ST QST108 User Manual

8 keys with individual key state outputs or I2C interface

LQFP32 (7x7 mm)

Features

■ Patented charge-transfer design

■ Up to 8 independent QTouch™ keys supported

■ Individual key state outputs or I

■ Fully “debounced” results

■ Patented AKS™ Adjacent Key Suppression

■ Self-calibration and auto drift compensation

■ Spread-spectrum bursts to reduce EMI

■ Up to 5 general-purpose outputs

■ ECOPACK® (RoHS compliant) package

Applications

This device specifically targets human interfaces and front panels for a wide range of applications such as PC peripherals, home entertainment systems, gaming devices, lighting and appliance controls, remote controls, etc.

QST devices are designed to replace mechanical switching/control devices and the reduced number of moving parts in the end product provides the following advantages:

■ Lower customer service costs

■ Reduced manufacturing costs

■ Increased product lifetime

Table 1. Device summary

C interface

QST108

Capacitive touch sensor device

Not For New Design

Description

The QST108 is the ideal solution for the design of capacitive touch sensing user interfaces.

Touch-sensitive controls are increasingly replacing electromechanical switches in home appliances, consumer and mobile electronics, and in computers and peripherals. Capacitive touch controls allow designers to create stylish, functional, and economical designs which are highly valued by consumers, often at lower cost than the electromechanical solutions they replace.

The QST108 QTouch™ sensor IC is a pure digital solution based on Quantum's patented chargetransfer (QProx™) capacitive technology.

QTouch™ and QProx™ are trademarks of the Quantum Research Group.

Order codes

Feature

QST108KT6

Operating supply voltage 2.4 to 5.5 V

Supported interfaces Individual key state outputs or I

Operating temperature -40° to +85° C

Package LQFP32 (7x7 mm)

July 2008 Rev 5 1/51

This is information on a product still in production but not recommended for new designs.

C Interface

www.st.com

Contents QST108

Contents

1 Device overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2 Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3 QST touch sensing technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.1 Functional description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.2 Spread-spectrum operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.3 Faulty and unused keys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.4 Detection threshold levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.5 Detection integrator filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.6 Self-calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.7 Fast positive recalibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.8 Forced key recalibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.9 Max On-Duration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.10 Drift compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.11 Adjacent key suppression (AKS™) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4 Device operating modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4.1 Reset and power-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4.2 Burst operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4.3 Low power mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4.4 Mode selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4.5 Stand-alone mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4.5.1 Main features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4.5.2 KOUT outputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

4.5.3 Option descriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

4.6 I2C mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

4.6.1 Main features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

4.6.2 General-purpose outputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

4.6.3 IRQ pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

4.6.4 Communication packet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

4.6.5 I2C address selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

4.7 Supported commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

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QST108 Contents

5 Design guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

5.1 CS sense capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

5.2 Sensitivity tuning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

5.2.1 Increasing sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

5.2.2 Decreasing sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

5.2.3 Key balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

5.3 Power supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

5.4 ESD protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

5.5 Crosstalk precautions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

5.6 PCB layout and construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

6 Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

6.1 Parameter conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

6.1.1 Minimum and maximum values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

6.1.2 Typical values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

6.1.3 Typical curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

6.1.4 Loading capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

6.1.5 Pin input voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

6.2 Absolute maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

6.3 EMC characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

6.3.1 Functional EMS (electro magnetic susceptibility) . . . . . . . . . . . . . . . . . 32

6.3.2 Electro magnetic interference (EMI) . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

6.3.3 Absolute maximum ratings (electrical sensitivity) . . . . . . . . . . . . . . . . . 33

6.4 Operating conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

6.5 Supply current characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

6.6 Capacitive sensing characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

6.7 KOUTn/OPTn/GPOn pin characteristics . . . . . . . . . . . . . . . . . . . . . . . . . 36

6.7.1 General characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

6.7.2 Output pin characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

6.8 RESET pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

6.9 I2C control interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

7 Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

7.1 Soldering information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

8 Part numbering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

9 Device revision information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

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Contents QST108

9.1 Device revision identification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

9.2 Device revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

9.2.1 Revision 2.1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

9.2.2 Revision 2.3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

10 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

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QST108 Device overview

1 Device overview

The QST108 capacitive touch sensor IC is a pure digital solution based on Quantum's patented charge-transfer (QProx™) capacitive technology.

This technology allows users to create simple touch panel sensing electrode interfaces for conventional or flexible printed circuit boards (PCB/FPCB). Sensing electrodes are part of the PCB layout (copper pattern or printed conductive ink) and may be used in various shapes (circle, rectangular, etc.).

By implementing the QProx™ charge-transfer algorithm, the QST108 detects finger presence (human touch) near electrodes behind a dielectric (glass, plastic, wood, etc.). Only one external sampling capacitor by channel is used in the measuring circuitry to control the detection.

QST technology also incorporates advanced processing techniques such as drift compensation, auto-calibration, noise filtering, and Quantum's patented Adjacent Key Suppression™ (AKS™) to ensure maximum usability and control integrity.

In order to meet environmental requirements, ST offers this device in ECOPACK® packages. These packages have a lead-free second level interconnect. The category of second level interconnect is marked on the package and on the inner box label, in compliance with JEDEC Standard JESD97.

The maximum ratings related to soldering conditions are also marked on the inner box label. ECOPACK is an ST trademark. ECOPACK specifications are available at: www.st.com.

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Pin description QST108

SNS_SCK1

32 31 30 29 28 27 26 25

24 23 22 21 20 19 18 17

9 101112131415

1 2 3 4 5 6 7 8

I2C_SCL/KOUT81) (HS)

RESET

DD_1

GPO4/KOUT4/OPT4 (HS) GPO5/KOUT5/OPT5 (HS)

IRQ/KOUT6/OPT6 (HS)

I2C_SDA/KOUT71) (HS)

SS_2VSS_3VSS_4

DD_2

SNSK_SCK1

SNS_SCK2

SS_1

SNS_SCK6 SNSK_SCK5

SNSK_SCK2

SNS_SCK3

SNS_SCK4

SNS_SCK5 SNSK_SCK4

SNSK_SCK3

SNS_SCK8

SNSK_SCK7

SNS_SCK7

SNSK_SCK6

GPO3/OPT3/KOUT3 (HS)

GPO2/OPT2/KOUT2 (HS)

GPO1/OPT1/KOUT1 (HS)

SNSK_SCK8

QST108KT6

(HS) 20 mA high sink capability (on N-buffer only)

2 Pin description

Figure 1. 32-pin package pinout

Table 2. Device pin description

Pin Pin name Type

1 GPO4/OPT4/KOUT4

2 GPO5/OPT5/KOUT5

3 OPT6/KOUT6/IRQ

4 KOUT7/I2C_SDA

5 KOUT8/I2C_SCL

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1. An external pull-up is required on these pins.

(1)

Stand-alone mode function I2C mode function If unused

(2)

PP (HS)

(2)

PP (HS)

(2)

(3)

PP/OD

(HS)

TOD (HS)

Key 4 output / BCD output 4 and MOD_0 option resistor

Key 5 output and MOD_1 option resistor

Key 6 output and OM_0 option resistor

Key 7 output I

Key 8 output I

General purpose output 4 and I²C address bit 2 option resistor

Option resistor

Open or

General purpose output 5

option resistor

Open or

Interrupt line (active low)

option resistor

C serial data Open

C serial clock Open

QST108 Pin description

Table 2. Device pin description (continued)

Pin Pin name Type

6 RESET

7 NC Not connected

(1)

Stand-alone mode function I2C mode function If unused

BD Reset (active low)

10nF capacitor to ground

10 V

11 V

12 V

13 V

DD_1

SS_1

SS_2

SS_3

SS_4

DD_2

S Supply voltage

S Ground voltage

S Supply voltage

14 SNS_SCK1 SNS Key 1 sense pin to Cs Open

15 SNSK_SCK1 SNS Key 1 sense pin to Cs/Rs Open

16 SNS_SCK2 SNS Key 2 sense pin to Cs Open

17 SNSK_SCK2 SNS Key 2 sense pin to Cs/Rs Open

18 SNS_SCK3 SNS Key 3 sense pin to Cs Open

19 SNSK_SCK3 SNS Key 3 sense pin to Cs/Rs Open

20 SNS_SCK4 SNS Key 4 sense pin to Cs Open

21 SNSK_SCK4 SNS Key 4 sense pin to Cs/Rs Open

22 SNS_SCK5 SNS Key 5 sense pin to Cs Open

23 SNSK_SCK5 SNS Key 5 sense pin to Cs/Rs Open

24 SNS_SCK6 SNS Key 6 sense pin to Cs Open

25 SNSK_SCK6 SNS Key 6 sense pin to Cs/Rs Open

26 SNS_SCK7 SNS Key 7 sense pin to Cs Open

27 SNSK_SCK7 SNS Key 7 sense pin to Cs/Rs Open

28 SNS_SCK8 SNS Key 8 sense pin to Cs Open

29 SNSK_SCK8 SNS Key 8 sense pin to Cs/Rs Open

(2)

PP (HS)

30 GPO1/OPT1/KOUT1

31 GPO2/OPT2/KOUT2

32 GPO3/OPT3/KOUT3

1. S: supply pin, BD: bidirectional pin, SNS: capacitive sensing pin, PP: Output push-pull, OD: Output open-drain, TOD: Output true open-drain and HS: 20mA high sink capability (on N-buffer only)

2. During the reset phase, these pins are floating and their state depends on the option resistor.

3. An external pull-up is required on these pins.

Key 1 output / BCD output 1 and MODE option resistor

Key 2 output / BCD output 2 and AKS option resistor

Key 3 output / BCD output 3 and LP option resistor

General purpose output 1 and MODE option resistor

General purpose output 2 and I2C address bit 0 option resistor

General purpose output 3 and I2C address bit 1 option resistor

Option resistor

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QST touch sensing technology QST108

Ai12569

SNS_SCKn

SNSK_SCKn

Sense capacitor C

(a few nF)

Cx (~20 pF)

CT (~5 pF)

Earth

Serial resistor R

(10 kΩ)

3 QST touch sensing technology

3.1 Functional description

QST devices employ bursts of charge-transfer cycles to acquire signals. Burst mode permits low power operation, dramatically reduces RF emissions, lowers susceptibility to RF fields, and yet permits excellent speed. Signals are processed using algorithms pioneered by Quantum which are specifically designed to provide reliable, trouble-free operation over the life of the product.

The QST switches and charge measurement hardware functions are all internal to the device. An external C then measured. Larger values of C rapidly, reducing available resolution. As a minimum resolution is required for proper operation, this can result in dramatically reduced gain. Larger values of C of differential voltage across it, increasing available resolution by permitting longer QST bursts. The value of C The device is responsive to both C changes in sensor gain.

Figure 2. QTouch™ measuring circuitry

capacitor accumulates the charge from sense-plate CX, which is

can thus be increased to allow larger values of CX to be tolerated.

cause the charge transferred into CS to rise more

reduce the rise

and CS, and changes in either can result in substantial

3.2 Spread-spectrum operation

The bursts operate over a spread of frequencies, so that external fields will have minimal effect on key operation and emissions are very weak. Spread-spectrum operation works with the Detection Integrator mechanism (DI) to dramatically reduce the probability of false detection due to noise.

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QST108 QST touch sensing technology

3.3 Faulty and unused keys

Any sensing channel that does not have its sense capacitor (CS) fitted is assumed to be either faulty or unused. This channel takes no further part in operation unless a Mastercommanded recalibration operation shows it to have an in-range burst count again. Faulty, unused or disabled keys are still bursted but not processed to avoid modifying the sensitivity of active keys.

This is important for sensing channels that have an open or short circuit fault across C Such channels would otherwise cause very long acquire bursts, and in consequence would slow the operation of the entire QST device.

To optimize touch response time and device power consumption, if some keys are not used, we recommend to try suppressing the ones which belong to the same burst. Bursts which do not have any keys implemented will then not be processed.

3.4 Detection threshold levels

The key capacitance change induced by the presence of a finger is sensed by the variation in the number of charge transfer pulses to load the capacitor. The difference in the pulse count number is compared to a threshold in order to detect the key as pressed or not.

Two different thresholds, one for detection and one for the end of detection, create an hysteresis in order to prevent erratic behavior.

The default threshold levels and hysteresis values are described in Section 6.6: Capacitive

sensing characteristics on page 35.

3.5 Detection integrator filter

The Detection Integrator (DI) filter mechanism works together with spread spectrum operation to dramatically reduce the effects of noise on key states. The DI mechanism requires a specified number of measurements that qualify as detections (and these must occur in a row) or the detection will not be reported.

In a similar manner, the end of a touch (loss of signal) also has to be confirmed over several measurements. It is called the End of Detection Integrator (EDI).

This process acts as a type of “debounce” mechanism against noise.

The default DI and EDI values for confirming start of touch and end of touch are described in

Section 6.6: Capacitive sensing characteristics on page 35.

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QST touch sensing technology QST108

Burst count

Time

Reference count

Reference + EofDeTh

Reference + DeThHysteresis

Key Detection signal

= Sampling point

Figure 3 shows an example of detection with DI=2 and EDI=2 meaning 3 consecutive

samples are necessary to trigger the key detection or end of detection

Figure 3. Detection signals

3.6 Self-calibration

On power-up, all keys are self-calibrated to provide reliable operation under almost any conditions. The calibration phase is used to compute a reference value per key which is then used by the process determining if a key is touched or not. The reference is an average of 8 single acquisitions. As a result, the calibration time of the system can be simply calculated using the following formula: t

= 8 * Burst_Period. The methodology used to measure the

CAL

burst period is described in application note AN2547. For a maximum calibration duration (t

), please refer to Section 6.6: Capacitive sensing characteristics on page 35.

CAL

3.7 Fast positive recalibration

The device autorecalibrates a key when its signal reflects a decrease in capacitance higher than a fixed threshold (PosRecalTh) for a defined number of acquisitions (PosRecalI).

3.8 Forced key recalibration

A recalibration of the device may be issued at any time by sending to the QST device the appropriate I

It is possible to recalibrate independently any individual key using an I

C command or by tying the RESET pin to ground.

C command.

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QST108 QST touch sensing technology

3.9 Max On-Duration

The device can time out and automatically recalibrate each key independently after a fixed duration of continuous touch detection. This prevents the keys from becoming ‘stuck on’ due to foreign objects or other sudden influences. This is known as the Max On-Duration feature.

After recalibration, the key will continue to operate normally, even if partially or fully obstructed. Max On-Duration works independently per channel: a timeout on one channel has no effect on another channel.

Infinite timeout is useful in applications where a prolonged detection can occur and where the output must reflect the detection no matter how long. In infinite timeout mode, the designer should take care to ensure that drift in C remain “stuck on” inadvertently even when the touching object is removed from the sense field. Timeout durations are not accurate and can vary substantially depending on V temperature values, and should not be relied upon for critical functions.

, CX, and VDD do not cause the device to

and

3.10 Drift compensation

Signal drift can occur because of changes in CX, CS, and VDD over time. Depending on the C

type and quality, the signal may vary substantially with temperature and veiling. If keys

are subject to extremes of temperature or humidity, the signal can also drift. It is crucial that drift be compensated, otherwise false detections, non detections, and sensitivity shifts will follow.

Drift compensation slowly corrects the reference level of each key while no detection is in effect. The rate of reference adjustment must be performed slowly or else legitimate detections can also be ignored. The device compensates drift on each channel independently using a maximum compensation rate to the reference level.

Once a touch is sensed, the drift compensation mechanism ceases since the signal is legitimately high, and therefore should not cause the reference level to change.

The signal drift compensation is “asymmetric”: the reference level compensates drift in one direction faster than it does in the other. Specifically, it compensates faster for increasing signals than for decreasing signals. Decreasing signals should not be compensated for quickly, since an approaching finger could be compensated for partially or entirely while approaching the sense electrode. However, an obstruction over the sense pad, for which the sensor has already made full allowance, could suddenly be removed leaving the sensor with an artificially elevated reference level and thus become insensitive to touch. In this latter case, the sensor will compensate for the object's removal very quickly, usually in only a few seconds.

Caution: When only one key is enabled or if keys are very close together, the common drift

compensation must be disabled or its rate must be reduced to ensure correct device operation.

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QST touch sensing technology QST108

Burst count

Reference + DeTh

Reference Count

Reference Count + PosRelTh

Time

Drift Compensation

Temperature Change

Figure 4 illustrates an example of the drift compensation algorithm following a temperature

change.

Figure 4. Drift compensation example

3.11 Adjacent key suppression (AKS™)

Adjacent key suppression (AKS™) is a Quantum-patented feature which prevents multiple keys from responding to a single touch. This can happen with closely spaced keys, or a scroll wheel that has buttons very near it.

The QST108 supports two AKS modes:

● Locking AKS

Once a key is considered as “touched”, all other keys are locked in an untouched state. To unlock these keys, the touched key must return to an untouched state. Then, the key having the lowest key ID number is declared as the “touched” one.

● Unlocking AKS

On each acquisition, the signal strengths from each key are compared and the key with the highest signal level is declared as the “touched” one.

In I

C mode, up to 8 AKS groups can be specified.

Note: All keys belonging to the same AKS group must have the same AKS mode.

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QST108 Device operating modes

4 Device operating modes

4.1 Reset and power-up

At power-up, the device configures itself according to the pull-up or pull-down option resistors present on pins OPT1 to OPT6. The device start-up and configuration may take up to t

When the power is established, it is possible to force a new device configuration by applying a negative pulse on the RESET

Setup

pin.

The RESET device resets itself (through an I²C command, for example).

A 10nF capacitor is recommended on the RESET immunity.

pin is a bidirectional pin with an internal pull-up. The line is forced low when the

4.2 Burst operation

The device operates in “Burst” mode. Each key touch is acquired using a burst of chargetransfer sensing pulses whose count varies depending on the value of the sense capacitor C

and the load capacitance CX. Key touches are acquired using two successive bursts of

pulses:

● Burst A: Keys 1, 2, 3, and 4

● Burst B: Keys 5, 6, 7, and 8

Bursts always operate in an A-B sequence. If Keys 5 to 8 are not implemented, the QST device will not perform the Burst B to improve the response time and reduce the power consumption when in Low Power (LP) mode.

In Low Power mode, the device sleeps in an ultra-low current state between bursts to conserve power.

4.3 Low power mode

pin to ensure reliable start-up and noise

In order to reduce the device power consumption, the QST family include scalable low power modes.

● Standard low power mode

When the device is in standard low power mode, a window with very low power consumption is inserted between the acquisition of the last active key and the following acquisition of the first active key.

This window duration is programmable as the 'sleep duration time'.

Note that the sleep window insertion is cancelled in the following conditions:

– If a change is detected on a key, in order to speed up the DI process, the sleep

window insertion is skipped until the end of the DI process.

–In I

– Inside an I

C mode, when a key change is actually detected and reported with a negative pulse on the IRQ command is received from the host.

period is skipped.

pin. In this case, the low power mode is disabled until a

C command, between the Write and the Read I2C frames, the sleep

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Device operating modes QST108

● Free run in detect

The behavior in this mode is the same as in the standard low power mode except that the sleep window insertion is always skipped if any of the active keys is detected as touched.

This is useful to improve the wheel response time.

● Deep Sleep mode

Caution: If an I

does not acknowledge the frame (even if it has an I

In Deep Sleep mode, the device enters a very low power mode indefinitely. The device resumes its operations after receiving an I

C frame is received while in Sleep or Deep Sleep mode, the device wakes up but

C frame with any address or a reset.

C frame with the device address). The

host must therefore send again the frame until it is taken in account and acknowledged.

4.4 Mode selection

The device options are configured by connecting pull-up or pull-down resistors on OPTn pins. The device operating mode is selected using option pin 1 (OPT1) while the device settings are configured using option pins OPT2 to OPT6 (Tab le 3 ). Option pins are sampled at power-up and after a reset.

To fit most applications, the QST108 device offers two different operating modes:

● Stand-alone mode

This mode allows the user to simply replace existing mechanical switches with a capacitive sensing solution. It is designed for maximum flexibility and can accommodate most popular sensing requirements via option resistors (AKS, Low power, Max On-Duration and output modes).

In this mode, the 8 output pins reflect the status of the 8 sensing channels.

● I

C mode

In this mode, which is the most open one, the device is driven using the I To avoid polling, the QST device features an output interrupt pin (IRQ reports all key changes to the Master device. The QST (Slave) device can drive up to five general-purpose outputs.

Table 3. Operating modes

C interface.

). The IRQ line

OPT1: Mode selection

Pin OPT1 is high at start-up Stand-alone mode AKS LP MOD_0 MOD_1 OM

Pin OPT1 is low at start-up I

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C mode ADD0 ADD1 ADD2 Unused Unused

Option resistor function

OPT2 OPT3 OPT4 OPT5 OPT6

QST108 Device operating modes

4.5 Stand-alone mode

This mode allows the user to simply replace existing mechanical switch interface with a capacitive sensing solution. It is designed for maximum flexibility and can accommodate most popular sensing requirements via option resistors (see Figure 5).

4.5.1 Main features

● Pins KOUT1 to KOUT8 directly reflect the state of keys

● Selectable global adjacent key suppression (AKS™)

● Selectable sleep duration

● Selectable Max On-Duration values

● Selectable BCD mode

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Ai12560

MOD_1/KOUT5

MOD_0/KOUT4

LP/KOUT3

AKS/KOUT2

SS_1

SS_2

SS_3

SS_4

DD_1

DD_2

RESET

UNREG

4.7µF 4.7µF

2.4~5.5V

Volt. Reg.

OM/KOUT6

KOUT7

KOUT8

MODE/KOUT1

Binarycoded Output Mode

10nF

10kΩ 10kΩ

To Host

100nF 100nF

SNS_SCK1

SNSK_SCK1

Key1

10kΩ

Keep these parts close to IC

SNS_SCK2

SNSK_SCK2

Key2

10kΩ

SNS_SCK3

SNSK_SCK3

Key3

10kΩ

SNS_SCK4

SNSK_SCK4

Key4

10kΩ

SNS_SCK5

SNSK_SCK5

Key5

10kΩ

SNS_SCK6

SNSK_SCK6

Key6

10kΩ

SNS_SCK7

SNSK_SCK7

Key7

10kΩ

SNS_SCK8

SNSK_SCK8

Key8

10kΩ

9 101112

1MΩ

KOUT6

1MΩ

KOUT5

1MΩ

KOUT4

1MΩ

KOUT3

1MΩ

KOUT2

1MΩ

KOUT1

KOUT8

KOUT7

Device operating modes QST108

Figure 5. Stand-alone mode typical schematic

4.5.2 KOUT outputs

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KOUTn outputs directly reflect the state of keys. These pins are push-pull outputs except for pins KOUT7 and KOUT8 which are true open-drain outputs. Under RESET, these pins are floating and their state depends on the option resistors. Pins KOUTn are active high meaning that when a key is “touched”, the corresponding KOUT pin outputs a ‘1’.

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