qt511 Quantum Research Group, qt511 Datasheet - Page 4

• Manufacturer Part Number: qt511
• Description: Qwheel? Touch Slider Ic
• Manufacturer: Quantum Research Group

Note: During calibration, the device cannot communicate.
DRDY will remain low during this interval.
1.7 Sensitivity Setting
The sensitivity of the slider area to finger detection is
dependent on the values of the three Cs capacitors (Section
2.2) and the threshold setting (Section 3.3.5). Larger values
of Cs increase sensitivity and also reduce granularity (missing
codes), at the expense of higher power consumption due to
longer acquisition bursts.
The threshold setting can be used to fine tune the sensitivity
of the sensing element. When setting the threshold, use the
smallest finger size for which detection is desired (normally a
6mm diameter spot), and probe at one of the two center
connection points where sensitivity is lowest. The stretches
between connection points are generally slightly higher in
sensitivity due to the collection of charge from two channels.
A ‘standard finger’ probe can be made by taking a piece of
metal foil of the required diameter, gluing it on the end of a
cylinder of sponge rubber, and connecting it to ground with a
wire. This probe is pressed against the panel centered on one
of the middle two connection points; the threshold parameter
is iterated until the sensor just detects. It is important to push
the probe into the panel quickly and not let it linger near the
electrode afterwards, so that the drift compensation
mechanism does not artificially create a threshold offset
during the iteration process. Between threshold changes, the
probe must be removed to at least 100mm from the panel.
1.8 Drift Compensation
The device features an ability to compensate for slow drift
due to environmental factors such as temperature changes or
humidity. Drift compensation is performed under host control
via a special drift command. See Section 3.3.3 for further
details.
1.9 Error Status
An error flag status is provided via a special command. An
error can only occur when a finger was touching the wheel
lQ
Figure 1-3 E510 PCB Layout (Applies also to QT511)
4
shown in Figure 1-3.
2.1 Electrode Construction
The wheel electrode should be a resistive element of
between 200K to 500K ohms (400K nominal target value)
between each set of connection points, of a suitable diameter
and width. Under heavy capacitive loading (for example if the
element must be placed immediately over a ground plane
within a millimeter), the resistance might need to be lowered.
Observe the sensing pulses for flatness on their tops in the
middle of a segment using a small coin and scope probe to
make sure the pulses fully settle before the falling edge (see
app note AN-KD02 Figure 7).
There are no known diameter restrictions other than those
governed by human factors.
The electrode can be made of a series chain of discrete
resistors with copper pads on a PCB, or from ITO (Indium Tin
Oxide, a clear conductor used in LCD panels and touch
screens) over a display. Thick-film carbon paste can also be
used, however linearity might be a problem as these films are
notoriously difficult to control without laser trimming or
scribing.
The linearity of the wheel is governed largely by the linearity
and consistency of the resistive element. Positional accuracy
to within 5% is routinely achievable with good grade resistors
and a uniform construction method.
Table 1-2 Recommended Cs vs. Materials
Thickness,
mm
0.4
0.8
1.5
2.5
3.0
4.0
electrode area during power-on or
recalibration, and then removed. In this
sequence of events, the finger is ‘calibrated
away’ and is not recognized as a touch.
When the finger is removed, the signals
from the wheel are inverted and a position is
reported as though the wheel has been
touched. However, this position report is in
error.
After any calibration event (i.e. a power-on
cycle or a CAL command) the next
detection event should be checked to see if
it is in error by using the special error
command. If it an error is reported, the
device should be immediately calibrated
again so that the wheel becomes properly
functional (Section 3.3.2).
2 Wiring & Parts
The device should be wired according to
Figure 1-1. An example PCB layout is
(
Acrylic
ε ε ε ε
100nF
10nF
22nF
47nF
R
=2.8)
-
-
QT511-ISSG R6.01/1005
Borosilicate glass
(
ε ε ε ε
100nF
5.6nF
10nF
22nF
39nF
47nF
R
=4.8)