MXT1664T3-ABESSQTP datasheet(24/98 Pages) MICROCHIP

24 / 98 page

MXT1664T3 2.0

DS40002054A-page 24

 2018 Microchip Technology Inc.

3.0

TOUCHSCREEN BASICS

3.1

Sensor Construction

A touchscreen is usually constructed from a number of transparent electrodes. These are typically on a glass or plastic

substrate. They can also be made using non-transparent electrodes, such as copper or carbon. Electrodes are constructed

from Indium Tin Oxide (ITO) or metal mesh. Thicker electrodes yield lower levels of resistance (perhaps tens to hundreds of

/

square) at the expense of reduced optical clarity. Lower levels of resistance are generally more compatible with capacitive

sensing. Thinner electrodes lead to higher levels of resistance (perhaps hundreds to thousands of

/square) with some of

the best optical characteristics.

Interconnecting tracks can cause problems. The excessive RC time constants formed between the resistance of the track and

the capacitance of the electrode to ground can inhibit the capacitive sensing function. In such cases, the tracks should be

replaced by screen printed conductive inks (non-transparent) outside the touchscreen viewing area.

3.2

Electrode Configuration

The specific electrode designs used in Microchip touchscreens are the subject of various patents and patent

applications. Further information is available on request.

The device supports various configurations of electrodes as summarized in Section 4.0 “Sensor Layout”.

3.3

Scanning Sequence

All nodes are scanned in sequence by the device. There is a full parallelism in the scanning sequence to improve overall

response time. The nodes are scanned by measuring capacitive changes at the intersections formed between the first X line

and all the Y lines. Then the intersections between the next X line and all the Y lines are scanned, and so on, until all X and Y

combinations have been measured.

The device can be configured in various ways. It is possible to disable some nodes so that they are not scanned at all. This

can be used to improve overall scanning time.

3.4

Touchscreen Sensitivity

3.4.1

ADJUSTMENT

Sensitivity of touchscreens can vary across the extents of the electrode pattern due to natural differences in the parasitic

capacitance of the interconnections, control chip, and so on. An important factor in the uniformity of sensitivity is the

electrode design itself. It is a natural consequence of a touchscreen pattern that the edges form a discontinuity and

hence tend to have a different sensitivity. The electrodes at the far edges do not have a neighboring electrode on one

side and this affects the electric field distribution in that region.

A sensitivity adjustment is available for the whole touchscreen. This adjustment is a basic algorithmic threshold that

defines when a node is considered to have enough signal change to qualify as being in detect.

3.4.2

MECHANICAL STACKUP

The mechanical stackup refers to the arrangement of material layers that exist above and below a touchscreen. The

arrangement of the touchscreen in relation to other parts of the mechanical stackup has an effect on the overall

sensitivity of the screen. QMatrix technology has an excellent ability to operate in the presence of ground planes close

to the sensor. QMatrix sensitivity is attributed more to the interaction of the electric fields between the transmitting (X)

and receiving (Y) electrodes than to the surface area of these electrodes. For this reason, stray capacitance on the X

or Y electrodes does not strongly reduce sensitivity.

Front panel dielectric material has a direct bearing on sensitivity. Plastic front panels are usually suitable up to about

2.2 mm, and glass up to about 4.5 mm (dependent upon the screen size and layout). The thicker the front panel, the

lower the signal-to-noise ratio of the measured capacitive changes and hence the lower the resolution of the

touchscreen. In general, glass front panels are near optimal because they conduct electric fields almost twice as easily

as plastic panels.

NOTE

Care should be taken using ultra-thin glass panels as retransmission effects can occur, which can

significantly degrade performance.

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