13892 datasheet(106/156 Pages) FREESCALE

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Analog Integrated Circuit Device Data

106

Freescale Semiconductor

13892

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

Inactive

Inputs TSX1, TSX2, TSY1, TSY2 can be used as general purpose ADC inputs

Interrupt

Interrupt detection is active. Generates an interrupt TSI when plates make

contact. TSI is dual edge sensitive and 30 ms debounced

Reserved

Reserved for a different interrupt mode

Touch Screen

ADC will control a sequential reading of 2 times a XY coordinate pair and 2 times

a contact resistance

Reserved

Reserved for a different reading mode

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

ADC Conversion

Signals sampled

Readout Address (73)

X position

000

X position

001

Dummy

010

Y position

011

Y position

100

Dummy

101

Contact resistance

110

Contact resistance

111

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'.

TSMOD2

TSMOD1

TSMOD0

Mode

Description

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