QT411 - Quantum Research Group

QT411-ISSG

LIDE

™ T

OUCH

LIDER

APPLICATIONS

yAutomotive controlsyClimate controlsyAppliance controlsyPersonal 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.

QT411-ISSG R6.01/1005

zQT401 QSlide™ enhancement - simplified calibration

zLinear finger-touch capacitive slider control

zRobust Charge-Transfer sensing method

zExtremely simple circuit - no external active components

zSPI slave-mode interface

zSelf-calibration and drift compensation

zSpread-spectrum operation for optimal EMC compliance

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

zEnhanced power supply & thermal drift rejection

z14-pin TSSOP Pb-free package

zCompatible with clear ITO over LCD construction

zInexpensive, simple 1-sided PCB construction possible

GND

DRDY

DETECT

SDI

SNS1A

SNS1B

SNS2ASNS2B

SNS3A

SNS3B

SCLK

/SS

SDO

VDD 1

QT411

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

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 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.

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.

2 QT411-ISSG R6.01/1005

Figure 1-1 QT411 Wiring Diagram

VDD

VSS

DETECT

SNS1A

SNS1B

SNS2A 8

Rs2 4.7k

Cs3

100nF

Rs3 4.7k

SNS2B 7

SNS3A 6

SNS3B 5

VIN VOUT

GND

Regulator

2.2uF

VIN

1= Detect Output

SPI BUS

1nF

100k

Cs2

100nF

Rs1 4.7k

Cs1

100nF

22k

1k SCLK

/SS

SDO

DRDY

SDI

Rs4 8.2k

Rs5 8.2k

127

~400k

'LEFT'

'RIGHT'

RESISTIVE SLIDER ELEMENT

~400k

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

~31ms ~31ms

Acquire Burst

<4ms ~30us

DRDY from QT

~25ms

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

3 QT411-ISSG R6.01/1005

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

Negative power pinGroundVSS14

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

ODRDY13

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

ODETECT12

Serial data inputISDI11

Sense pin (to Cs1)I/OSNS1A10

Sense pin (to Cs1, Rs1); connects to 33% point (from left) of sliderI/OSNS1B9

Sense pin (to Cs2)I/OSNS2A8

Sense pin (to Cs2, Rs2); connects to 66% point (from left) of sliderI/OSNS2B7

Sense pin (to Cs3)I/OSNS3A6

Sense pin (to Cs3, Rs3); connects to both slider ends, each via separate additional 8.2K ohm resistorsI/OSNS3B5

Serial clock input. Clock idles highISCLK4

Slave Select pin. This is an active low input that enables serial communicationsI/SS3

Serial data outputOSDO2

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

DESCRIPTIONTYPENAMEPIN

Table 1-1 Pin Descriptions

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.

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

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).

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

4 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.

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,

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).

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 sensing strip is governed largely by the

linearity and consistency of the resistive element. Position

accuracy to within 5% is routinely achievable with good grade

resistors and a uniform construction method.

2.2 Cs Sample Capacitors

Cs1, Cs2 and Cs3 are the charge sensing sample capacitors;

normally they are identical in nominal value. They should be

of type X7R dielectric.

The optimal Cs values 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 sample

capacitors.

A table of suggested values 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 Cs improve granularity at the expense of

longer burst lengths and hence more average power.

Cs1, Cs2 and Cs3 should be X7R type, matched to within

10% of each other (ie, 5% tolerance) for best accuracy. The

PCB reference layout (Figure 1-3) is highly recommended. If

the Cs capacitors are poorly matched, position accuracy will

be affected and there could also be missing codes.

2.3 Rs Resistors

See Figure 1-1. Rs1, Rs2, and Rs3 are low value (typically

4.7K) resistors used to suppress the effects of ESD and

assist with EMC compliance. They are optional in most

cases.

In addition, there are two 8.2K resistors required to split

channel SNS3B into the two constituent ends. These two

resistors should be placed close to the ends of the slider

strip.

2.4 Power Supply

The usual power supply considerations with QT parts applies

also to the QT411. The power should be very clean and come

from a separate regulator if possible. This is particularly

critical with the QT411 which reports continuous position as

opposed to just an on/off output.

A ceramic 0.1µF 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 QT411 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 negative effect on acquisition accuracy.

To assist with this problem, the QT411 waits 500µs after the

400µs taken to come 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.

The QT411 has specially enhanced power supply rejection

built in. This means that it is often possible to share the

regulator with other circuits. However, it is always advised to

be sure that Vdd is free from spikes and transients, and is

filtered sufficiently to prevent detection problems.

During development it is wise to first design a regulator onto

the PCB just for (and next to) the QT411, but allow for it to be

‘jumpered out’. If in development it is clear that there are no

problems with false detection or ‘angle noise’ even without a

separate regulator for the QT411, then the regulator can be

safely omitted.

2.5 PCB Layout and Mounting

One form of PCB layout is shown in Figure 1-3. This is a

1-sided 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 position error due to capacitive ‘hand

shadow’ effects and will also have poorer EMC performance.

The Figure 1-3 layout uses a series copper pads connected

with intervening series resistors in a row. The total resistance

between any two connection points can be in the range of

100K to 500K, with ~400K being a suitable target value .

Resistance values at the higher end of this range will

generate more sensitivity provided there is no ground plane

close underneath the electrode strip.

A human finger interpolates between the copper pads (if the

pads are narrow enough) to make a smooth output with no

apparent steps. The lateral dimension along the centre of

each electrode should be no wider than the expected

smallest diameter of finger touch, to prevent stepping of the

position response (if it matters).

It is also possible create an interleaved electrode array with

only 3 resistors between each channel’s connection point on

the strip. Interleaving eliminates stepping while reducing the

number of required resistors. Consult Quantum for further

details.

Resistive inks (such as ITO, Agfa Orgacon

etc.) can also be

used if the resistance between connection points is in the

recommended range.

The electrode strip can be made in various lengths up to at

least 80mm. The electrode width should be about 12mm wide

or more, as a rule. The strip can also be an arc or other

irregular shape. For a 360 degree wheel, use the QT511 or

consult Quantum for other options.

5 QT411-ISSG R6.01/1005

The SMT components should be oriented perpendicular to

the direction of bending so that they do not fracture when the

PCB is flexed during bonding to the panel.

Additional ground area or a ground plane on the PCB will

compromise signal strength and is to be avoided. A single

sided PCB can be made of FR-2 or CEM-1 for low cost.

‘Handshadow’ effects: With thicker and wider panels an

effect known as ‘handshadow’ can become noticeable. If the

capacitive coupling from finger to electrode element is weak,

for example due to a narrow electrode width 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 element. Thinner panels and those

with a smaller diameter 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.

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

EMC and Related Issues

Please refer to Quantum app note AN-KD02 for further

information on ESD and EMC matters.

3 Serial Communications

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

compatible with multi-drop operation, i.e. 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 QT411 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 QT411.

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.

From power up to first communication, allow a total of 550ms

in startup delay.

3.2 SPI Timing

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

found in Figure 3-1. 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

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

return data from a command or request is always one SPI

cycle behind.

An acquisition burst always happens about 920µs after /SS

goes high after coming out of Sleep mode . SPI clocking

lasting more than 15ms can cause the chip to self-reset.

6 QT411-ISSG R6.01/1005

Figure 3-1 SPI Timing Diagram

~31ms ~31ms

Acquire Burs

<1ms <1ms

Sleep Mode awake low-power sleep awake sleep

400us typ

3-state if left to float

DRDY from QT >13us, <100us

>12us, <100us

>12us, <100us >20us

<35us >1us, <5us

SS from host

sleep until automatic wake (~3s)

wake up on /SS line

CLK from Host

data hold >=12us

after last clock

Host Data Output

(Slave Input - MOSI)

response byte

QT Data Output 3-state 3-state

(Slave Out - MISO)

output driven output floats

<12us after /SS before DRDY

goes low goes low

0? 7654321

76543210

Data shifts out on falling edge

Data sampled on rising edge

command byte

<10us delay

edge to data

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 be pulsed high before being

pulsed low for 1-5 µs. This must be 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 pulsed, 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 the

DETECT pin 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 after the rising

edge of /SS without requiring an actual SPI transmission. In

order to ensure the sampling capacitors have enough time to

discharge after a short /SS pulse, DRDY is held high for

approximately 700µs before the burst occurring.

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 QT411 when the QT411 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 QT411 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 = Cs3 =100nF).

While DRDY is a push-pull output ; however, this pin floats

after power-up and after wake from Sleep mode, for ~400µs

(typical at Vdd = 3.3V). 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 QT411. 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. Before entering sleep mode, the

device will lower DRDY. If another immediate acquisition

burst is desired, /SS should be pulsed at least 35µs after the

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

necessary to pulse /SS after an even longer duration. During

this time, the QT411 will wake up approximately every 3

seconds and burst before going back to sleep. This allows

the QT411 to compensate for thermal changes.

Changes on CLK will also cause the device to wake, however

the device will not cause an acquire burst to occur if /SS has

also gone low and high again.

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

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

adjusting the percentage of time that the device spends in

sleep.

The delay between the wake signal and the following burst is

1ms max to allow power to stabilize. The DETECT and DRDY

lines will float for ~400µs (typical at Vdd = 3.3V) during 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)

Bit 6 = 0 to indicate linear type sensor

= { 1 to indicate wheel chip }

Bits 5, 4, 3, 2: unused (report 0)

Bits 1, 0 reserved (report 0 or 1)

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

Bits 0..6: Contains 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).

Bit 6 indicates the type of device: ‘1’ means that the device is

a wheel (e.g. QT501 or QT511), and ‘0’ means the device is a

linear type (e.g. QT411, or QT401).

There are 5 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, returned on the next SPI shift.

7 QT411-ISSG R6.01/1005

This command is predominant once the device has been

calibrated and is running normally.

3.3.2 0x01 - Calibrate

10000000

01234567

This command takes ~325ms @ 3.3V to complete.

0x01 causes the device 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

reported via the 0x04 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, returned on the next SPI shift. During calibration,

device communications are suspended.

3.3.3 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 10th

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

The maximum drift compensation slew rate in the reference

level is -

50ms x 10 x 10 = 5.0 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 element. Most

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

seconds per count.

The response to this command is the Standard Response

byte, returned on the next SPI shift.

3.3.4 0x04 - Error Status

00100000

01234567

This command is used to read the current status of the

device. In particular it is used to detect if there is a sensing

error caused by a calibration or power-on at a bad time, ie

when a finger is on the sensing strip and thereafter removed.

The reported bits are as follows:

Bit 7 = 1 indicates touch;

= 0 indicates no touch

Bit 6 = 0 indicates Linear type (QT401 or QT411)

= 1 indicates Wheel type (QT510 or QT511)

Bits 5, 4, 3, 2: unused (0)

Bit 1 = 1 if calibration error

Bit 0 reserved (reports 0 or 1)

All bits except Bit 1 can be safely ignored.

The status byte should be read the first time there is a

detection just after a power-on reset or after a 0x01

calibration. If Bit 1 = 1, there was a calibration error and the

device should be immediately calibrated again using the 0x01

command. After the second calibration it should be checked

yet again (and so on) until there is no error.

If there is no error according to the sequence of the above

paragraph, it is not required to read this byte again.

The error byte is returned on the following SPI shift.

8 QT411-ISSG R6.01/1005

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

Power up default value = 10

Threshold0x8T

On the following SPI shift, returns the error status of the part; causes acquire burst. See Section 3.3.4.Error Status0x04

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

Force recalibration of reference; causes 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

3.3.5 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 value 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 for this to work.

The response to this command is the Standard Response

byte, returned on the next SPI shift.

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. 0x8T - Set touch threshold (optional)

3. An endlessly repeating mixture of:

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

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

command with 0x03 where typically, n = 10

c. 0x04 (Error status) - use after any detection just

after a calibration or power-up to see if there is a

calibration error.

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

is no need for fast updates.

9 QT411-ISSG R6.01/1005

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, Cs3................................................................................................. 100nF

Cs1, Cs2, Cs3 relative matching.................................................................................. ±5%

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

kHz37 5SPI clock rateF

SPI

ms550Power-up delay to operateT

ms1QT Burst spacingT

Modulated spread-spectrum (chirp)kHz1049892Sample frequencyF

Variable parameter under host controlpF 0.6Touch SensitivityS

Under host controlms-Response timeT

NotesUnitsMaxTypMinDescriptionParameter

4.5 Signal Processing and Output

Depends on element linearity, layout%±3Position linearity

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

% of threshold setting%12.5Hysteresis, touch sensingH

% of threshold setting%0Hysteresis, prox sensingH

Host controlled variable631Threshold, wheel touchT

Host controlled variable631Threshold, proxT

Both prox and touch detectioncounts1Detection integrator countsDI

NotesUnitsMaxTypMinDescriptionParameter

10 QT411-ISSG R6.01/1005

4.6 TSSOP Package

4.7 Ordering Information

QT411-40

C ~ +85

CTSSOP-14QT411-ISSG

MARKINGTEMP RANGEPACKAGEPART NO.

11 QT411-ISSG R6.01/1005

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

Patented and patents pending

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www.qprox.com

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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 corresponding 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, Matthew Trend