qt60320c Quantum Research Group, qt60320c Datasheet - Page 2

• Manufacturer Part Number: qt60320c
• Description: 32-key Qmatrix? Charge-transfer Ic
• Manufacturer: Quantum Research Group

1 - OVERVIEW
The QT60320 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).
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.
The QT60320 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.
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.
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.
The QT60320 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
Figure 1-1 Field flow between X and Y elements
elem e nt
X
overly ing panel
elem ent
Y
2
of the X drive pulse. The charge emitted by the X electrode is
partly received onto the corresponding Y electrode which is
then processed. The QT60320 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.
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
manipulated to achieve the required sensitivity on a
key-by-key basis during scanning.
If the voltage on Cs rises excessively it can fall outside of the
ADC's range. To reduce the voltage again without affecting
gain, one of two (or both) Cz capacitors can be switched to
Figure 1-3 Fields With a Conductive Film
Figure 1-2 Field Flows When Touched
element
X
QT60320C R1.08/01.03
ove rly in g p an e l
elem ent
Y