion inserted between the different readings is to allow the references in the system to be pre-biased for
the change in touch screen plate polarity and will read out as '0'.
13892
Analog Integrated Circuit Device Data
106
Freescale Semiconductor
FREESCALE [ Freescale ]
