lQ
QT401
QS
LIDE
™ T
OUCH
S
LIDER
IC
1-dimensional finger-touch slider
Extremely simple circuit - no external active components
Completely passive sensing strip: no moving parts
Compatible with clear ITO over LCD construction
SPI slave-mode interface
Self-calibration and drift compensation modes
Proximity mode for wake up of a product
Spread-spectrum operation for optimal EMC compliance
2.5 - 5.5V single supply operation; very low power
14-pin SOIC, TSSOP lead-free packages
Inexpensive, simple 1-sided PCB construction possible
E401 reference design board available
VDD
SDO
/SS
SCLK
SDI
SNS1A
SNS1B
1
2
3
4
5
6
7
14
13
GND
DRDY
DETECT
PROX
N/A
SNS2A
SNS2B
QT401
12
11
10
9
8
APPLICATIONS
Lighting controls
Appliance controls
Touch-screens
Automotive controls
The QT401 QSlide™ IC is a 1-dimensional position sensor IC designed for human interfaces. This unique IC allows designers
to create speed or volume controls, menu bars, and other more exotic forms of human interface on the panel of an appliance
or over an LCD display.
The device uses a simple, inexpensive resistive sensing strip between two connection end points. The strip element can be an
arc or a semicircle or simply linear. The strip can also be used as a proximity sensor out to several centimeters, to wake up an
appliance or display from a sleep mode in a dramatic fashion.
The QT401 can report a single rapid touch anywhere along the slider element, or, it can track a finger moving laterally along
the slider strip in real time. The device self-calibrates under command from a host controller in one of two modes.
The QT401 is a new type of capacitive sensor based on Quantum’s patented charge-transfer methods. This device uses two
channels of simultaneous sensing across a resistive element to determine finger position, using mathematical analysis. The
accuracy of QSlide™ is theoretically the same as a conventional potentiometer. A positional accuracy of 5% (or better) is
relatively easy to achieve.
The acquisitions are performed in a burst mode which uses proprietary spread-spectrum modulation for superior noise
immunity and low emissions.
The output of the QT401 can also be used to create discrete controls on a strip, by interpreting sets of number ranges as
buttons. For example, the number range 0..19 can be button A, 30..49 button B, 60..79 button C etc. Continuous slider action
and discrete controls can be mixed on a single strip, or, the strip can be reinterpreted differently at different times, for example
when used below or on top of an LCD to act as a menu input device that dynamically changes function in context. In this
fashion the QT401 can be used to create ultra-simple, extremely inexpensive ‘touch screens’. The device is compatible with
ITO (Indium Tin Oxide) overlays on top of various displays.
T
A
-40 C ~ +85 C
0
0
AVAILABLE OPTIONS
SO-14
QT401-ISG
TSSOP-14
QT401-ISSG
LQ
Copyright © 2004 QRG Ltd
QT401 R10.04/0505
1 Operation
The QT401 uses a SPI slave mode interface for control
and data communications with a host controller.
Acquisition timings and operating parameters are
under host control; there are no option jumpers and the
device cannot operate in a stand-alone mode.
The positional output data is a 7-bit binary integer
(0...127) indicating position from left (0) to right (127).
Like all QProx™ devices, the QT401 operates using
bursts of charge-transfer pulses; burst mode permits
an unusually high level of control over spectral
modulation, power consumption, and response time.
The QT401 modulates its bursts in a spread-spectrum
fashion in order to heavily suppress the effects of
external noise, and to suppress RF emissions.
VIN
C1
2.2uF
Figure 1-1 QT401 Wiring Diagram
Regulator
VIN
VOUT
GND
C2
2.2uF
R1
22k
R2
100k
13
2
3
4
5
1 QT401
VDD
8
SNS2B
SNS2A 9
DRDY
SDO
/SS
SCLK
SDI
Cs2
100nF
127
Slider Element
60K~150K ohms
total resistance
SNS1A 6
SNS1B 7
SPI BUS
R3
1K
Proximity
Touch Detect
C3 1nF
Cs1
100nF
11 PROX
0
1.1 Synchronized Mode
Refer also to Figure 3-1, page 6.
Sync mode allows the host device to control the rep etition
rate of the acquisition bursts, which in turn govern response
time and power consumption. The maximum spacing from the
end of one burst to the start of the next in this mode is 1
second.
In sync mode, the device will wait for the SPI slave select line
/SS to fall and rise and will then do an acquisition burst;
actual SPI clocks and data are optional. The /SS pin thus
becomes a ‘sync’ input in addition to acting as the SPI
framing control.
Within 35µs of the last rising edge of CLK, the device will
enter a low power sleep mode. The rising edge of /SS must
occur after this time; when /SS rises, the device wakes from
sleep, and shortly thereafter does an acquisition burst. If a
more substantial sleep time is desired, /SS should be made
to rise some delay period later.
By increasing the amount of time spent in sleep mode, the
host can decrease the average current drain at the expense
of response time. Since a burst typically requires 31ms (at
3.3V, reference circuit), and an acceptable response time
might be ~100ms, the power duty cycle will be 31/100 or 31%
of peak current.
If power is not an issue the device can run constantly under
host control, by always raising /SS after 35µs from the last
rising edge of CLK. Constant burst operation can be used by
the host to gather more data to filter the position data further
to suppress noise effects , if required.
12 DETECT
VSS
14
C4 1nF
Mains Sync:
Sync mode can be used to sync to mains
frequency via the host controller, if mains interference is
possible (ie, running as a lamp dimmer control). The host
should issue SPI commands synchronously with the mains
frequency. This form of operation will heavily suppress
interference from low frequency sources (e.g. 50/60Hz),
which are not easily suppressed using spread-spectrum burst
modulation.
Cross-talk suppression:
If two more QT401’s are used in
close proximity, or there are other QTouch™ type device(s)
close by, the devices can interfere strongly with one another
to create position jitter or false triggering. This can be
suppressed by making sure that the devices do not perform
acquisition bursts at overlapping times. The host controller
can make sure that all such devices operate in distinctly
different timeslots, by using a separate /SS line or Sync
signal for each part.
1.2 Free-Run Mode
If /SS stays high, the device will acquire on its own
repetitively approximately every 60ms (Figure 1-2). This
mode can be used to allow the part to function as a prox or
touch detector first, perhaps to wake a host controller. Either
the PROX or DETECT can be used as a wakeup.
In free-run mode, the device does not sleep between acquire
bursts. In this mode the QT401 performs automatic drift
compensation at the maximum rate of one count per 180
acquisition burst cycles, or about one count every 3 seconds
without host intervention. It is not possible to change this
Figure 1-2 Free-Run Timing Diagram ( /SS = high )
~31ms
Acquire Bur
~3.8ms
~30us
~31ms
DRDY from QT
~25ms
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QT401 R10.04/0505
Table 1-1 Pin Descriptions
PIN
1
2
3
4
5
6
7
8
9
10
11
12
13
14
NAME
VDD
SDO
/SS
SCLK
SDI
SNS1A
SNS1B
SNS2B
SNS2A
N/A
PROX
DETECT
DRDY
VSS
TYPE
Power
O
I
I
I
I/O
I/O
I/O
I/O
O
O
O
O
Ground
DESCRIPTION
Positive power pin (+2.5 .. +5V)
Serial data output
Slave Select pin. Active low input to enable serial clocking. 1K ohms in series recommended.
Serial clock input. Clock idles high
Serial data input
Sense pin (to Cs1)
Sense pin (to Cs1, Rs1); connects to ‘0’ end of slider element
Sense pin (to Cs2, Rs2); connects to ‘127’ end of slider element
Sense pin (to Cs2)
Leave open
Active high when a hand is near the slider. May be left unconnected.
Note (1)
Active high when slider is touched. May be left unconnected.
Note (1)
Data ready output. Goes high to indicate it is possible to communicate with the QT401.
Note (1)
Negative power pin
Note (1):
Pin floats briefly after wake from Sleep mode.
setting of drift compensation in Free-Run mode. See also
Section 3.3.4.
1.5 Position Data
The position value is internally calculated and can be
accessed only when the slider is touched (Detect pin is high).
The position data is a 7-bit number (0..127) that is computed
in real time; the end numbers (0, left; 127, right) map to the
physical ends by one of two possible calibration methods
(see Section 1.6). The position data will update either with a
single rapid touch or will track if the finger is moved
lengthwise along the surface of the slider element. The
position data ceases to be reported when touch detection is
no longer sensed.
1.3 Sleep Mode
After an SPI transmission, the device will enter a low power
sleep state; see Figure 3-1, page 6, and Section 3.2.4, page
7 for details. This sleep state can be extended in order to
lower average power, by simply delaying the rise of /SS.
Coming out of sleep state when /SS rises, the PROX,
DETECT, and DRDY pins will float for ~400µs; it is
recommended that these pins be pulled low to Vss to avoid
false signalling if they being monitored during this time .
Note: Pin /SS clamps to Vss for 250ns after coming out of
sleep state as a diagnostic pulse. To prevent a possible pin
drive conflict, /SS should either be driven by the host as an
open-drain pull-high drive (e.g. with a 100K pullup resistor), or
there should be a ~1K resistor placed in series with the /SS
pin. See Figure 1-1, R3.
1.6 Calibration
Calibration is possible via two methods:
1) Power up or power cycling (there is no reset input).
2) On command from host via SPI (Command 0x01: see
Section 3.3.2).
The calibration period requires 10 burst cycles, which are
executed automatically without the need for additional SPI
commands from the host. The spacing between each Cal
burst is 2ms, and the bursts average about 23ms each when
Cs1, Cs2 are 100nF, ie the Cal command requires ~220ms to
execute. Lower values of Cs will result in shorter bursts and
hence shorter cal times.
In addition to the basic calibration, it is also possible to
request that the QT401 adjust its reported data to achieve
physically calibrated end points (0, 127) via a serial command
(command 0x02: Section 3.3.3). This requires an immediately
preceding reference calibration command (command 0x01:
see Section 3.3.2) in order to work correctly.
Calibration should be performed when there is no hand
proximity to the element, or the results may be in error.
Should this happen, the error flag (bit 1 of the standard
response, see Section 3.3) will activate when the hand is
withdrawn again. In most cases this condition will self-correct
if drift compensation is used, and it can thus be ignored. See
also Section 1.8 below.
1.4 PROX, DETECT Outputs
There are two active-high output pins for detection of hand
proximity and slider position:
PROX output:
This pin goes high when a hand is detected
in free space near the slider. This condition is also found
as bit 0 in the standard response when there is no touch
detection (Section 3.3).
DETECT output:
This pin goes high when the signal is
large enough to allow computation of finger position. This
condition is also found as bit 7 in the standard response
(Section 3.3).
The sensitivities of these functions can be set using serial
commands (Sections 3.3.5 and 3.3.6).
These outputs will float for ~400µs after wake from Sleep
mode (see Section 1.3). If Sleep mode is used, it is
recommended that PROX and DETECT (if used) be shunted
to ground with 1nF capacitors to hold their states during the
400µs float interval when emerging from Sleep.
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QT401 R10.04/0505
Figure 1-3 E401 PCB Layout (1-sided, 144 x 20 x 0.6mm)
1.7 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 completely under
host control via a special drift command. See Section 3.3.4
for further details.
glass. The worst panel is thick plastic. Granularity due to poor
coupling can be compensated for by the use of larger values
of Cs1 and Cs2.
A table of suggested values for Cs1 and Cs2 for no missing
position values is shown in Table 1-2. Values of Cs smaller
than those shown in the table can cause skipping of position
codes. Code skipping may be acceptable in many
applications where fine position data is not required. Smaller
Cs capacitors have the advantage of requiring shorter
acquisition bursts and hence lower power drain.
Larger values of Cs1 and Cs2 improve granularity at the
expense of longer burst lengths and hence more average
power. Conversely where power is more important than
granularity, Cs1 and Cs2 can be reduced to save power at
the expense of resolution. Optimal values depends on the
user application, and some experimentation is necessary.
Cs1 and Cs2 should be matched to within 10% of each other
(ie, 5% tolerance, X7R dielectric) for best left-right end zone
balance, using the E401 reference layout (Figure 1-3). See
also Section 2.3. Linearity is not greatly affected by Cs
mismatching. If the error is too extreme, one of the end
locations could attempt to exceed the physical limits of the
slider. At or below this 10% guideline, the device will correctly
calibrate the end locations to within 1 or 2 millimeters for a
100mm slider.
In critical applications, the capacitors should be sort-matched,
or, the host device should store end location calibration
correction data based on a one-time factory calibration
procedure. Alternatively the Rs end resistors can be factory
adjusted to determine end locations more precisely.
1.8 Error Flag
An error flag bit (bit 1) is provided in the standard response
byte but only when there is no touch detection present
(Section 3.3); if the Error bit is high, it means the signal has
fallen significantly below the calibration level when not
touched. If this happens the device could report somewhat
inaccurate position values when touched.
This condition can self-correct via the drift compensation
process after some time under host control (Section 3.3.4).
Alternatively, the host controller can cause the device to
recalibrate immediately by issuing a calibration command
(Section 3.3.2), perhaps also followed by an end-calibrate
command (Section 3.3.3) if desired.
2 Wiring & Parts
The device should be wired according to Figure 1-1. An
example PCB layout (of the E401 eval board) is shown in
Figure 1-3.
2.1 Slider Strip Construction
The slider should be a resistive strip of about 100K ohms
+/-50%, from end to end, of a suitable length and width. Arcs
and semicircles are also possible. There are no known length
restrictions.
The slider 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. Carbon thick-film 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 slider is governed largely by the linearity
and consistency of the resistive slider element. Positional
accuracy to within 5% is routinely achievable with good grade
resistors and a uniform construction method.
2.3 Rs End Resistors
In auto end-cal mode, Rs1 and Rs2 are used only for EMC
and ESD protection; they should be no more than ~1K ohms.
However they are optional, and in the E401 eval board they
are set to 0.
In fixed cal mode, Rs1 and Rs2 can be varied to adjust the
ends of the slider outwards. Typically they will range from 10K
to 20K each. In fixed cal mode, the end resistors should be
selected to achieve a reasonable 0..127 position
correspondence with the desired mechanical range; in
particular, they should be adjusted so that the reported
2.2 Cs Sample Capacitors
Cs1 and Cs2 are the charge sensing capacitors , of type X7R.
The optimal values of Cs1 and Cs2 depend on the thickness
of the panel and its dielectric constant. Lower coupling to a
finger caused by a low dielectric constant and/or thicker panel
will cause the position result to become granular and more
subject to position errors. The ideal panel is made of thin
Table 1-2 Recommended Cs vs. Materials
Thickness,
mm
0.4
0.8
1.5
2.5
3.0
4.0
Acrylic
(
ε
R
=2.8)
10nF
22nF
47nF
100nF
-
-
Borosilicate glass
(
ε
R
=4.8)
5.6nF
10nF
22nF
39nF
47nF
100nF
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4
QT401 R10.04/0505
values 0 and 127 can be easily achieved by both large and
small fingers at the ends.
Increasing the Rs values will move the reported ends
‘outwards’. If they are too large, the values 0 and/or 127 will
not be reportable. The Rs resistors can have differing values.
Having well-defined ends is important in most applications, so
that the user can select the absolute minimum and maximum
values (ie OFF, MAX etc) reliably. If the numerical ends
cannot be achieved the user can have difficulty in controlling
the product.
The end zones should be defined to be physically large
enough so that over a wide range of values of Cs, Rslider etc
a usable set of ends are always preserved.
End zone tolerances can be affected by Cs1 / Cs2
capacitance matching and the values of Rs1 and Rs2 if fixed
end-cal is used. See also Section 2.2.
Other geometries are possible, for example arcs and
semicircles over a small scale (50mm radius max
recommended semicircle, or any radius as a shallow arc).
The strip can be made longer or shorter and with a different
width. The electrode strip should be about 10mm wide or
more, as a rule. Other features of the PCB layout are:
The components are oriented perpendicular to the strip
length so that they do not fracture easily when the PCB is
flexed during bonding to the panel.
The slider end connections should have a symmetrical
layout; note the dummy end trace connected to Rs1 just
below the slider element, to replicate the upper end trace
connected to Rs2. Without this the slider will be
unbalanced and will tend to skew its result to one side.
The ground ring around the slider measures 2mm thick
and is spaced 1mm from the long end traces. The end
traces should be placed as close as possible to the slider
element and be of the thinnest possible trace thickness.
0-ohm 0805 jumpers are used to connect the ground ring
back to circuit ground. These bridge over the two end
traces.
Additional ground area or a ground plane on the PCB’s
rear will compromise signal strength and is to be avoided.
The slider should normally be used in a substantially
horizontal orientation to reduce tracking accuracy
problems due to capacitive ‘hand shadow’ effects. Thinner
panels and an electrode strip on the back of the PCB (so
it has less material to penetrate) will reduce these effects.
‘Handshadow’ effects:
With thicker or wider panels an effect
known as ‘handshadow’ can become noticeable. If the
capacitive coupling from finger to electrode strip is weak, for
example due to a narrow electrode strip or a thick, low
dielectric constant panel, the remaining portion of the human
hand can contribute a significant portion of the total
detectable capacitive load. This will induce an offset error,
which will depend on the proximity and orientation of the hand
to the remainder of the strip. Thinner panels will reduce this
effect since the finger contact surface will strongly domina te
the total signal and the remaining handshadow capacitance
will not contribute significantly to create an error offset.
Slider strips placed in a vertical position are more prone to
handshadow problems than those that are horizontal.
PCB Cleanliness:
All capacitive sensors should be treated
as highly sensitive circuits which can be influenced by stray
conductive leakage paths. QT devices have a basic
resolution in the femtofarad range; in this region, there is no
such thing as ‘no clean flux’. Flux absorbs moisture and
becomes conductive between solder joints, causing signal
drift and resultant false detections or temporary loss of
sensitivity. Conformal coatings will trap in existing amounts of
moisture which will then become highly temperature
sensitive.
The designer should specify ultrasonic cleaning as part of the
manufacturing process, and in extreme cases, the use of
conformal coatings after cleaning.
2.4 Power Supply
The usual power supply considerations with QT parts applies
also to the QT401. The power should be very clean and come
from a separate regulator if possible. This is particularly
critical with the QT401 which reports continuous position as
opposed to just an on/off output.
A ceramic 0.1uF bypass capacitor should be placed very
close to the power pins of the IC.
Regulator stability:
Most low power LDO regulators have
very poor transient stability, especially when the load
transitions from zero current to full operating current in a few
microseconds. With the QT401 this happens when the device
comes out of sleep mode. The regulator output can suffer
from hundreds of microseconds of instability at this time,
which will have a deleterious effect on acquisition accuracy.
To assist with this problem, the QT401 waits 500µs after
coming out of sleep mode before acquiring to allow power to
fully stabilize. This delay is not present before an acquisition
burst if there is no preceding sleep state.
Use an oscilloscope to verify that Vdd has stabilized to within
5mV or better of final settled voltage before a burst begins.
2.5 PCB Layout and Mounting
The E401 PCB layout (Figure 1-3) should be followed if
possible. This is a 1-sided, 144 x 20 x 0.6mm board; the
blank side is simply adhered to the inside of a 2mm thick (or
less) control panel. Thicker panels can be tolerated with
additional positional error due to capacitive ‘hand shadow’
effects and will also have poorer EMC performance.
This layout uses 18 copper pads connected with 17
intervening series resistors in a chain. The end pads are
larger to ensure a more robust reading of 0 (left) and 127
(right). The finger interpolates between the copper pads (if
the pads are narrow enough) to make a smooth, 0..127 step
output with no apparent stair-casing. A wide ground border
helps to suppress the sense field outside of the strip area,
which would otherwise affect position accuracy.
The small electrodes of this PCB measure about 12.5 x
5.2mm. The lateral (eg 5.2mm) dimension of these electrodes
should be no wider than the expected smallest diameter of
finger touch, to prevent stair-casing of the position response.
2.6 ESD Protection
Since the electrode is always placed behind a dielectric
panel, the IC will be protected from direct static discharge.
However even with a panel transients can still flow into the
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QT401 R10.04/0505
微控制器 MCU
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