Revision V1.7/October 2010

©2010 Semtech Corp.

SX8652

www.semtech.com

Page 13

ADVANCED COMMUNICATIONS & SENSING

DATASHEET

15kV ESD Low Power 4-Wire / 5-Wire Resistive

Touchscreen Controller with SPI Interface

4.3. Pressure Measurement

The 4-wire touchscreen allows pressure measurement. The contact resistance between the two sheets are a function of

the pressure applied on the top sheet. Indeed, the a low pressure applied with the finger will create a small contact area.

The measurement setup given in Table 7 allows to find Z1 and Z2.

Arranging Z1 and Z2 with Rxtot and Rytot allows the

An alternative calculation method is using Xpos and Ypos.

Table 7. Measurement setup

4.4. Pen Detection

The pen detection circuitry is used to detect a user action on the

touchscreen. The contact between the two layers generates an interrupt or

starts an acquisition sequence.

Doing a pen detection prior to conversion avoids feeding the host with

dummy data and saves power.

no current will flow so long as pressure is not applied to the surface (see

Figure 10).

When a pressure is applied, a current path is created and brings X+ to the

Figure 10. 4-wire pen detection circuitry

can also improve the detection on panel with high resistance.

The pen detection will set the PENIRQ bit of the RegStat register. The PENIRQ bit will be cleared and the NIRQ will be de-

asserted as soon as the host reads the status register.

In PENDET mode, the pen detection will set NIRQ low.

Measurement

Vref +

Vref-

ADC +

X

X+

X-

Y+

Y

Y+

Y-

X+

Z1

X+

Y-

Y+

Z2

X+

Y-

X-

z1

4095

R4

R1

R4

R

T

+

+

---------------------------------

⋅

=

z2

4095

R4

Rt

+

R1

R4

R

T

+

+

---------------------------------

⋅

=

R

T

Rytot

Ypos

4095

------------

z2

z1

-----

1

–

⋅

⋅

=

R

T

Rytot Y

⋅ pos

4095

-------------------------------

4095

z1

------------

1

–

Rxtot

1

Xpos

4095

-------------

–

⋅

–

⋅

=

X-

X+

R2

R1

Vref

+

-

Y-

Y+

R4

R3

Sb

Rb

IRQ

Q

Q

Internal

logic