QT401-ISSG - Quantum Research Group

lQQT401

LIDE

™ T

OUCH

LIDER

APPLICATIONS

yAutomotive controlsyTouch-screensyAppliance controlsyLighting 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.

QT401 R10.04/0505

z1-dimensional finger-touch slider

zExtremely simple circuit - no external active components

zCompletely passive sensing strip: no moving parts

zCompatible with clear ITO over LCD construction

zSPI slave-mode interface

zSelf-calibration and drift compensation modes

zProximity mode for wake up of a product

zSpread-spectrum operation for optimal EMC compliance

z2.5 - 5.5V single supply operation; very low power

z14-pin SOIC, TSSOP lead-free packages

zInexpensive, simple 1-sided PCB construction possible

zE401 reference design board available

SDO

/SS

SCLK

SDI

SNS1A

SNS1B SNS2B

SNS2A

N/A

PROX

DETECT

DRDY

VDD GND

QT401

QT401-ISSGQT401-ISG-40

C ~ +85

TSSOP-14SO-14T

AVAILABLE OPTIONS

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.

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.

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

2 QT401 R10.04/0505

Figure 1-1 QT401 Wiring Diagram

QT401

SCLK

/SS

SDO

VDD

VSS

DRDY

PROX

SDI

SNS1A

SNS1B

SNS2A

Cs2

100nF

SNS2B

VIN VOUT

GND

Regulator

2.2uF

VIN

Proximity

SPI BUS

100k

Cs1

100nF

22k 127

Slider Element

60K~150K ohms

total resistance

DETECT

Touch Detect

C3 1nF C4 1nF

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

~31ms ~31ms

Acquire Bur

~3.8ms ~30us

DRDY from Q

~25ms

setting of drift compensation in Free-Run mode. See also

Section 3.3.4.

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.

1.4 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.

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.

1.6 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.

3 QT401 R10.04/0505

Note (1): Pin floats briefly after wake from Sleep mode.

Negative power pinGroundVSS14

Data ready output. Goes high to indicate it is possible to communicate with the QT401. Note (1)

ODRDY13

Active high when slider is touched. May be left unconnected. Note (1)

ODETECT12

Active high when a hand is near the slider. May be left unconnected. Note (1)

OPROX11

Leave openON/A10

Sense pin (to Cs2)I/OSNS2A9

Sense pin (to Cs2, Rs2); connects to ‘127’ end of slider elementI/OSNS2B8

Sense pin (to Cs1, Rs1); connects to ‘0’ end of slider elementI/OSNS1B7

Sense pin (to Cs1)I/OSNS1A6

Serial data inputISDI5

Serial clock input. Clock idles highISCLK4

Slave Select pin. Active low input to enable serial clocking. 1K ohms in series recommended.I/SS3

Serial data outputOSDO2

Positive power pin (+2.5 .. +5V)PowerVDD1

DESCRIPTIONTYPENAMEPIN

Table 1-1 Pin Descriptions

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.

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.

2.2 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

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.

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

4 QT401 R10.04/0505

Figure 1-3 E401 PCB Layout (1-sided, 144 x 20 x 0.6mm)

Table 1-2 Recommended Cs vs. Materials

100nF-

4.0

47nF-

3.0

39nF100nF

2.5

22nF47nF

1.5

10nF22nF

0.8

5.6nF10nF

0.4

Borosilicate glass

(

εε

=4.8)

Acrylic

(

εε

=2.8)

Thickness,

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.

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.

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.

2.6 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

5 QT401 R10.04/0505

electrode via induction, or in extreme cases via dielectric

breakdown. Porous materials may allow a spark to tunnel

right through the material. Testing is required to reveal any

problems. The device has diode protection on its terminals

which will absorb and protect the device from most ESD

events; the usefulness of the internal clamping will depending

on the panel's dielectric properties and thickness.

One method to enhance ESD suppression is to insert

resistors Rs1, Rs2 in series with the strip as shown in Figure

1-1; these can be as high as 1K ohms. Normally these are

not required, and in the E401 eval board they are 0 ohms.

Diodes or semiconductor transient protection devices or

MOV's on the electrode trace are not advised; these devices

have extremely large amounts of nonlinear parasitic

capacitance which will swamp the capacitance of the

electrode and cause false detections and other forms of

instability. Diodes also act as RF detectors and will cause

serious RF immunity problems.

See also Section 2.7, below

2.7 EMC and Related Noise Issues

External AC fields (EMI) due to RF transmitters or electrical

noise sources can cause false detections or unexplained

shifts in sensitivity.

The influence of external fields on the sensor can be reduced

by means of the 1K series end resistors described in Section

2.6. The Cs capacitor and Rs1, Rs2 (Figure 1-1) form a

natural low-pass filter for incoming RF signals; the roll-off

frequency of this network is defined by -

2✜R

If for example Cs = 22nF, and Rs = 1K, the EMI rolloff

frequency is ~7.2kHz, which is vastly lower than most noise

sources (except for mains frequencies i.e. 50 / 60 Hz). The

resistance from the sensing strip itself is actually much higher

on average, since the strip is typically 100K ohms from end to

end. A more credible value for Rs is about 10K.

Rs and Cs must both be placed very close to the body of the

IC so that the lead lengths between them and the IC do not

form an unfiltered antenna at very high frequencies.

PCB layout, grounding, and the structure of the input circuitry

have a great bearing on the success of a design to withstand

electromagnetic fields and be relatively noise-free.

These design rules should be adhered to for best ESD and

EMC results:

1. Use only SMT components.

2. Keep all Cs, Rs, and the Vdd bypass cap close to the IC.

3. Do not place the electrode or its connecting trace near

other traces, or near a ground plane.

4. Do use a ground plane under and around the QT401

itself, back to the regulator and power connector (but not

beyond the Cs capacitor).

5. Do not place an electrode (or its wiring) of one QT401

device near the electrode or wiring of another device, to

prevent cross interference.

6. Keep the electrode (and its wiring) away from other

traces carrying AC or switched signals.

6 QT401 R10.04/0505

Figure 3-1 SPI Timing Diagram

~31ms

Acquire Burst

<1ms, ~920µs typ

Sleep State

400µs typ

3-state

DRDY from QT >13

s, <100

>12µs, <100µs

<30µs

SS from host

>35µs

CLK from Host

data hold >12µs

after last clock

Host Data Output

(Slave Input - MOSI)

response byte

QT Data Output 3-state 3-state

(Slave Out - MISO)

out

ut driven out

ut floats

<11µs after /SS before DRDY

goes low goes low

32107654

4321? 765

Data shifts out on falling edge

Data sampled on rising edge

command byte

sleep state: 1s maxawake awake

7. If there are LEDs or LED wiring near the electrode or its

wiring (ie for backlighting of the key), bypass the LED

wiring to ground on both its ends.

8. Use a voltage regulator just for the QT401 to eliminate

noise coupling from other switching sources via Vdd.

Make sure the regulator’s transient load stability provides

for a stable voltage just before each burst commences.

9. If Mains noise (50/60 Hz noise) is present, use the Sync

feature to suppress it if possible (see Section 1.1).

For further tips on construction, PCB design, and EMC issues

browse the application notes and faq at www.qprox.com

3 Serial Communications

The serial interface is a SPI slave-only mode type which is

compatible with multi-drop operation, ie the MISO pin will float

after a shift operation to allow other SPI devices (master or

slave) to talk over the same bus. There should be one

dedicated /SS line for each QT401 from the host controller.

A DRDY (‘data ready’) line is used to indicate to the host

controller when it is possible to talk to the QT401.

3.1 Power-up Timing Delay

Immediately after power-up, DRDY floats for approximately

20ms, then goes low. The device requires ~525ms thereafter

before DRDY goes high again, indicating that the device has

calibrated and is able to communicate.

3.2 SPI Timing

The SPI interface is a five-wire slave-only type; timing is

found in Figure 3-1, page 6.

The phase clocking is as follows:

5kHz min, 40kHz maxClock rate:

8 bits, MSB shifts firstBit length & order:

Low from QT inhibits hostData Ready DRDY:

Negative level frame from hostSlave Select /SS:

Rising edge of CLK from hostInput data read on:

Falling edge of CLK from hostData out changes on:

HighClock idle:

The host can shift data to and from the QT on the same cycle

(overlapping commands). Due to the nature of SPI, the return

data from a command or action is always one SPI cycle

behind.

An acquisition burst always happens about 920µs after /SS

goes high after coming out of Sleep mode .

3.2.1 /SS Line

/SS acts as a framing signal for SPI data clocking under host

control. See Figure 3-1.

After a shift operation /SS must go high again, a minimum of

35µs after the last clock edge on CLK. The device

automatically goes into sleep state during this interval, and

wakes again after /SS rises. If /SS is simply held low after a

shift operation, the device will remain in sleep state up to the

maximum time shown in Figure 3-1. When /SS is raised,

another acquisition burst is triggered.

If /SS is held high all the time, the device will burst in a

free-running mode at a ~17Hz rate. In this mode a valid

position result can be obtained quickly on demand, and/or

one of the two OUT pins can be used to wake the host. This

rate depends on the burst length which in turn depends on

the value of each Cs and load capacitance Cx. Smaller

values of Cs or higher values of Cx will make this rate faster.

Dummy /SS Burst Triggers: In order to force a single burst,

a dummy ‘command’ can be sent to the device by pulsing /SS

low for 10µs to 10ms; this will trigger a burst on the rising

edge of /SS without requiring an actual SPI transmission.

DRDY will fall within 56µs of /SS rising again, and then a

burst will occur 1mS later (while DRDY stays low).

After the burst completes, DRDY will rise again to indicate

that the host can get the results.

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.

3.2.2 DRDY Line

The DRDY line acts primarily as a way to inhibit the host from

clocking to the QT401 when the QT401 is busy. It also acts to

signal to the host when fresh data is available after a burst.

The host should not attempt to clock data to the QT401 when

DRDY is low, or the data will be ignored or cause a framing

error.

On power-up, DRDY will first float for about 20ms, then pull

low for ~525ms until the initial calibration cycle has

completed, then drive high to indicate completion of

calibration. The device will be ready to communicate in

typically under 600ms (with Cs1 = Cs2 = 100nF).

While DRDY is a push-pull output, it does float for ~400µs

after power-up and after wake from Sleep mode. It is

desirable to use a pulldown resistor on DRDY to prevent false

signalling back to the host controller; see Figure 1-1 and

Section 1.3.

3.2.3 MISO / MOSI Data Lines

MISO and MOSI shift on the falling edge of each CLK pulse.

The data should be clocked in on the rising edge of CLK. This

applies to both the host and the QT401. The data path follows

a circular buffer, with data being mutually transferred from

host to QT, and QT to host, at the same time. However the

return data from the QT is always the standard response byte

regardless of the command.

The setup and hold times should be observed per Figure 3-1.

3.2.4 Sleep Mode

Please refer to Figure 3-1, page 6.

The device always enters low-power sleep mode after an SPI

transmission (Figure 3-1), at or before about 35µs after the

last rising edge of CLK. Coincident with the sleep mode, the

device will lower DRDY. If another immediate acquisition

burst is desired, /SS should be raised again at least 35 µs

after the last rising edge of CLK. To prolong the sleep state, it

is only necessary to raise /SS after an even longer duration.

In sleep mode, the device consumes only a few microamps of

current. The average current can be controlled by the host, by

7 QT401 R10.04/0505

adjusting the percentage of time that the device spends in

sleep.

The delay between the rising edge of /SS and the following

burst is <1ms to allow Vdd to stabilize. If the maximum spec

on /SS low (1s) is exceeded, the device will eventually come

out of sleep and calibrate again on its own. The 1s is a

minimum design guide, not a precise number; the actual time

can vary considerably from device to device and should not

be relied upon.

The DETECT, PROX, and DRDY lines will float for ~400µs

after wake from Sleep mode; see Section 1.3 for details.

After each acquisition burst, DRDY will rise again to indicate

that the host can do another SPI transmission.

3.3 Commands

Commands are summarized in Table 3-1. Commands can be

overlapped, i.e. a new command can be used to shift out the

results from a prior command.

All commands cause a new acquisition burst to occur when

/SS is raised again after the command byte is fully clocked.

Standard Response: All SPI shifts return a ‘standard

response’ byte which depends on the touch detection state:

No touch detection: Bit 7 = 0 (0= not touched)

Bits 6, 5, 4, 3, 2: unused

Bit 1 = 1 if signal polarity error

Bit 0 = 1 if prox detection only

Is touch detection: Bit 7 = 1 (1= is touched)

Bits 0..6: Contain calculated position

Note that touch detection calculated position is based on the

results of the prior burst, which is triggered by the prior /SS

rising edge (usually, from the prior command, or, from a

dummy /SS trigger - see Section 3.2.1).

There are 6 commands as follows.

3.3.1 0x00 - Null Command

00000000

01234567

The Null command will trigger a new acquisition (if /SS rises),

otherwise, it does nothing. The response to this command is

the Standard Response byte.

This command is predominant once the device has been

calibrated and is running normally.

3.3.2 0x01 - Calibrate

10000000

This command takes ~525ms to complete with the circuit

shown in Figure 1-1. This time can be reduced by using

smaller Cs capacitors. Smaller Cs capacitors may result in

loss of resolution unless the panel thickness is also reduced.

0x01 causes the sensor to do a basic recalibration. After the

command is given the device will execute 10 acquisition

bursts in a row in order to perform the recalibration, without

the need for /SS to trigger each of the bursts. The host should

wait for DRDY to rise again after the calibration has

completed before shifting commands again.

This command should be given if there is an error flag (bit 1

of the response byte when no touch detection is present).

Note that this command cancels the 0x02 ‘End Calibrate’

command if 0x02 was previously issued to the part; if end

calibration is desired, the 0x02 command must be reissued

again after the 0x01 command.

On power-up the device calibrates itself automatically and so

a 0x01 command is not required on startup.

The response to this command is the Standard Response

byte.

3.3.3 0x02 - End Calibrate

01000000

01234567

The command takes ~500ms to complete.

The 0x02 command should preferably only be performed

after the basic calibration (0x01) is done. If it is done at

another time, the end calibration may be inaccurate.

0x02 causes the sensor to relocate the reported endpoints of

the slider to automatically correspond to the physical ends,

using a special calibration process. After the command is

given the device will execute 20 acquisition bursts in a row in

order to perform the calibration, without the need for /SS

cycles to trigger each of the bursts.

The host should remain quiet during this period and obey

DRDY which will remain low until the process is done before

shifting the next command.

This command is optional, however if it is not used, two Rs

resistors should be used to set the end zones of the slider

(See Section 2.3).

Executing a calibrate command (0x01) cancels the End

Calibrate mode, and therefore the End Calibrate command

has to be performed again if desired. However, once the

0x01 / 0x02 command sequence is performed, it should

8 QT401 R10.04/0505

Set touch threshold; causes acquire burst. Bottom 6 bits (‘T’) are the touch threshold value. (10TT TTTT)

Power up default value = 10

Touch Thresh0x8T

Set prox threshold; causes acquire burst. Bottom 6 bits (‘P’) are the prox threshold value. (01PP PPPP)

Power up default value = 10

Prox Thresh0x4P

Drift compensation request; causes acquire burst. Max drift rate is 1 count per ten 0x03’s.Drift Comp0x03

Auto-find slider end points; cause 20 sequential bursts.

Power up default value = disabled

End Calibrate0x02

Force recalibration of reference using fixed slider ends; cause 10 sequential bursts

Power up default value = calibrated

Calibrate0x01

Shift out data; cause acquire burst (if /SS rises again)Null0x00

What it doesCommandHex

TABLE 3-1 - Command Summary

normally not be necessary to ever repeat the sequence

unless an error flag is found or the part is powered down and

back up again.

3.3.4 0x03 - Drift Compensate

11000000

01234567

0x03 causes the sensor to perform incremental drift

compensation. This command must be given periodically in

order to allow the sensor to compensate for drift. The more

0x03 commands issued as a percentage of all commands,

the faster the drift compensation will be.

The 0x03 command must be given 10 times in order for the

device to do one count of drift compensation in either

direction. The 0x03 command should be used in substitution

of the Null command periodically.

Example: The host causes a burst to occur by sending a

0x00 Null command every 50ms (20 per second). Every 6th

command the host sends is a 0x03 (drift) command.

The maximum drift compensation slew rate in the reference

level is -

50ms x 6 x 10 = 3.0 seconds

The actual rate of change of the reference level depends on

whether there is an offset in the signal with respect to the

reference level, and whether this offset is continuous or not.

It is possible to modulate the drift compensation rate

dynamically depending on circumstances, for example a

significant rate of change in temperature, by varying the mix

of Drift and Null commands.

If the Drift command is issued while the device is in touch

detection (ie bit 7 of the Standard Response byte =1), the drift

function is ignored.

Drift compensation during Free-Run mode is fixed at 6, which

results in a maximum rate of drift compensation rate of about

3secs / count; see Section 1.2.

The drift compensation rate should be made slow, so that it

does not interfere with finger detection. A drift compensation

rate of 3s ~ 5s is suitable for almost all applications. If the

setting is too fast, the device can become u nnecessarily

desensitized when a hand lingers near the strip. Most

environmental drift rates are of the order of 10's or 100's of

seconds per count.

3.3.5 0x4P - Set Proximity Threshold

01234567

This command is optional, but if it is not given, the proximity

detection function will work at a default setting of 10.

The lower 6 bits of this command (P5..P0) are used to set the

proximity threshold level. Higher numbers are less sensitive

(ie the signal has to travel further to cross the threshold).

Operand ‘P’ can range in value from 0 to 63. Zero (0) should

never be used. Very low settings can cause excessive flicker

in the proximity result due to low level noise and drift.

The host device can require that the Proximity output be

active many times in a row to confirm a detection, to make

prox detection more robust.

P is normally in the range from 6 to 10. The prox threshold

has no hysteresis and should only be used for non-critical

applications where occasional detection bounce is not a

problem, like power activation (i.e. to turn on an appliance or

a display).

Both the prox bit in the standard response and the PROX pin

will go high if the signal exceeds this threshold. The PROX

pin can be used to wake an appliance or display as a hand

approaches the slider, however the /SS line must remain high

so that the device acquires continuously, or /SS has to be at

least pulsed regularly (see Section 3.2.1) for this to work.

0x4P power-up default setting: 10

3.3.6 0x8T - Set Touch Threshold

01234567

The lower 6 bits of this command (T5..T0) are used to set the

touch threshold level. Higher numbers are less sensitive (ie

the signal has to travel further to cross the threshold).

Operand ‘T’ can range from 0 to 63. Internally the number is

multiplied by 4 to achieve a wider range. 0 should never be

used.

This number is normally set to 10, more or less depending on

the desired sensitivity to touch and the panel thickness.

Touch detection uses a hysteresis equal to 12.5% of the

threshold setting.

Both the touch bit (bit 7) in the standard response and the

DETECT pin will go high if this threshold is crossed. The

DETECT pin can be used to indicate to the host that the

device has detected a finger, without the need for SPI polling.

However the /SS line must remain high constantly so that the

device continues to acquire continuously, or /SS has to be at

least pulsed regularly (see Section 3.2.1) for this to work.

0x8T power-up default setting: 10

3.4 SPI - What to Send

The host should execute the following commands after

powerup self-cal cycle has completed: (assuming a 50ms SPI

repetition rate):

1. 0x01 - Basic calibration (optional as this is done

automatically on power-up)

2. 0x02 - End calibration (optional)

3. 0x4P - Set prox threshold (optional)

4. 0x8T - Set touch threshold (optional)

5. An endlessly repeating mixture of:

a. 0x00 (Null) - all commands except:

b. 0x03 (Drift compensate) - replace every nth Null

command where typically, n = 6

c. If there is ever an error bit set, send a 0x01 and

optionally, a 0x02.

If the error occurs frequently, then perhaps the ratio of drift

compensation to Nulls should be increased.

Note: the Null can be replaced by an empty /SS pulse if there

is no need for fast updates.

9 QT401 R10.04/0505

4.1 Absolute Maximum Specifications

Operating temperature range, Ta....................................................................... -40

C to +85

Storage temperature range, Ts........................................................................ -55

C to +125

VDD.....................................................................................................-0.5 to +7.0V

Max continuous pin current, any control or drive pin .............................................................. ±20mA

Short circuit duration to ground, any pin ..........................................................................infinite

Short circuit duration to VDD, any pin.............................................................................infinite

Voltage forced onto any pin................................................................... -0.6V to (Vdd + 0.6) Volts

4.2 Recommended Operating Conditions

VDD..................................................................................................... +2.5 to 5.0V

Supply ripple+noise..................................................................................... 5mV p-p max

Cs1, Cs2.............................................................................................. 22nF to 100nF

Cs1, Cs2 relative matching....................................................................................... 10%

Output load, max............................................................................................. ±0.5mA

4.3 DC Specifications

Vdd = 5.0V, Cs1 = Cs2 = 100nF, 100ms rep rate, Ta = recommended range, all unless otherwise noted

bits7Acquisition resolutionA

µA±1Input leakage currentI

1mA sourceVVdd-0.7High output voltageV

4mA sinkV0.6Low output voltageV

V2.2High input logic levelV

V0.8Low input logic levelV

Required for proper startup and calibrationV/s100Supply turn-on slopeV

DDS

@ 3VµA110Average supply currentI

@ 5VµA180Average supply currentI

@ 3VmA0.60.45Peak supply currentI

@ 5VmA1.50.75Peak supply currentI

NotesUnitsMaxTypMinDescriptionParameter

4.4 AC Specifications

Vdd = 5.0V, Cs1 = Cs2 = 100nF, Ta = recommended range, unless otherwise noted

kHz40 5SPI clock rateFspi

ms500Power-up delay to operateTd

µs500QT Burst spacingTbs

Modulated spread-spectrumµs2.421.8QT Pulse widthTqt

Modulated spread-spectrum (chirp)kHz1008775Sample frequencyFqt

Variable parameter under host controlpF 0.6Slide SensitivitySs

Variable parameter under host controlpF0.15Prox SensitivitySp

Under host controlms-Response timeTr

NotesUnitsMaxTypMinDescriptionParameter

4.5 Signal Processing and Output

Depends on element linearity, layout%±3Position linearity

% of bursts; host controlled%±10Drift compensation rateDr

% of threshold setting%12.5Hysteresis, touch sensingHt

% of threshold setting%0Hysteresis, prox sensingHp

Host controlled variable631Threshold, slider touchTt

Host controlled variable631Threshold, proxTp

Both prox and touch detectioncounts1Detection integrator countsDI

NotesUnitsMaxTypMinDescriptionParameter

10 QT401 R10.04/0505

4.6 Typical Position Response - Raw Cal (0x01) Only

4.7 Typical Position Response - After Auto End Cal (0x01 + 0x02)

11 QT401 R10.04/0505

100

120

10 20 30 40 50 60 70 80 90 100

DISTANCE FROM END (mm)

REPORTED POSITION

100

120

10 20 30 40 50 60 70 80 90 10

DISTANCE FROM END (mm)

REPORTED POSITION

4.8 Small Outline (SO) Package

4.9 TSSOP Package

12 QT401 R10.04/0505

Base level

Seating level

Wø

ß×45º

SYMBOL Millimeters Inches

Min Max Notes Min Max Notes

Package Type: 14 Pin SOIC

0.0200.0140.510.36L

0.008

0.347

0.010

0.244

0.050

0.0200.014

0.016

0.150

0.050

0.228

0.004

0.337

0.31 0.33

0.010

0.51

1.27

3.99

1.27

6.20

0.25

8.81

1.75

0.25

0.41

3.81

1.27

5.79

0.10

8.56

1.35

0.20e

0.157

BSC BSC

MIN NOM MAX MIN NOM MAX

Number of Pins n 14 14

Pitch p 0.026 0.65

Overall Height A 0.043 1.10

Standoff A1 0.002 0.004 0.006 0.05 0.10 0.15

Overall Width E 0.246 0.251 0.256 6.25 6.38 6.50

Moulded Package W idth E1 0.169 0.173 0.177 4.30 4.40 4.50

Moulded Package Length D 0.193 0.197 0.201 4.90 5.00 5.10

Foot Length L 0.020 0.024 0.028 0.50 0.60 0.70

FootAngle 048048

Lead Thickness c 0.004 0.006 0.008 0.09 0.15 0.20

Lead W idth B 0.007 0.010 0.012 0.19 0.25 0.30

MouldDraftAngleTop a05100510

MouldDraftAngleBottom 0 5 10 0 5 10

MILLIMETERSINCHES

Dimension Limits

Units

4.10 Ordering Information

QT401TSSOP-14-40

C ~ +85

CQT401-ISSG

QT401SO-14-40

C ~ +85

CQT401-ISG

MARKINGPACKAGETEMP RANGEPART NO.

13 QT401 R10.04/0505

5 Product Pictures

14 QT401 R10.04/0505

Figure 5.2 - A Clear ITO Slider Sensing Strip

(Courtesy Click-Touch NV, Belgium)

Figure 5.1 - E401 Eval Product (front, back, pcb rear)

NOTES

15 QT401 R10.04/0505

Patented and patents pending

Corporate Headquarters

1 Mitchell Point

Ensign Way, Hamble SO31 4RF

Great Britain

Tel: +44 (0)23 8056 5600 Fax: +44 (0)23 80565600

www.qprox.com

North America

651 Holiday Drive Bldg. 5 / 300

Pittsburgh, PA 15220 USA

Tel: 412-391-7367 Fax: 412-291-1015

This device covered under one or more of the following United States and international patents: 5,730,165, 6,288,707, 6,377,009, 6,452,514,

6,457,355, 6,466,036, 6,535,200. Numerous further patents are pending which may apply to this device or the applications thereof.

The specifications set out in this document are subject to change without notice. All products sold and services supplied by QRG are subject

to our Terms and Conditions of sale and supply of services which are available online at www.qprox.com and are supplied with every order

acknowledgment. QProx, QTouch, QMatrix, QLevel, QWheel, QView, QScreen, and QSlide are trademarks of QRG. QRG products are not

suitable for medical (including lifesaving equipment), safety or mission critical applications or other similar purposes. Except as expressly set

out in QRG's Terms and Conditions, no licenses to patents or other intellectual property of QRG (express or implied) are granted by QRG in

connection with the sale of QRG products or provision of QRG services. QRG will not be liable for customer product design and customers

are entirely responsible for their products and applications which incorporate QRG's products.

Development Team: Martin Simmons, Samuel Brunet, Luben Hristov