of the X drive pulse. The charge emitted by the X electrode is
partly received onto the corresponding Y electrode which is
1 - OVERVIEW
The QT60320D is a digital burst mode charge-transfer (QT)
sensor designed specifically for matrix geometry touch
controls; it includes all signal processing functions necessary
to provide stable sensing under a wide variety of changing
conditions. Only a few low cost external parts are required for
operation. The entire circuit can be built within about 1
square inch of PCB area (smt).
then processed. The QT60320D matrix uses
8
'X'
edge-driven rows and 4 'Y' sense columns to allow up to 32
keys.
The charge flows are absorbed by the touch of a human
finger (Figure 1-2) resulting in a decrease in coupling from X
to Y; coupled charge increases in the presence of a
conductive film like water (Figure 1-3) which acts to bridge
the two elements. Increasing signals due to water films are
quite easy to discern and are not detected by the IC.
Figure 1-1 Field flow between X and Y elements
1.2 CIRCUIT MODEL
An electrical circuit model is shown in Figure 1-4. The
coupling capacitance between X and Y electrodes is
represented by Cx. While the reset switch is open, a
sampling switch is gated so that it transfers charge flows only
from the rising edge of X into the sample capacitor Cs. Cs is
a large value capacitor, typically in the range of 1 - 50nF. The
voltage rise captured on Cs after each X edge is quite small,
on the order of a millivolt, while changes due to touch are on
typically the order of 10's of microvolts. The X pulse can be
repeated in a burst consisting of up to several hundred
pulses to build up the voltage (and the change in voltage due
to touch) to a larger value. Longer bursts increase system
gain by collecting more charge; gain can thus be digitally
overlying panel
X
Y
element
element
manipulated to achieve the required sensitivity on
key-by-key basis during scanning.
a
The 60320 uses burst-mode charge transfer methods
pioneered and patented by Quantum, including charge
cancellation methods which allow for a wide range of key
sizes and shapes to be mixed together in a single keypanel.
These features permit the construction of entirely new
classes of keypanels never before contemplated, such as
touch-sliders, back-illuminated keys, and arbitrary shape
keypanels, all at very low cost.
If the voltage on Cs rises excessively it can fall outside of the
ADC's range. To reduce the voltage again without affecting
Figure 1-2 Field Flows When Touched
The QT60320D uses an asynchronous serial (uart) interface
running at 9600 baud to allow key data to be extracted and to
permit individual key parameter setup. The interface protocol
uses simple ASCII commands and responds with either
ASCII or binary results depending on the command.
In addition to normal operating and setup commands the
device can also report back actual key signal strength and
error codes. Spare eeprom memory (over 80 bytes) can also
be written to and read to save the system designer from
having to install and interface to a separate eeprom.
overlying panel
The IC also includes 4 readable input (I1..I4) pins and 8
settable output (O1..O4) pins which can be used in any way
desired, including to scan a secondary keypad of up to 32
contact closures. Alternatively they can be used to remotely
activate panel LEDs, buzzers, or other types of indicators.
X
Y
element
element
QmBtn software for the PC can be used to program a board
containing the IC as well as read back key status and signal
levels in real time.
Figure 1-3 Fields With a Conductive Film
The QT60320D employs transverse charge-transfer ('QT')
sensing, a new technology that senses the charge forced
across an electrode set by a digital edge.
1.1 FIELD FLOWS
Figure 1-1 shows how charge is transferred across the
electrode set to permeate the overlying panel material; this
charge flow exhibits a rapid dQ/dt during the edge transitions
LQ
2
QT60320D R1.11/12.07.03
QUANTUM [ QUANTUM RESEARCH GROUP ]
