PC34709VKR2 datasheet(79/142 Pages) FREESCALE

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Analog Integrated Circuit Device Data



Freescale Semiconductor

34709

Functional Block Description

Touch Screen Pen detection bias can be enabled via the TSPENDETEN bit in the AD0 register. When this bit is enabled and a

pen touch is detected, the TSPENDET bit in the Interrupt Status 0 register is set and the INT pin is asserted - unless the interrupt

is masked. Pen detection is only active when TSEN is low.

The reference for the touch screen (Touch Bias) is TSREF and is powered from VCORE. During touch screen operation, TSREF

is a dedicated regulator. No loads other than the touch screen should be connected here. When the ADC performs non touch

screen conversions, the ADC does not rely on TSREF and the reference is disabled.

The readouts are designed such that the on chip switch resistances are of no influence on the overall readout. The readout

scheme does not account for contact resistances, as present in the touch screen connectors. The touch screen readings will

have to be calibrated by the user or the factory, where one has to point with a stylus to the opposite corners of the screen. When

reading the X-coordinate, the 10-bit ADC reading represents a 10-bit coordinate, with ‘0’ for a coordinate equal to X-, and full

scale ‘1023’ when equal to X+. When reading the Y-coordinate, the 10-bit ADC reading represents a 10-bit coordinate, with ‘0’

for a coordinate equal to Y-, and full scale ‘1023’ when equal to Y+. When reading contact resistance, the 10-bit ADC reading

represents the voltage drop over the contact resistance created by the known current source, multiplied by 2.

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 pin of the touch screen and through

the TSX2 pin, 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 determine if the touch

screen is touched with a finger or a stylus.

The TSSELx[1:0] allows the application processor to select its own reading sequence. The TSSELx[1:0] determines what is read

during the touch screen reading sequence, as shown in Table 70. The touch screen will always start at TSSEL0 and read up to

and including the channel set by TSSEL at the TSSTOP[2:0] bits. For example when TSSTOP[2:0] = 010, it will request the ADC

to read channels indicated in TSSEL0, TSSEL1, and TSSEL2. When TSSTOP[2:0] = 111, all eight addresses will be read.

The touch screen readings can be repeated, as in the following example readout sequence, to reduce the interrupt rate and to

allow for easier noise rejection. The dummy conversion inserted between the different readings allows the references in the

system to be pre-biased for the change in touch screen plate polarity. It will read out as ‘0’.

A touch screen reading will take precedence over an ADC sequence. If an ADC reading is triggered during a touch screen event,

the ADC sequence will be overwritten by the touch screen data.

The first touch screen conversion can be delayed from 0 (default) to 600

s by programming the TSDLY1[3:0] bits. The

TSDLY2[3:0] controls the delay between each of the touch screen conversions from 0 to 600

s. TSDLY[2:0] sets the delay after

the last address is converted. TSDLY1, 2, and 3 are set to 0 by default.

Table 70. Touch Screen Action Select

TSSELx[1:0]

Signals Sampled

Dummy to discharge TSREF cap

X - plate

Y - plate

Contact

Table 71. TSDLYx[3:0]

TSDLYx[3:0]

Delay in uS

0000

0001

0010

0011

120

0100

160

0101

200

0110

240

PC34709VKR2 Datasheet(PDF) 79 Page - Freescale Semiconductor, Inc

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