SX8674ICSTRT datasheet(16/64 Pages) SEMTECH

16 / 64 page

ADVANCED COMMUNICATIONS & SENSING

Rev 1 – 25

th July 2011

www.semtech.com

SX8674/SX8675/SX8676

Haptics Enabled Multitouch 4/5-Wire

Resistive Touchscreen Controller with Proximity Sensing

Rxtot

Rytot

Ypos

4095

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

⋅

Re-arranging z1 and z2 gives:

------

–

⋅

This finally results in:

Rytot

Ypos

4095

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

------

–

⋅

The touch resistance calculation above hence requires three channel measurements (Ypos, z2 and z1) and one

specification data (Rytot).

An alternative calculation method is using Xpos, Ypos, one z channel and both Rxtot and Rytot as shown in the

next calculations.

R1 is inversely proportional to the X coordinate:

Rxtot

Xpos

4095

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

–

⋅

Substituting R1 and R4 into z1 and rearranging terms gives:

Rytot Y

⋅ pos

4095

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

4095

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

–

Rxtot

Xpos

4095

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

–

⋅

–

⋅

Please note that the chip only outputs z1, z2, etc. The calculation of RT itself with the formulas above must be

performed by the host.

4.3.1

Bias Time (POWDLY)

In order to perform correct measurements, some time must be given for the touch screen to reach a proper Vref

bias level before the conversion is actually performed. It is a function of the PCB trace resistance connecting the

chip to the touchscreen and also the capacitance of the touchscreen. If tau is this RC time constant, then

POWDLY duration must be programmed to 10 tau to reach 12 bits accuracy.

Adding a capacitor from the touch screen electrodes to ground may also be used to minimize external noise (if

the touchscreen is used as the proximity sensor, make sure you do not exceed the maximum capacitive load for

required for proper proximity sensing operation). The low-pass filter created with the capacitor may increase

settling time requirement. Therefore, POWDLY can be used to stretch the acquisition period and delay

conversion appropriately.

POWDLY can be estimated by the following formula:

PowD ly

Rtouch

Ctouch

4.4

Pen Detection

The pen detection circuitry is used both to detect a user action and generate an interrupt or start an acquisition in

PENDET and PENTRG mode respectively. Doing pen detection prior to conversion avoids feeding the host with

dummy data and saves power. Pen detection is also used to disable and resume proximity sensing. For more

details on pen detection usage please refer to §4.7.

A 4-wire touchscreen will be powered between X+ and Y- through a resistor RPNDT so no current will flow as

long as no pressure is applied to the surface (see figure below). When a touch occurs, a current path is created

bringing X+ to the level defined by the resistive divider determined by RPNDT and the sum of R1, RT and R4.

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