Quantum QT411-ISSG User Manual

l

lQ

Q

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zzzz QT401 QSlide™ enhancement - simplified calibration

zzzz Linear finger-touch capacitive slider control

z Robust Charge-Transfer sensing method

zzzz Extremely simple circuit - no external active components

zzzz SPI slave-mode interface

zzzz Self-calibration and drift compensation

z Spread-spectrum operation for optimal EMC compliance

zzzz 2.5 - 5.5V single supply operation; very low power

z Enhanced power supply & thermal drift rejection

zzzz 14-pin TSSOP Pb-free package

z Compatible with clear ITO over LCD construction

z Inexpensive, simple 1-sided PCB construction possible

APPLICATIONS

QS

LIDE

QT411-ISSG

™ T

SCLK

SNS3B

SNS3A

OUCH SLIDER

VDD 1

SDO

/SS

2

3

4

5

6

7

QT411

14

13

12

11

10

9

8

IC

GND

DRDY

DETECT

SDI

SNS1A

SNS1B

SNS2ASNS2B

y Automotive controlsy Climate controlsy Appliance controlsy Personal electronics

The QT411 QSlide™ IC is a new type of linear capacitive touch ‘slider’ sensor IC based on Quantum’s patented
charge-transfer (‘QT’) methods. 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 personal electronic device. Generally it can be used to
replace any form of linear control, through a completely sealed panel.

The device uses a simple, inexpensive resistive sensing element between four connection points. The sense element can be a
straight line or curved. The device can report a single rapid touch anywhere along the sense element, or, it can track a finger
moving along the sensing surface in real time.

This device uses three channels of synchronous sensing across a resistive element to determine touch position, using
mathematical analysis. 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 ultra-low RF low emissions.

The output of the QT411 can also be used to create discrete controls buttons in a line, 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 number-range based discrete control points can be mixed on a single element, or, the element can be reinterpreted
differently at different times, for example when used adjacent to or on top of an LCD to act as a menu input device that
dynamically changes function in context. The device is compatible with ITO (Indium Tin Oxide) overlays on top of various
displays or simply to provide for a backlighting effect.

The QT411 is significantly more stable with temperature and other environmental influences than the QT401 which it is
designed to replace. In particular it can tolerate extreme temperature swings without false detection or shifts in reported touch
position. Also it does not require special calibration of the endpoints of the slider area. However, unlike the QT401 the QT411
does not have a proximity detection function.

LQ

Copyright © 2005 QRG Ltd

QT411-ISSG R6.01/1005

1 Operation

The QT411 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 output data is a 7-bit binary
number (0...127) indicating angular
position.

Like all QProx™ devices, the QT411

1= Detect Output

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 QT411 modulates its bursts in a spread-spectrum
fashion in order to heavily suppress the effects of external
noise, and to suppress RF emissions.

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.

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 3 1/100 or 31%
of peak current.

VIN

C1

2.2uF

SPI BUS

Regulator

VIN VOUT

GND

Figure 1-1 QT411 Wiring Diagram

1

C2

2.2uF

R2

100k

R3
1k

C3
1nF

R1
22k

13

DRDY

2

SDO

3

/SS

4

SCLK

11

SDI

12

DETECT

VDD

VSS

SNS3B

SNS3A

SNS2A

SNS2B

SNS1A

SNS1B

4

1

Rs3 4.7k

5

Cs3
100nF

6

8

Cs2
100nF

7

Rs2 4.7k

10

Cs1
100nF

9

Rs1 4.7k

Rs5 8.2k

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.

Synchronized mode also allows the host device to control the
rate of drift compensation, by periodically sending a ‘drift’
command to the device.

Mains Sync: Sync mode can and should 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 pulse
modulation.

Cross-talk suppression: If two or more QT411’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 for each part.

~400k

~400k

~400k

Rs4 8.2k

'RIGHT'

'LEFT'

127

83

45

0

RESISTIVE SLIDER ELEMENT

Acquire Bur st

DRDY from QT

lQ

Figure 1-2 Free-Run Timing Diagram ( /SS = high )

~31ms ~31ms

<4ms ~30us

~25ms

2 QT411-ISSG R6.01/1005

Table 1-1 Pin Descriptions

DESCRIPTIONTYPENAMEPIN

Positive power pin (+2.5 .. +5V)PowerVDD1
Serial data outputOSDO2
Slave Select pin. This is an active low input that enables serial communicationsI/SS3
Serial clock input. Clock idles highISCLK4
Sense pin (to Cs3, Rs3); connects to both slider ends, each via separate additional 8.2K ohm resistorsI/OSNS3B5
Sense pin (to Cs3)I/OSNS3A6
Sense pin (to Cs2, Rs2); connects to 66% point (from left) of sliderI/OSNS2B7
Sense pin (to Cs2)I/OSNS2A8
Sense pin (to Cs1, Rs1); connects to 33% point (from left) of sliderI/OSNS1B9
Sense pin (to Cs1)I/OSNS1A10
Serial data inputISDI11

ODETECT12

Active high touch detected. May be left unconnected. Note (1)

ODRDY13

Data ready output. Goes high to indicate it is possible to communicate with the QT411. Note (1)
Negative power pinGroundVSS14

Note (1): Pin floats ~400µs after wake from Sleep mode.

1.2 Free-Run Mode

If /SS stays high, the device will acquire on its own repetitively
after a timeout of about 30ms (Figure 1-2). In this mode, the
DETECT pin can be used to wake up the host when it goes
high upon touch.

In free-run mode, the device does not sleep between bursts.
In this mode the QT411 performs automatic drift
compensation at the maximum rate of one count per 1 20
acquisition burst cycles, or about one count every 7 seconds
without host intervention. It is not possible to change this
setting of drift compensation in Free-Run mode. See also
Section 3.3.3.

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 is pulsed, the DETECT
and DRDY pins will float for ~400µs. It is recommended that
the DRDY pin be pulled to Vss with a resistor and DETECT
by bypassed with a capacitor to avoid false signalling if they
are being monitored during this time ; see Section 1.4.

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.

Note that activity on SCLK will also wake the QT411, which
in turn will then wait for the /SS to rise. For lowest possible
operation in Sleep mode, do not pulse on SCLK until after
/SS goes low.

1.4 DETECT Output Pin

This pin drives high when touch is detected and the chip is
reporting an angular position. This condition is also found as
bit 7 in the standard response.

This output will float for ~400µs during wake from Sleep mode
(see Section 1.3). It is recommended that the DETECT pin (if
it is used) be shunted to ground with a 1nF capacitor to hold

its state during the 400µs float interval when emerging from
Sleep.

Note that in the QT411, detection occurs when one or two of
the sensing channels becomes imbalanced with respect to
the other channel(s). A touch at one position will always
cause such an imbalance. However, a signal change that is
balanced among all 3 channels will not cause a detection. For
example, if a book is placed on top of the slider element, the
channels will all change in the same way and as a result,
detection will be suppressed. This feature is significantly
different from the way the QT401 operates.

1.5 Position Data

The position value is internally calculated and can be
accessed only when the sensor is touched (Detect pin high).

Direction convention: ‘Left’ is defined as the side closest to
the connection made by SNS1, and ‘Right’ is defined as the
side closest to the SNS2 connection. The ends are both
connected to SNS3, each via a resistor which allows the chip
to identify left and right as separate positions. See Figure 1-1.

The use of the terms ‘left’ and ‘right’ should not be interpreted
to mean the device can only be used in one orientation. In
fact the strip can be oriented backwards, vertically, or at any
angle.

The position on the left end reports as 0, while the position at
the right reports as 127. The device reports 45 when touched
at the SNS1 node and 83 at SNS2. The position data is a
7-bit number (0..127) that is computed in real time and is
returned via a status command.

End stops: The QT411 defines end zones of the slider
element as saturated ‘end stops’, which consist of fixed
regions where only a reading of ‘0’ or ‘127’ is returned. This is
to allow robust position detection of these important locations,
so that it is easy for a user to select ‘full off’ and ‘full on’. The
left slider end allocates 10% of the slider’s length to location
‘0’, and the right end similarly allocates 10% of the slider’s
length to location ‘127’. Only the center 80% of the slider’s
length will track changes in touch position in the range of

1..126.

The position data will update either with a single rapid touch
or will track if the finger is moved along the surface of the

lQ

3 QT411-ISSG R6.01/1005

Figure 1-3 Conventional PCB Layout (1-sided)

Copper side faces away from the panel; the bare side is glued to the inside of the product.

element. The position data ceases to be reported when touch
detection is no longer sensed.

1.6 Calibration

Calibration is possible via two methods:

1) Power up or power cycling (there is no reset input).

2) On command from the host via the SPI port
(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 1ms, and the bursts average about 31ms each, i.e.
the Cal command requires ~325ms to execute. The power up
calibration has 6 extra bursts to allow for power supply
stabilization, and requires a total of ~550ms to begin normal
operation.

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. In most cases this condition will self-correct if drift
compensation is used, and it can thus be ignored. See
Section 1.9 below.

Note: During calibration, the device cannot communicate.
DRDY will remain low during this interval.

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 sensing
strip 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 device are inverted and a position is reported
as though the strip 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 to restore normal function (Section 3.3.2).

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 weakest. The linear
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

lQ

2 Wiring & Parts

The device should be wired according to Figure 1-1. An
examples of a PCB layout is shown in Figure 1-3.

2.1 Electrode Construction

The strip 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 length and width.
Under heavy capacitive loading (for example if the element

Table 1-2 Recommended Cs vs. Materials

Thickness,

mm

0.4

0.8

1.5

2.5

3.0

4.0

4 QT411-ISSG R6.01/1005

Acrylic

(

εεεε

=2.8)

R

Borosilicate glass

εεεε

=4.8)

(

R

5.6nF10nF
10nF22nF
22nF47nF
39nF100nF
47nF-

100nF-