1 Overview
QMatrix devices are digital burst mode charge-transfer (QT)
sensors designed specifically for matrix geometry touch
controls; they include all signal processing functions
necessary to provide stable sensing under a wide variety of
changing conditions. Only a few external parts are required
for operation. The entire circuit can be built within 5 square
centimeters of single-sided PCB area.
QMatrix parts employ transverse charge-transfer ('QT')
sensing, a technology that senses changes in electrical
charge forced across an electrode by a digital edge (Figure
1-1).
QMatrix devices allow for a wide range of key sizes and
shapes to be mixed together in a single touch panel.
The devices use both UART and SPI interfaces to allow key
data to be extracted and to permit individual key parameter
setup. The interface protocol uses simple single byte
commands and responds with single byte responses in most
cases. The command structure is designed to minimize the
amount of data traffic while maximizing the amount of
information conveyed.
In addition to normal operating and setup functions the device
can also report back actual signal strengths and error codes.
QmBtn software for the PC can be used to program the
operation of the IC as well as read back key status and signal
levels in real time.
The parts are electrically identical with the exception of the
number of keys which may be sensed.
1.1 Part differences
Versions of the device are capable of a maximum of 16, 24,
32, and 48 keys.
The QT60xx6 devices are identical to one another in all
respects, except that each device is capable of only the
number of keys specified for each device. These keys can be
located anywhere within the electrical grid of 8 X and 6 Y
scan lines. Unused keys are always pared from the burst
sequence in order to optimize timing performance.
Even with a given part type, such as QT60486, a lesser
number of enabled keys will cause any unused acquisition
burst timeslots to be pared. Thus, if only 40 keys are actually
enabled, only 40 timeslots are used for scanning.
2 Hardware
2.1 Matrix Scan Sequence
The circuit operates by scanning each key sequentially, key
by key. Key scanning begins with location X=0 / Y=0. X axis
keys are known as rows while Y axis keys are referred to as
columns. Keys are scanned sequentially by row, for example
the sequence Y0X0 Y0X1 .... Y0X3, Y1X0 Y1X1... etc.
Each key is sampled up to 64 times in a burst whose length is
determined by the Setups parameter BL, which can be set on
a per-key basis. A burst is completed entirely before the next
key is sampled; at the end of each burst the resulting signal is
converted to digital form and processed. The burst length
directly impacts key gain; each key can have a unique burst
length in order to allow tailoring of key sensitivity on a key by
key basis.
2.2 Oscillator
The oscillator can use either a quartz crystal or a ceramic
resonator. In either case, the XT1 and XT2 must both be
loaded with 22pF capacitors to ground. 3-terminal resonators
having onboard ceramic capacitors are commonly available
and are recommended. An external TTL-compatible
frequency source can also be connected to XT1 in which
case, XT2 should be left unconnected.
The frequency of oscillation should be 16MHz +/-1% for
accurate UART transmission timing.
2.3 Sample Capacitors
The charge sampler capacitors on the Y pins should be the
values shown. They can be X7R ceramic type. The value of
these capacitors is non-critical and can vary from 3.3nF to
10nF; 4.7nF is acceptable in most cases. Heavy Cx load
capacitances may necessitate the use of larger Cs
capacitors.
The Cs capacitor values have no effect on conversion gain.
Unused Y lines should have a 1nF dummy capacitor
connected as shown.
2.4 Sample Resistors
There are 6 sample resistors (Rs) used to perform
single-slope ADC conversion of the acquired charge on each
Cs capacitor. These resistors are directly linked with
acquisition gain. Larger values of Rs will proportionately
increase signal gain. Values of Rs can range from 220K✡ to
1M✡. 220K✡ is a reasonable typical value for most
purposes.
Larger values for Rs will also increase conversion time and
may reduce the fastest possible key sampling rate, which can
impact response time especially with larger numbers of
enabled keys.
2.5 Signal Levels
Using Quantum’s QmBtn™ software it is easy to observe the
absolute level of signal received by the sensor on each key.
The signal values should normally be in the range from 250 to
750 counts with properly designed key shapes (see
appropriate Quantum app note on matrix key design).
QmBtn software is available free of charge on Quantum’s
website.
lQ
2 QT60486-AS 0.07/1103
Advanced information; subject to change
Figure 1-1 Field flow between X and Y elements
overlying panel
X
element
Y
elem ent
