maxtouch 768-channel touchscreen controller mxt768e-ab...

maXTouch™ 768-channel Touchscreen Controller

mXT768E-ABAutomotive Version

9855CX–AT42–01/15

Features• maXTouch™ Touchscreen

– Two touchscreens with true 12-bit multiple touch reporting and real-time XY tracking for up to 16 concurrent touches per touchscreen

– Up to 10.1 inch diagonal screen size supported • Number of Channels

– Electrode grid configurations of up to 24 X and 32 Y lines supported– Touchscreens up to 768 channels (subject to other configurations)– Up to 64 channels can be allocated as fixed keys (subject to other configurations)– Two separate self-capacitance channels for use as touch keys or proximity

sensors.• Signal Processing

– Advanced digital filtering using both hardware engine and firmware– Self-calibration– Auto drift compensation– Adjacent Key Suppression® (AKS®) technology– Noise cancellation algorithms for display noise suppression– Grip suppression– Suppression of unintentional touches– Reports one-touch and two-touch gestures– Down-scaling and clipping support to match LCD resolution – Ultra-fast start-up and calibration for best user experience– Supports axis flipping and axis switch-over for portrait and landscape modes– Fast and powerful 32-bit processor core

• Scan Speed– Maximum single touch >120 Hz, subject to configuration– Maximum 10 touches >100 Hz, subject to configuration– Configurable to allow power/speed optimization– Programmable timeout for automatic transition from active to idle states

• Response Times– Initial latency

FOR RELEASE ONLY UNDER NON-DISCLOSURE AGREEMENT (NDA)

1. Overview of the mXT768E-AB

1.1 IntroductionThe Atmel® maXTouch family of touch controllers has set a new industry benchmark forcapacitive touchscreens with their low current consumption, fast response time and high levelsof accuracy. The mXT768E-AB’s single-chip solution offers the benefits of the maXTouchenhanced architecture on devices with touchscreens up to 10.1 in. diagonal:

• Patented capacitive sensing method – The mXT768E-AB uses a unique charge-transfer acquisition engine to implement Atmel’s patented QMatrix® capacitive sensing method. This allows the measurement of up to 768 mutual capacitance nodes. Coupled with a state-of-the-art CPU, the entire touchscreen sensing solution can measure, classify and track individual finger touches with a high degree of accuracy.

• Capacitive Touch Engine (CTE) – The mXT768E-AB features the E series acquisition engine, which uses an optimal measurement approach to ensure almost complete immunity from parasitic capacitance on the receiver inputs (Y lines). The engine includes sufficient dynamic range to cope with touchscreen mutual capacitances spanning 0.63 pF to 5 pF. This allows great flexibility for use with Atmel’s proprietary ITO pattern designs. One and two layer ITO sensors are possible using glass or PET substrates.

• Noise filtering – Hardware noise processing in the capacitive touch engine provides enhanced autonomous filtering and allows a broad range of noise profiles to be handled. The result is good performance in the presence of charger and LCD noise.

• Processing power – The main CPU has two powerful, yet low power, microsequencer coprocessors under its control. These combine to allow the signal acquisition, preprocessing, postprocessing and housekeeping to be partitioned in an efficient and flexible way. This gives ample scope for sensing algorithms, touch tracking or advanced shape-based filtering. An in-circuit reflash can be performed over the chip’s hardware-driven interface.

• Interpreting user intention – Atmel's mutual capacitance method provides unambiguous multitouch performance. Algorithms in the mXT768E-AB provide optimized touchscreen position filtering for the smooth tracking of touches. Stylus support allows stylus touches to be detected and distinguished from other touches, such as finger touches. The suppression of unintentional touches from the user’s gripping fingers, resting palm or touching cheek or ear also help ensure that the user’s intentions are correctly interpreted.

• Self-capacitance channels – Two sets of self-capacitance channels allow for the implementation of additional touch keys or proximity sensors using Atmel’s patented QTouch® technology.

1.2 Understanding Unfamiliar ConceptsIf some of the concepts mentioned in this datasheet are unfamiliar, see the following sectionsfor more information:

• Appendix C on page 47 for a glossary of terms

• Appendix D on page 49 for QMatrix technology

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mXT768E-AB

1.3 ResourcesThe following document provides essential information on configuring the device:

• mXT768E-AB 2.2 Protocol Guide

The following documents may also be useful (available by contacting Atmel’s TouchTechnology division):

• Configuring the device:

– Application Note: QTAN0058 – Rejecting Unintentional Touches with the maXTouch Touchscreen Controllers

– Application Note: QTAN0078 – maXTouch Stylus Tuning

• Miscellaneous:

– Application Note QTAN0050 – Using the maXTouch Debug Port

– Application Note QTAN0061 – maXTouch™ Sensitivity Effects for Mobile Devices

• Touchscreen design and PCB/FPCB layout guidelines:

– Application Note QTAN0054 – Getting Started with maXTouch Touchscreen Designs

– Application Note QTAN0048 – maXTouch PCB/FPCB Layout Guidelines

– Application Note QTAN0080 – Touchscreens Sensor Design Guide

• Other documents – The device uses the same core technology as the mXT224E, so the following documents may also be useful (available by contacting Atmel’s Touch Technology division):

– Application Note: QTAN0083 – mXT224E Power and Speed Considerations

– Application Note QTAN0052 – mXT224 Passive Stylus Support

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2. Pinout and Schematic

2.1 Pinout Configuration

51525354555657585960616263646566676869707172737475

48 49 504746454443424137 38 39 4031 32 33 34 35 363029282726

78 77 767980818283848589 88 87 8695 94 93 92 91 9096979899100

AVDD

Y27Y28Y29Y30Y31

GND

GND

VDDReserved

ReservedNC

VDD_INPUT

GND

RESET

SDASCL

VDD

GPIO2/SYNCReserved

GPIO1/DBG_CLK

GPIO3GPIO4GPIO5GPIO6

123456789101112

25

1314151617181920

24232221

Y10

Y18

Y11

Y19

Y12

Y20

Y14

Y15

Y5

Y6

Y7

Y8

Y16

Y9

Y17

Y13

Y21

Y23

Y24

Y25

Y26

Y22

AV

DD

NC

GN

D

Y2Y1

Y3Y4

X22X23

X15X14

X12

AVDDX17

X18

X13

X20

X21

X19

AVDD

AVDD

Y0

GND

GND

GNDGND

GND

X16X

8X

9X

10 X11

GN

D

GN

D

GN

D X5

GP

IO0/

DB

G_D

ATA

NC

NC

CH

G

VD

D

VD

DV

DD

CO

RE

AD

DS

EL

X7

X6

X4

X3X1

AV

DD

XV

DDX2

X0

mXT768E-AB

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2.2 Pinout Descriptions I

Table 2-1. Pin Listing

Pin Name Type Comments If Unused, Connect To...

1 Y27 I Y line connection Leave open





6 AVDD P Analog power –

7 GND P Ground –

8 VDD P Digital Power –

9 Reserved – Always connect to GND –

10 NC – No connection Leave open


12 GND P Ground –

13GPIO1

DBG_CLKSNS0

I/OGeneral purpose I/O – Input: GND, Output: leave openDebug clockSelf-capacitance sense pin

Make it available as a test pin

14GPIO2/SYNCSNSK0

I/OGeneral purpose I/OExternal synchronizationSelf-capacitance sense pin

Input: GNDOutput: leave open


16 GPIO3 I/O General purpose I/OInput: GND

Output: leave open

17GPIO4SNS1

I/OGeneral purpose I/OSelf-capacitance sense pin


18GPIO5

CHRG_INSNSK1

I/OGeneral purpose I/OCharger present inputSelf-capacitance sense pin


19 GPIO6 I/O General purpose I/OInput: GND

Output: leave open

20 VDD P Digital power –

21 GND P Ground –

22 RESET I Reset low; has internal 20 k to 60 k pull-up resistor Vdd (1)

23 VDD_INPUT – For factory use only – connect to VDD VDD

24 SDA (1) OD Serial Interface Data VDD

25 SCL (1) OD Serial Interface Clock VDD

26 CHG (2) OD State change interrupt Leave open

27 NC – No connection –

28 NC – No connection –

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30 GND P Ground –

31 ADDSEL I I2C-compatible address select –

32GPIO0

DBG_DATAI/O

General purpose I/O – Input: GND, Output: leave openDebug data

Make it available as a test pin


34 VDDCORE PDigital core power. Must be connected as in schematics (see Section 2.3 on page 9)

–

35 GND P Ground –

36 X0 O X matrix drive line Leave open






42 GND P Ground –








50 XVDD PX line regulator bypass – see schematics in Section 2.3 on page 9

–








58 GND P Ground –




62 GND P Ground –

Table 2-1. Pin Listing (Continued)


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mXT768E-AB

63 GND P Ground –





68 GND P Ground –

69 GND P Ground –
















85 NC – No connection Leave open



88 GND P Ground –












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I Input only OD Open drain output O Output only, push-pullP Ground or power




1. It is recommend that RESET is connected to the host system.2. CHG is momentarily set (approximately 100 ms) as an input after power-up or reset for diagnostic purposes.



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2.3 Schematics

2.3.1 I2C-compatible Mode – Digital Supply 2.5–3.3V

RESET

GPIO0/DBG_DATA

GPIO1/DBG_CLK

GPIO2/SYNC

GPIO3

GPIO4

GPIO5

VDD

GPIO6

GND

GND

VDD

CS0

CS1

RS1

RS0

SDA

SCL

ADDSEL

CHG

GND

GND

100n

FC12

100n

FC11

100n

FC10

100nFC14

1uF

C9

1uFC15

100n

FC13

PROXIMITYKEY 0

PROXIMITYKEY 1

X0 36

X1 37

GPIO5/CHRG_IN/SNSK118

X3 39

X4 40

X5 41

X6 43

X7 44

X8 45

X9 46

X10 47

X11 48

X12 51

X13 52

X14 53

X15 54

X16 55

X17 56

Y0 70Y1 71Y2 72Y3 73Y4 74Y5 76Y6 77Y7 78Y8 79Y9 80

Y10 81Y11 82Y12 83Y13 84Y14 86Y15 87Y16 90Y17 91Y18 92

AVDD

75

Y20 94Y21 95Y22 96Y23 97Y24 98Y25 99Y26 100Y27 1Y28 2Y29 3

SDA24

SCL25

CHG26

VDD_INPUT23

NC27

NC28

GND

7

Reserved9

ADDSEL31

GPIO0/DBG_DATA32

GPIO1/DBG_CLK/SNS013

GPIO2/SYNC/SNSK014

Reserved15

GPIO316

GPIO4/SNS117

X2 38

VDD

33

VDD

8

VDD

20

VDDCORE

34

Y19 93

AVDD

89

XVDD

50

AVDD

49

Reserved11

GND

30

GND

12

GND

42GND

35

GND 68

GND

21

RESET22

X18 59

X19 60

X20 61

X21 64

X22 65

Y30 4Y31 5

GPIO619

X23 66

GND

58

GND

62

GND

63

GND

69

AVDD

67

AVDD

6

VDD

29

AVDD

GNDGND

100n

FC6

100n

FC5

100n

FC4

100n

FC3

1uF

C2

100n

FC8

GND100nFC1

0RRn AVDDSee Notes

57AVDD

100n

FC7

GND

GND

88

NCNC

8510

mXT768E-AB

I2C-COMPATIBLE

MAT

RIX

X D

RIV

EM

ATR

IX Y

SC

AN

IN

< 5 mm away from chip

NOTES

1. Capacitors C1 to C14 must be X7R or X5R and placed


3. Touchscreen Basics

3.1 Sensor ConstructionA touchscreen is usually constructed from a number of transparent electrodes. These are typically ona glass or plastic substrate. They can also be made using non-transparent electrodes, such ascopper or carbon. Electrodes are normally formed by etching a material called Indium Tin Oxide(ITO). This is a brittle ceramic material, of high optical clarity and varying sheet resistance. ThickerITO yields lower levels of resistance (perhaps tens to hundreds of /square) at the expense ofreduced optical clarity. Lower levels of resistance are generally more compatible with capacitivesensing. Thinner ITO leads to higher levels of resistance (perhaps hundreds to thousands of/square) with some of the best optical characteristics.

Interconnecting tracks formed in ITO can cause problems. The excessive RC time constantsformed between the resistance of the track and the capacitance of the electrode to ground caninhibit the capacitive sensing function. In such cases, ITO tracks should be replaced by screenprinted conductive inks (non-transparent) outside the touchscreen’s viewing area.

A range of trade-offs also exist with regard to the number of layers used for construction. Atmel haspioneered single-layer ITO capacitive touchscreens. For many applications these offer a nearoptimum cost/performance balance. With a single layer screen, the electrodes are all connectedusing ITO out to the edges of the sensor. From there the connection is picked up with printed silvertracks. Sometimes two overprinted silver tracking layers are used to reduce the margins betweenthe edge of the substrate and the active area of the sensor.

Two-layer designs can have a strong technical appeal where ultra-narrow edge margins arerequired. They are also an advantage where the capacitive sensing function needs to have a veryprecise cut-off as a touch is moved to just off the active sensor area. With a two-layer design theQMatrix transmitter electrodes are normally placed nearest the bottom and the receiver electrodesnearest the top. The separation between layers can range from hundreds of nanometers tohundreds of microns, with the right electrode design and considerations of the sensing environment.

3.2 Electrode ConfigurationThe specific electrode designs used in Atmel's touchscreens are the subject of various patentsand patent applications. Further information is available on request.

The device supports various configurations of electrodes as summarized below:

Touchscreens: 2 Touchscreens allowed3 X × 3 Y minimum (depends on screen resolution)24 X × 32 Y maximum (subject to other configurations)

Keys: 2 Key Arrays allowedEach up to 32 keys (subject to other configurations)

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3.3 Scanning SequenceAll channels are scanned in sequence by the device. The channels are scanned by measuringcapacitive changes at the intersections formed between the first X line and all the odd Y lines,followed by the intersections between the first X line and all the even Y lines. Then the intersectionsbetween the next X line and the odd and even Y lines are scanned, and so on, until all X and Ycombinations have been measured.

The device can be configured in various ways. It is possible to disable some channels so that theyare not scanned at all. This can be used to improve overall scanning time.

3.4 Touchscreen Sensitivity

3.4.1 AdjustmentSensitivity of touchscreens can vary across the extents of the electrode pattern due to naturaldifferences in the parasitics of the interconnections, control chip, and so on. An importantfactor in the uniformity of sensitivity is the electrode design itself. It is a natural consequence ofa touchscreen pattern that the edges form a discontinuity and hence tend to have a differentsensitivity. The electrodes at the far edges do not have a neighboring electrode on one sideand this affects the electric field distribution in that region.

A sensitivity adjustment is available for the whole touchscreen. This adjustment is a basicalgorithmic threshold that defines when a channel is considered to have enough signal changeto qualify as being in detect.

The device supports mixed configurations of different touch objects, each having independentthreshold controls to allow fine tuning with mixed configurations.

3.4.2 Mechanical StackupThe mechanical stackup refers to the arrangement of material layers that exist above andbelow a touchscreen. The arrangement of the touchscreen in relation to other parts of themechanical stackup has an effect on the overall sensitivity of the screen. QMatrix technologyhas an excellent ability to operate in the presence of ground planes close to the sensor.QMatrix sensitivity is attributed more to the interaction of the electric fields between thetransmitting (X) and receiving (Y) electrodes than to the surface area of these electrodes. Forthis reason, stray capacitance on the X or Y electrodes does not strongly reduce sensitivity.

Front panel dielectric material has a direct bearing on sensitivity. Plastic front panels areusually suitable up to about 1.2 mm, and glass up to about 2.5 mm (dependent upon thescreen size and layout). The thicker the front panel, the lower the signal-to-noise ratio of themeasured capacitive changes and hence the lower the resolution of the touchscreen. Ingeneral, glass front panels are near optimal because they conduct electric fields almost twiceas easily as plastic panels.

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4. Detailed Operation

4.1 Power-up/ResetThere is an internal Power-on Reset (POR) in the device.

The device must be held in RESET (active low) while both the digital and analog powersupplies (Vdd and AVdd) are powering up. If a slope or slew is applied to the digital or analogsupplies, Vdd and AVdd must reach their nominal values before the RESET signal isdeasserted (that is, goes high). This is shown in Figure 4-1. See Section 7.2 on page 29 fornominal values for Vdd and AVdd.

Figure 4-1. Power Sequencing on the mXT768E-AB

Note that there are no specific power-up, or power-down sequences required for themXT768E-AB. This means that the digital or analog supplies can be applied independentlyand in any order during power-up.

After power-up, the device takes ~46.2 ms before it is ready to start communications. Vddmust drop to below 1.45V in order to effect a proper POR. See Section 7 for furtherspecifications.

If the RESET line is released before the AVDD supplies have reached their nominal voltage(see Figure 4-2), then some additional operations need to be carried out by the host. Thereare two options open to the host controller:

• Start the part in deep sleep mode and then send the command sequence to set the cycle time to wake the part and allow it to run normally. Note that in this case a calibration command is also needed.

• Send a reset command.

> 90 ns

Nom

Vdd

Nom

Vdd

(Vdd)

RESET

See note below

Su

pp

ly V

olta

ge

s

Note: Vdd and AVdd can be powered up in either order.There is no prerequisite for the length of time between Vdd and AVdd powering up.

A

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mXT768E-AB

Figure 4-2. Power Sequencing on the mXT768E-AB – Late rise on AVDD

The RESET pin can be used to reset the device whenever necessary. The RESET pin mustbe asserted low for at least 90 ns to cause a reset. After releasing the RESET pin the devicetakes ~46.2 ms before it is ready to start communications. It is recommended to connect theRESET pin to a host controller to allow it to initiate a full hardware reset without requiring apower-down.

Note that the voltage level on the RESET pin of the device must never exceed Vdd (digitalsupply voltage).

A software reset command can be used to reset the chip (refer to the Command Processorobject in the mXT768E-AB 2.2 Protocol Guide). A software reset takes ~194 ms. After the chiphas finished it asserts the CHG line to signal to the host that a message is available. The resetflag is set in the Message Processor object to indicate to the host that it has just completed areset cycle. This bit can be used by the host to detect any unexpected brownout events. Thisallows the host to take any necessary corrective actions, such as reconfiguration.

A checksum check is performed on the configuration settings held in the nonvolatile memory.If the checksum does not match a stored copy of the last checksum, then this indicates thatthe settings have become corrupted. This is signaled to the host by setting the configurationerror bit in the message data for the Command Processor object (refer to the mXT768E-AB2.2 Protocol Guide for more information).

Note that the CHG line is momentarily set (approximately 100 ms) as an input after power-upor reset for diagnostic purposes. It is therefore particularly important that the line should beallowed to float high via the CHG line pull-up resistors during this period. It should not bedriven by the host.

Nom

Vdd

Nom

Vdd

(Vdd)

RESET

Su

pp

ly V

olta

ge

s

Send command

A

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4.2 CalibrationCalibration is the process by which a sensor chip assesses the background capacitance oneach channel. Channels are only calibrated on power-up and when:

• The channel is enabled (that is, activated).

OR

• The channel is already enabled and one of the following applies:

– The channel is held in detect for longer than the Touch Automatic Calibration setting (refer to the mXT768E-AB 2.2 Protocol Guide for more information on TCHAUTOCAL setting in the Acquisition Configuration object).

– The signal delta on a channel is at least the touch threshold (TCHTHR) in the anti-touch direction, while no other touches are present on the channel matrix (refer to the mXT768E-AB 2.2 Protocol Guide for more information on the TCHTHR field in the Multiple Touch Touchscreen and Key Array objects).

– The host issues a recalibrate command.

– Certain configuration settings are changed.

A status message is generated on the start and completion of a calibration.

Note that the device performs a global calibration; that is, all the channels are calibratedtogether.

4.3 Operational ModesThe device operates in two modes: active (touch detected) and idle (no touches detected).Both modes operate as a series of burst cycles. Each cycle consists of a short burst (duringwhich measurements are taken) followed by an inactive sleep period. The difference betweenthese modes is the length of the cycles. Those in idle mode typically have longer sleepperiods. The cycle length is configured using the IDLEACQINT and ACTVACQINT settings inthe Power Configuration object. In addition, an Active to Idle timeout (ACTV2IDLETO) settingis provided.

Refer to the mXT768E-AB 2.2 Protocol Guide for full information on how these modesoperate, and how to use the settings provided.

4.4 Touchscreen LayoutThe physical matrix can be configured to have one or more touch objects. These are configured using the appropriate touch objects (Multiple Touch Touchscreen,Key Array,Proximity Sensor). It is not mandatory to have all the allowable touch objects present. Theobjects are disabled by default so only those that you wish to use need to be enabled. Refer tothe mXT768E-AB 2.2 Protocol Guide for more information on configuring the touch objects.

When designing the physical layout of the touch panel, obey the following rules:

• Each touch object should be a regular rectangular shape in terms of the lines it uses.

• Touch objects can share X and Y lines, as necessary. Note, however, that the first instance (instance 0) of the Multiple Touch Touchscreen T9 object cannot share Ylines if the Shieldless T56 object is enabled.

• The design of the touch objects does not physically need to be on a strict XY grid pattern.

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mXT768E-AB

4.5 Signal Processing

4.5.1 Adjacent Key Suppression TechnologyAdjacent Key Suppression (AKS) technology is a patented method used to detect which touchobject is touched when objects are located close together. A touch in a group of AKS objectsis only indicated on the object in that group that is touched first. This is assumed to be theintended object. Once an object in an AKS group is in detect, there can be no furtherdetections within that group until the object is released. Objects can be in more than one AKSgroup.

Note that AKS technology works best when it operates in conjunction with a detect integrationsetting of several acquisition cycles.

The device has two levels of AKS. The first level works between the touch objects (MultipleTouch Touchscreen T9, Key Array T15, Proximity Key T52). The touch objects are assigned toAKS groups. If a touch occurs within one of the touch objects in a group, then touches withinother objects inside that group are suppressed. For example, if a Touchscreen and Key Arrayare placed in the same AKS group, then a touch in the Touchscreen will suppress touches inthe Key Array, and vice versa.

The second level of AKS is internal AKS within an individual Key Array object (note thatinternal AKS is not present on other types of touch objects, only a Key Array). If internal AKS isenabled, then when one key is touched, touches on all the other keys within the Key Array aresuppressed.

AKS is configured using the touch objects (Multiple Touch Touchscreen T9,or Key Array T15,Proximity Key T52). Refer to the mXT768E-AB 2.2 Protocol Guide for more information.

Note: If a touch is in detect and then AKS is enabled, that touch will not be forced out of detect. It will not go out of detect until the touch is released. AKS will then operate normally. This applies to both levels of AKS.

4.5.2 Detection IntegratorThe device features a touch detection integration mechanism. This acts to confirm a detectionin a robust fashion. A counter is incremented each time a touch has exceeded its thresholdand has remained above the threshold for the current acquisition. When this counter reachesa preset limit the sensor is finally declared to be touched. If, on any acquisition, the signal isnot seen to exceed the threshold level, the counter is cleared and the process has to start fromthe beginning.

The detection integrator is configured using the appropriate touch objects (Multiple TouchTouchscreen T9, Key Array T15). Refer to the mXT768E-AB 2.2 Protocol Guide for moreinformation.

4.5.3 Digital Filtering and Noise SuppressionThe mXT768E-AB supports the on-chip filtering of the acquisition data received from thesensor. Specifically, the Noise Suppression T48 object provides an algorithm to suppress theeffects of noise (for example, from a noisy charger plugged into the user’s product). Thisalgorithm can automatically adjust some of the acquisition parameters on-the-fly to filter theanalog-to-digital conversions (ADCs) received from the sensor. The algorithm can make useof a Grass Cutter (which rejects any samples outside a predetermined limit) and a MedianFilter (which rejects the highest and lowest sets of accumulated ADCs, leaving the center set).

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Noise suppression is triggered when a noise source is detected (typically when a charger isturned on). A hardware trigger can be implemented using the CHRG_IN pin. Alternatively, thehost’s driver code can indicate when a noise source is present.

A Mean Acquisition Mode can be used for acquisition when the noise suppression algorithm isin its off state. This aids stylus operation, as the quieter reported position gives better linearity.

An alternative burst mode on the X lines, known as Dual X Drive, is provided. This improvesthe signal-to-noise ratio (SNR) on a closely spaced X sensor matrix (when finger touches arelikely to cover more than one X line).

Refer to the mXT768E-AB 2.2 Protocol Guide for more information on the Noise SuppressionT48 object.

4.5.4 Stylus SupportThe mXT768E-AB allows for the particular characteristics of stylus touches, whilst still allowingconventional finger touches to be detected. Stylus touches are configured by the Stylus T47object. There is one instance of the Stylus T47 object for each Multiple Touch Touchscreen T9object present on the device.

For example, stylus support ensures that the small touch area of a stylus registers as a touch,as this would otherwise by considered too small for the touchscreen. Additionally, there arecontrols to distinguish a stylus touch from an unwanted approaching finger (such as on thehand holding the stylus).

The touch sensitivity and threshold controls for stylus touches are configured separately fromthose for conventional finger touches so that both types of touches can be accommodated.

4.5.5 Grip SuppressionThe mXT768E-AB has a grip suppression mechanism to suppress false detections when theuser grips a handheld device.

Grip suppression works by specifying a boundary around a touchscreen, within which touchescan be suppressed whilst still allowing touches in the center of the touchscreen. This ensuresthat a “rolling” hand touch (such as when a user grips a mobile device) is suppressed. A “real”(finger) touch towards the center of the screen is allowed.

Grip suppression is configured using the Grip Suppression T40 object. There is one instanceof the Grip Suppression T40 object for each Multiple Touch Touchscreen T9 object present onthe device. Refer to the mXT768E-AB 2.2 Protocol Guide for more information.

4.5.6 Unintentional Touch SuppressionThe Touch Suppression T42 object provides a mechanism to suppress false detections fromunintentional touches from a large body area, such as from a face, ear or palm. Thismechanism is enhanced by another mechanism (known as distance touch suppression) thatoperates in conjunction with large object suppression to suppress false touches only if they areless than a specified distance from a suppressed touch, whilst any touches greater than thisdistance remain unsuppressed. The Touch Suppression T42 object also provides MaximumTouch Suppression to suppress all touches if more than a specified number of touches hasbeen detected. There is one instance of the Touch Suppression T42 object for each MultipleTouch Touchscreen T9 object present on the device. Refer to the mXT768E-AB 2.2 ProtocolGuide for more information.

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mXT768E-AB

4.5.7 GesturesThe device supports the on-chip processing of touches so that specific gestures can bedetected. These may be a one-touch gesture (such as a tap or a drag) or they may be atwo-touch gesture (such as a pinch or a rotate).

Gestures are configured using the One-touch Gesture Processor T24 and the Two-touchGesture Processor T27 objects. There is one instance of the One-touch Gesture ProcessorT24 and the Two-touch Gesture Processor T27 objects for each Multiple Touch TouchscreenT9 object present on the device. Refer to the mXT768E-AB 2.2 Protocol Guide for moreinformation on gestures and their configuration.

4.6 GPIO PinsThe mXT768E-AB has seven GPIO pins, which can be set as either input or output pins, asrequired. Four of the GPIO pins can be set as PWM pins. The GPIO pins are configured usingthe GPIO/PWM Configuration T19 object.

4.7 Self-capacitance ChannelsThe mXT768E-AB has two self-capacitance channels for the implementation of individualtouch keys or proximity sensors. These are separate from the touchscreen matrix and insteaduse Atmel’s QTouch technology specifically designed for touch-key applications.

By increasing the sensitivity, these channels can be used as effective proximity sensors,allowing the presence of a nearby object (typically a hand) to be detected. In this scenario alarge electrode size can be used, which is extremely effective in increasing the sensitivity ofthe detector. The value of the Cs sampling capacitor (see Section 4.8.6 on page 18) will alsoneed to be increased to ensure improved sensitivity.

With the proper electrode and circuit design, the mXT768E-AB will project a touch or proximityfield to several centimeters through any dielectric.

4.8 Circuit Components

4.8.1 Bypass CapacitorsEach power supply (Vdd and AVdd) requires a 1 µF bypass capacitor. In addition, thereshould be a 100 nF bypass capacitor on each power trace. The capacitors should be ceramicX7R or X5R. See the schematics in Section 2.3 on page 9 for more details.

The PCB traces connecting the bypass capacitors to the pins of the device must not exceed5 mm in length. This limits any stray inductance that would reduce filtering effectiveness. Seealso Section 7.8 on page 36.

4.8.2 Supply QualityWhile the device has good Power Supply Rejection Ratio properties, poorly regulated and/ornoisy power can significantly reduce performance. See Section 7.8 on page 36.

Always operate the device with a well-regulated and clean AVdd supply. It supplies thesensitive analog stages in the device.

4.8.3 Supply SequencingVdd and AVdd can be powered independently of each other without damage to the device.Vdd and AVdd should be supplied with the same voltage unless specified by Atmel.

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Make sure that any lines connected to the device are below or equal to Vdd during power-up.For example, if RESET is supplied from a different power domain to the mXT768E-AB’s Vddpin, make sure that it is held low when Vdd is off. If this is not done, the RESET signal couldparasitically couple power via the mXT768E-AB’s RESET pin into the Vdd supply.

4.8.4 OscillatorAn oscillator is not required.

4.8.5 Decoupling RequirementsCertain pins have specific decoupling requirements:

• Pin 23 (VDD_INPUT) should be connected to VDD.

• Pin 34 (VDDCORE) is a decoupling connection for an internal circuit and should not be powered.

• Pin 67 (AVDD) should be closely connected to pin 68 (GND) by a decoupling capacitor.

• For future enhancements, boards should be laid out so that Rn and C1 can be removed.

See also the schematics in Section 2.3 on page 9.

4.8.6 Cs Sample CapacitorsThe Cs capacitors are the charge sensing sample capacitors for the two self-capacitancechannels. The optimal Cs values depend on the corresponding key electrode design, thethickness of the panel and its dielectric constant, and the purpose of the design (touch key orproximity sensor). Thicker panels require larger values of Cs. Larger values of Cs demandhigher stability and better dielectric to ensure reliable sensing. Values can be in the range4.7 nF for a key to 100 nF for a proximity sensor.

The value of the Cs capacitors should be chosen such that a light touch on a key mounted in aproduction unit or a prototype panel causes a reliable detection. The chosen Cs value shouldnever be so large that the key signals exceed ~1750, as reported by the chip in the debugdata.

The Cs capacitors should be a stable type, such as X7R ceramic or PPS film. For moreconsistent sensing from unit to unit, 5 percent tolerance capacitors are recommended.

4.8.7 Rs Series ResistorsThe Rs series resistors in the self-capacitance circuit are inline with the electrode connections(close to the mXT768E-AB chip) and are used to limit electrostatic discharge (ESD) currentsand to suppress radio frequency (RF) interference. The value should be chosen so that thesensor is fully charged. This can be measured using a coin. Refer to QTAN0079, Buttons,Sliders and Wheels Sensor Design Guide, (downloadable from the Touch Technology area ofAtmel’s website) for details.

Although these resistors may be omitted, the device may become susceptible to externalnoise or radio frequency interference (RFI). For details on how to select these resistors refer toQTAN0079, Buttons, Sliders and Wheels Sensor Design Guide.

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mXT768E-AB

4.8.8 PCB CleanlinessModern no-clean-flux is generally compatible with capacitive sensing circuits.

4.9 PCB LayoutSee Appendix A on page 41 for general advice on PCB layout.

4.10 DebuggingThe device provides a mechanism for obtaining raw data for development and testingpurposes by reading data from the Diagnostic Debug T37 object. Refer to the mXT768E-AB2.2 Protocol Guide for more information on this object.

A second mechanism is provided that allows the host to read the real-time raw data using thelow-level debug port. This can be accessed via the SPI interface. Refer to QTAN0050, Usingthe maXTouch Debug Port, for more information on the debug port.

There is also a Self Test T25 object that runs self-test routines in the mXT768E-AB to findhardware faults on the sense lines and the electrodes. Refer to the mXT768E-AB 2.2 ProtocolGuide and QTAN0059, Using the maXTouch Self Test Feature, for more information.

4.11 CommunicationsCommunication with the host is achieved using the I2C-compatible interface (see Section E onpage 51). Connect the COMMSEL pin to GND to select the I2C-compatible interface.

4.12 Configuring the DeviceThe device has an object-based protocol that organizes the features of the device into objectsthat can be controlled individually. This is configured using the Object Protocol common tomany of Atmel’s touch sensor devices. For more information on the Object Protocol and itsimplementation on the device, refer to the mXT768E-AB 2.2 Protocol Guide.

CAUTION: If a PCB is reworked to correct soldering faults relating to any ofthe device, or to any associated traces or components, be sure that you fullyunderstand the nature of the flux used during the rework process. Leakagecurrents from hygroscopic ionic residues can stop capacitive sensors fromfunctioning. If you have any doubts, a thorough cleaning after rework may be theonly safe option.

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5. I2C-compatible Communications

5.1 Communications ProtocolThe device can use an I2C-compatible interface for communication. See Appendix E onpage 51 for details of the I2C-compatible protocol.

The I2C-compatible interface is used in conjunction with the CHG line. The CHG line goingactive signifies that a new data packet is available. This provides an interrupt-style interfaceand allows the device to present data packets when internal changes have occurred.

5.2 I2C-compatible Addresses

5.2.1 I2C-compatible AddressesThe mXT768E-AB supports two I2C-compatible device addresses that are selected using theADDSEL line at start-up. The two internal I2C-compatible device addresses are 0x4C(ADDSEL low) and 0x4D (ADDSEL high). These are shifted left to form the SLA+W or SLA+Raddress when transmitted over the I2C-compatible interface, as shown in Table 5-1.

5.3 Writing To the DeviceA WRITE cycle to the device consists of a START condition followed by the I2C-compatibleaddress of the device (SLA+W). The next two bytes are the address of the location into whichthe writing starts. The first byte is the Least Significant Byte (LSByte) of the address, and thesecond byte is the Most Significant Byte (MSByte). This address is then stored as the addresspointer.

Subsequent bytes in a multibyte transfer form the actual data. These are written to the locationof the address pointer, location of the address pointer +1, location of the address pointer + 2,and so on. The address pointer returns to its starting value when the WRITE cycle’s STOPcondition is detected.

Figure 5-1 shows an example of writing four bytes of data to contiguous addresses starting at0x1234.

Figure 5-1. Example of a Four-byte Write Starting at Address 0x1234

Table 5-1. Format of SLA+W and SLA+R

Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0

Address: 0x4C or 0x4D Read/write

Write Data

START SLA+W 0x34 0x12 0x96 0x9B 0xA0 0xA5 STOP

Write Address(LSB, MSB)

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5.4 I2C-compatible Writes in Checksum ModeIn I2C-compatible checksum mode an 8-bit CRC is added to all I2C-compatible writes. TheCRC is sent at the end of the data write as the last byte before the STOP condition. All thebytes sent are included in the CRC, including the two address bytes. Any command or datasent to the device is processed even if the CRC fails.

To indicate that a checksum is to be sent in the write, the most significant bit of the MSByte ofthe address is set to 1. For example, the I2C-compatible command shown in Figure 5-2 writesa value of 150 (0x96) to address 0x1234 with a checksum. The address is changed to 0x9234to indicate checksum mode.

Figure 5-2. Example of a Write To Address 0x1234 With a Checksum

5.5 Reading From the DeviceTwo I2C-compatible bus activities must take place to read from the device. The first activity isan I2C-compatible write to set the address pointer (LSByte then MSByte). The second activityis the actual I2C-compatible read to receive the data. The address pointer returns to its startingvalue when the read cycle’s NACK is detected.

It is not necessary to set the address pointer before every read. The address pointer isupdated automatically after every read operation. The address pointer will be correct if thereads occur in order. In particular, when reading multiple messages from the MessageProcessor T5 object, the address pointer is automatically reset to allow continuous reads (seeSection 5.6 on page 22).

The WRITE and READ cycles consist of a START condition followed by the I2C-compatibleaddress of the device (SLA+W or SLA+R respectively).

Figure 5-3 shows the I2C-compatible commands to read four bytes starting at address 0x1234.

Figure 5-3. Example of a Four-byte Read Starting at Address 0x1234

Write Data

START SLA+W 0x34 0x92 0x96 Checksum

Write Address(LSB, MSB)

STOP

Read Data

START SLA+R 0x96 0x9B 0xA0 0xA5 STOP

START SLA+W 0x34 0x12

Read Address(LSB, MSB)

Set Address Pointer

Read Data

STOP

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5.6 Reading Status Messages with DMAThe device facilitates the easy reading of multiple messages using a single continuous readoperation. This allows the host hardware to use a direct memory access (DMA) controller forthe fast reading of messages, as follows:

1. The host uses a write operation to set the address pointer to the start of the Message Count T44 object, if necessary. (1) If a checksum is required on each message, the most significant bit of the MSByte of the read address must be set to 1.

2. The host starts the read operation of the message by sending a START condition.

3. The host reads the Message Count T44 object (one byte) to retrieve a count of the pending messages (refer to the mXT768E-AB 2.2 Protocol Guide for details).

4. The host calculates the number of bytes to read by multiplying the message count by the size of the Message Processor T5 object. (2)

Note that the size of the Message Processor T5 object as recorded in the Object Table includes a checksum byte. If a checksum has not been requested, one byte should be deducted from the size of the object. That is: number of bytes = count × (size-1).

5. The host reads the calculated number of message bytes. It is important that the host does not send a STOP condition during the message reads, as this will terminate the continuous read operation and reset the address pointer. No START and STOP conditions must be sent between the messages.

6. The host sends a STOP condition at the end of the read operation after the last message has been read. The NACK condition immediately before the STOP condition resets the address pointer to the start of Message Count T44 object.

Figure 5-4 shows an example of using a continuous read operation to read three messagesfrom the device without a checksum. Figure 5-5 on page 24 shows the same example with achecksum.

1. The STOP condition at the end of the read resets the address pointer to its initial location, so it may already be pointing at the Message Count T44 object following a previous message read.

2. The host should have already read the size of the Message Processor T5 object in its initialization code.

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Figure 5-4. Continuous Message Read Example – No Checksum

START SLA+W LSB MSB

Start Address ofMessage Count Object

STOP

Set Address Pointer

Read Message Count

START SLA+R Count = 3

Message Count Object

Read Message Data

Report ID Data Data

Message Processor Object Message # 1–

( 1) bytessize –

Report ID Data Data


Report ID Data Data


STOP

ContinuousRead

Notes: 1. STOP and START conditions are not necessary between messages.

2. The address pointer is automatically reset to the start of the Message Processor Object between reads.

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Figure 5-5. Continuous Message Read Example – I2C-compatible Checksum Mode

There are no checksums added on any other I2C-compatible reads. An 8-bit CRC can beadded, however, to all I2C-compatible writes, as described in Section 5.4 on page 21.

An alternative method of reading messages using the CHG line is given in Section 5.7.

5.7 CHG LineThe CHG line is an active-low, open-drain output that is used to alert the host that a newmessage is available in the Message Processor T5 object. This provides the host with aninterrupt-style interface with the potential for fast response times. It reduces the need forwasteful I2C-compatible communications.

The CHG line remains low as long as there are messages to be read. The host should beconfigured so that the CHG line is connected to an interrupt line that is level-triggered. Thehost should not use an edge-triggered interrupt as this means adding extra softwareprecautions.

Start Address ofMessage Count Object

Set Address Pointer

Read Message Count

START SLA+R Count = 3

Message Count Object

Read Message Data

Report ID Data Data


size bytes

Report ID Data Data


Report ID Data Data


STOP

ContinuousRead

START SLA+W LSB MSB |0x80 Checksum STOP

Checksum

Checksum

Checksum

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The CHG line should be allowed to float during normal usage. This is particularly importantafter power-up or reset (see Section 4.1 on page 12).

A pull-up resistor is required, typically 10 k to Vdd.

The CHG line operates in two modes, as defined by the Communications Configuration T18object (refer to the mXT768E-AB 2.2 Protocol Guide).

In Mode 0:

1. The CHG line goes low to indicate that a message is present.

2. The CHG line goes high when the first byte of the first message (that is, its report ID) has been sent and acknowledged (ACK sent) and the next byte has been prepared in the buffer.

3. The STOP condition at the end of an I2C-compatible transfer causes the CHG line to stay high if there are no more messages. Otherwise the CHG line goes low to indicate a further message.

Mode 0 allows the host to continually read messages. Messaging reading ends when a reportID of 255 (“invalid message”) is received. Alternatively the host ends the transfer by sending aNACK after receiving the last byte of a message, followed by a STOP condition. If and whenthere is another message present, the CHG line goes low, as in step 1. In this mode the stateof the CHG line does not need to be checked during the I2C-compatible read.

In Mode 1:

1. The CHG line goes low to indicate that a message is present.

2. The CHG line remains low while there are further messages to be sent after the current message.

3. The CHG line goes high again only once the first byte of the last message (that is, its report ID) has been sent and acknowledged (ACK sent) and the next byte has been prepared in the output buffer.

Figure 5-6. CHG Line Modes for I2C-compatible Transfers

Mode 0

I2C-compatible Interface ACK NACK

START SLA-R B0 B1 ... Bn B0 B1 ... Bn ... B0 B1 ... Bn STOP

Message #1 Message #2 Message #m

CHG Line

...

CHG line high or low; see text

Mode 1

I2C-compatible Interface ACK

START SLA-R B0 B1 ... Bn B0 B1 ... Bn ... B0 B1 ... Bn STOP

Message #1 Message #2 Message #m

CHG Line

...

CHG line high or low; see text

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Mode 1 allows the host to continually read the messages until the CHG line goes high, and thestate of the CHG line determines whether or not the host should continue receiving messagesfrom the device.

Note: The state of the CHG line should be checked only between messages and not between the bytes of a message. The precise point at which the CHG line changes state cannot be predicted and so the state of the CHG line cannot be guaranteed between bytes.

The Communications Configuration T18 object can be used to configure the behavior of theCHG line. In addition to the CHG line operation modes described above, this object allows theuse of edge-based interrupts, as well as direct control over the state of the CHG line. Refer tothe mXT768E-AB 2.2 Protocol Guide for more information.

5.8 SDA, SCLThe I2C-compatible bus transmits data and clock with SDA and SCL, respectively. These areopen-drain. The device can only drive these lines low or leave them open. The terminationresistors (Rp) pull the line up to Vdd if no I2C-compatible device is pulling it down.

The termination resistors commonly range from 1 k to 10 k. They should be chosen so thatthe rise times on SDA and SCL meet the I2C-compatible specifications (see Section 7.6 onpage 36).

5.9 Clock StretchingThe device supports clock stretching in accordance with the I2C specification. It may alsoinstigate a clock stretch if a communications event happens during a period when the device isbusy internally. The maximum clock stretch is approximately 10 – 15 ms.

The device has an internal bus monitor that can reset the internal I2C-compatible hardware ifSDA or SCL is stuck low for more than 200 ms. This means that if a prolonged clock stretch ofmore than 200 ms is seen by the device, then any ongoing transfers with the device may becorrupted. The bus monitor is enabled or disabled using the Communications ConfigurationT18 object. Refer to the mXT768E-AB 2.2 Protocol Guide for more information.

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6. Getting Started With the mXT768E-AB

6.1 Establishing Contact

6.1.1 Communication with the HostThe host can use the I2C-compatible interface (see Section 5.1 on page 20) to communicatewith the device.

6.1.2 I2C-compatible InterfaceOn power-up, the CHG line goes low to indicate that there is new data to be read from theMessage Processor T5 object. If the CHG line does not go low, there is a problem with thedevice.

The host should attempt to read any available messages to establish that the device is presentand running following power-up or a reset. Examples of messages include reset or calibrationmessages. The host should also check that there are no configuration errors reported.

6.2 Using the Object ProtocolThe device has an object-based protocol that is used to communicate with the device. Typicalcommunication includes configuring the device, sending commands to the device, andreceiving messages from the device. Refer to the mXT768E-AB 2.2 Protocol Guide for moreinformation.

The host must perform the following initialization so that it can communicate with the device:

1. Read the start positions of all the objects in the device from the Object Table and build up a list of these addresses.

2. Use the Object Table to calculate the report IDs so that messages from the device can be correctly interpreted.

6.3 Writing to the DeviceThere are two mechanisms for writing to the device:

• Using an I2C-compatible write operation (see Section 4.3 on page 14).

To communicate with the device, you write to the appropriate object:

• To send a command to the device, you write the appropriate command to the Command Processor T6 object (refer to the mXT768E-AB 2.2 Protocol Guide).

• To configure the device, you write to an object. For example, to configure the device’s power consumption you write to the global Power Configuration T7 object, and to set up a touchscreen you write to a Multiple Touch Touchscreen T9 object. Some objects are optional and need to be enabled before use. Refer to the mXT768E-AB 2.2 Protocol Guide for more information on the objects.

6.4 Reading from the Device Status information is stored in the Message Processor T5 object. This object can be read toreceive any status information from the device. There are two mechanisms that provide aninterrupt-style interface for reading messages in the Message Processor T5 object:

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• Using the I2C-compatible interface, the CHG line is asserted whenever a new message is available in the Message Processor T5 object (see Section 5.7 on page 24). See Section 5.5 on page 21 for information on the format of the I2C-compatible read operation.

Note that the host should always wait to be notified of messages. The host should not poll thedevice for messages.

6.5 Configuring the Device The objects are designed such that a default value of zero in their fields is a “safe” value thattypically disables functionality. The objects must be configured before use and the settingswritten to the nonvolatile memory using the Command Processor T6 object. Refer to themXT768E-AB 2.2 Protocol Guide for more information.

The following objects must be configured before use:

• Power Configuration

– Set up the Idle Acquisition Interval, Active Acquisition Interval and Active to Idle Timeout.

• Acquisition Configuration

The following objects should also be configured and enabled, as required:

• Touch objects: Multiple Touch Touchscreen T9, Key Array T15, Proximity Key T52

– Enable the object.

– Configure the origin and the number of channels it occupies, if applicable. Configure the other fields in the object, as required. For example, set up the AKS group(s), specify the burst length and threshold.

– Enable reporting to receive touch messages from the object.

• Signal processing objects: One-touch Gesture Processor T24, Two-touch Gesture Processor T27, Grip Suppression T40, Stylus T47, Noise Suppression T48, Adaptive Threshold T55, SlimSensor T56, Extra Touchscreen Data T57

– Enable the object.

– Configure the fields in the object, as required.

– Enable reporting to receive signal processing messages from the object.

• Support objects: Communications Configuration T18, GPIO/PWM Configuration T19, CTE Configuration T46, Self Test T25, Generic Data, User Data T38,

– Enable the object, if the object requires it.

– Configure the fields in the object, as required.

– Enable reporting, if the object supports messages, to receive messages from the object.

Refer to the mXT768E-AB 2.2 Protocol Guide for more information on configuring the objects.

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7. Specifications

7.1 Absolute Maximum Specifications

7.2 Recommended Operating Conditions

7.3 DC Specifications

7.3.1 Digital Power (Vdd)

7.3.2 Analog Power (AVdd)

Vdd 3.6 V

AVdd 3.6 V

Max continuous pin current, any control or drive pin 40 mA

Voltage forced onto any pin –0.3 V to (Vdd or AVdd) + 0.3 V

Configuration parameters maximum writes 10,000

CAUTION: Stresses beyond those listed under Absolute Maximum Specifications may cause permanent damage to the device. This is a stress rating only and functional operation of the device at these or other conditions beyond those indicated in the operational sections of this specification is not implied. Exposure to absolute maximum specification conditions for extended periods may affect device reliability.

Operating temp –40°C to +105°C

Storage temp –65°C to +150°C

Vdd 2.7 V to 3.3 V (±5%)

AVdd 2.7 V to 3.3V (±5%)

Vdd vs AVdd power sequencing No sequencing required

Vdd Supply ripple See Section 7.8 on page 36

Cx transverse load capacitance per channel 0.63 pF to 5 pF

GPIO maximum output current 2.7 mA

Parameter Description Min Typ Max Units Notes

Vdd Operating limits 2.57 3.47 V

Rate of rise (1)

1. These values are based on simulation and characterization of other maXTouch devices manufactured using the same process technology. These values are not covered by test limits in production

0.002 2.5 V/µs


AVdd Operating limits 2.57 3.47 V See Section 4.8.2 on page 17

Rate of rise (1)

1. These values are based on simulation and characterization of other maXTouch devices manufactured using the same process technology. These values are not covered by test limits in production

2.5 V/µs

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7.3.3 Input/Output

Ta (ambient temperature) = recommended range, unless otherwise noted


Input (RESET, SDA, SCL, GPIO)

Vil Low input logic level –0.3 0.3 × Vdd V Vdd = 2.57 V to 3.47 V

Vih High input logic level 0.7 × Vdd Vdd + 0.3 V Vdd = 2.57 V to 3.47 V

Iil Low input leakage current 1 µA Pull-up resistors disabled

Iih High input leakage current 1 µA

Output (CHG, SDA, SCL, GPIO)

Vol Low output voltage 0.2 × Vdd V Vdd = 2.57 V to 3.47 V

Voh High output voltage 0.8 × Vdd V Vdd = 2.57 V to 3.47 V

Iol Low output current 1.35 mA Vdd = 3.0 V

Ioh High output current 1.35 mA Vdd = 3.0 V

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7.4 Current Consumption

7.4.1 Analog Current

Acquisition Rate (ms) 0 Touches 1 Touch 2 Touches 3 Touches 5 TouchesFree-run 16.50 12.28 11.81 11.39 10.50

10 9.32 9.33 9.33 9.32 9.4016 5.83 5.84 5.84 5.85 5.8532 2.92 2.91 2.93 2.91 3.1864 1.47 1.47 1.55 1.41 1.66128 0.66 0.77 1.02 0.74 0.98254 0.43 0.37 0.39 0.44 0.66

Sleep 0.0015

I2C Interface, AVdd

Active to Idle Timeout = 2 seconds, CHRGTIME = 60, Gain = 160, XSIZE = 22, YSIZE = 32, Noise Suppression T48 Off, Ta (ambient temperature) = recommended range, unless otherwise noted

0

2

4

6

8

10

12

14

16

18

20

Free-run 10 16 32 64 128 254 Sleep

Cur

rent

Con

sum

ptio

n (m

A)

Acquisition Rate (ms)

0 Touches

1 Touch

2 Touches

3 Touches

5 Touches

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10 11.55 13.02 12.68 12.31 11.6016 9.83 8.22 8.27 8.26 8.2432 9.32 3.49 3.50 3.49 3.6764 4.86 4.57 2.74 2.89 2.98128 2.46 2.41 1.05 1.27 1.05254 1.46 1.50 0.47 0.39 1.11

Sleep 0.0012

I2C Interface, AVdd

Active to Idle Timeout = 2 seconds, CHRGTIME = 60, Gain = 160, XSIZE = 22, YSIZE = 32, Noise Suppression T48 On, Ta (ambient temperature) = recommended range, unless otherwise noted

0

2

4

6

8

10

12

14

16

18

20

Free-run 10 16 32 64 128 254 Sleep

Cur

rent

Con

sum

ptio

n (m

A)


0 Touches

1 Touch

2 Touches

3 Touches

5 Touches

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7.4.2 Digital Current


10 6.02 8.72 9.20 9.63 10.5916 3.77 5.54 5.69 6.06 6.6432 1.95 2.76 2.95 3.19 3.5164 1.01 1.54 1.52 1.75 2.21128 0.52 0.85 1.19 1.08 0.98254 0.37 0.98 1.26 0.85 1.23

Sleep 0.09

I2C Interface, DVdd

Active to Idle Timeout = 2 seconds, CHRGTIME = 60, Gain = 160, XSIZE = 22, YSIZE = 32, Noise Suppression T48 Off, Ta (ambient temperature) = recommended range, unless otherwise noted

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

Free-run 10 16 32 64 128 254 Sleep

Cur

rent

Con

sum

ptio

n (m

A)


0 Touches

1 Touch

2 Touches

3 Touches

5 Touches

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10 8.53 10.68 10.89 10.98 11.1916 7.13 7.25 7.25 7.41 8.1932 7.03 4.13 3.37 3.56 3.9664 4.10 4.21 2.18 2.47 2.91128 1.77 1.99 1.81 1.57 1.89254 0.90 1.21 1.24 0.92 0.67

Sleep 0.09

I2C Interface, DVdd

Active to Idle Timeout = 2 seconds, CHRGTIME = 60, Gain = 160, XSIZE = 22, YSIZE = 32, Noise Suppression T48 On, Ta (ambient temperature) = recommended range, unless otherwise noted

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

Free-run 10 16 32 64 128 254 Sleep

Cur

rent

Con

sum

ptio

n (m

A)


0 Touches

1 Touch

2 Touches

3 Touches

5 Touches

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7.5 Timing Specifications

7.5.1 Touch Latency

7.5.2 Speed

7.5.3 Reset Timings

Touches = 1, XSIZE = 24, YSIZE = 32, CHRGTIME = 2.5 µs, Ta (ambient temperature) = recommended range, unless otherwise noted


Tlatency

83 Hz 7.1 18.5 msIDLEACQINT = ACTVACQINT = 12



Acquisition Rate vs Number of Moving TouchesXSIZE = 24, YSIZE = 32, CHRGTIME = 75 (2.5 µs), IDLESYNCSPERX/ACTSYNCSPERX = 16,

Noise Suppression T48 disabled, SlimSensor T56 = On

100

110

120

130

140

150

160

170

180

0 1 2 3 4 5 6 7 8 9 10

Ref

resh

Rat

e (H

z)

Number of Moving Touches

Refresh Rate (Hz)

XSIZE = 24, YSIZE = 32, CHRGTIME = 2.5 µs, Ta (ambient temperature) = recommended range, unless otherwise noted

Parameter Min Typ Max Units Notes

Power on to CHG line low 46.6 66 ms

Hardware reset to CHG line low 46.2 66 ms

Software reset to CHG line low 194 277 ms

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7.6 I2C-compatible Specifications

7.7 Touch Accuracy and Repeatability

7.8 Power Supply Ripple and Noise

The test circuit used for the charts is shown in Figure 7-1.

Figure 7-1. Circuit Used for Power Supply Ripple Characterization Charts

Parameter Operation

Addresses 0×4C or 0×4D

Maximum bus speed (SCL) 400 kHz

I2C specification Version 2.1

Touchscreen pitch 5 mm, front panel = 1 mm, touch size = 8 mm

Parameter Min Typ Max Units Notes

Linearity ±0.5 mm

Accuracy ±1 mm

Accuracy at edge ±2 mm

Repeatability ±0.25 % X axis with 12-bit resolution

Parameter Min Typ Max (1) Units Notes

Vdd ±50 mVAcross frequency range 1Hz to 1 MHz

AVdd ±10 mVAcross frequency range1Hz to 1 MHz

1. Note that the values are not measured at production, but characterized from typical devices

SignalGenerator

CurrentBuffer

3�

50�

VDD/AVDD

Note: Bypass capacitors are

mXT768E-AB

7.9 ESD Information

7.10 Thermal Packaging

7.10.1 Thermal Data

7.10.2 Junction TemperatureThe average chip junction temperature, TJ in °C can be obtained from the following:

1. TJ = TA + (PD × JA)2. TJ = TA + (PD × (HEATSINK + JC))where:

• JA= package thermal resistance, Junction to ambient (°C/W), provided in Section 7.10.1.

• JC = package thermal resistance, Junction to case thermal resistance (°C/W), provided in Section 7.10.1.

• HEATSINK = cooling device thermal resistance (°C/W), provided in the device datasheet.

• PD = device power consumption (W) estimated from data provided in Section 7.8 on page 36.

• TA is the ambient temperature (°C).

From the first equation, the user can derive the estimated lifetime of the chip and decide if acooling device is necessary or no

t. If a cooling device is required, the second equation should be used to calculate the resultingaverage chip-junction temperature TJ in °C.

1.5 k/100 pF/3 pulses

Parameter Value Reference Standard

Electrostatic Discharge HBM ±2000V MIL– STD883 Method 3015.7

Electrostatic Discharge CDM ± 250 V JEDEC-22A TEST METHOD C101-A

Parameter Typ Unit Condition Package

Junction to ambient thermal resistance 49.7 °C/W Still air TQFP 100, 14 X 14 mm

Junction to case thermal resistance 14.2 °C/W TQFP 100, 14 X 14 mm

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7.11 Soldering Profile

Profile Feature Green Package

Average Ramp-up Rate (217°C to Peak) 3°C/s max

Preheat Temperature 175°C ±25°C 150 – 200°C

Time Maintained Above 217°C 60 – 150s

Time within 5°C of Actual Peak Temperature 30s

Peak Temperature Range 260°C

Ramp down Rate 6°C/s max

Time 25°C to Peak Temperature 8 minutes max

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7.12 Mechanical Dimensions

Note: Dimensions D, D1, E and E1 have a tolerance of ±0.1 mm.

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7.13 Part Marking

7.14 Part Number

7.15 Moisture Sensitivity Level (MSL)

Pin 1 ID

Abbreviation ofPart Number

Lot Code(variable text)

ATMELMXT768E-BYYWW V CCLOTCODE Die Revision

For example V shown

Date(year and week) Country Code

Part Number Reference Number Description

ATMXT768E-AB(trays)

HDQ100-pin 14 × 14 × 1 mm TQFP RoHS compliant

ATMXT768E-ABR(tape and reels)

HDQ100-pin 14 × 14 × 1 mm TQFP RoHS compliant

MSL Rating Peak Body Temperature Specifications

MSL3 260oC IPC/JEDEC J-STD-020

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mXT768E-AB

Appendix A. PCB Design Considerations

A.1 IntroductionThe following sections give the design considerations that should be adhered to whendesigning a PCB layout for use with the mXT768E-AB. Of these, power supply and groundtracking considerations are the most critical.

By observing the following design rules, and with careful preparation for the PCB layoutexercise, designers will be assured of a far better chance of success and a correctlyfunctioning product.

A.2 Printed Circuit BoardAtmel recommends the use of a four layer printed circuit board for mXT768E-AB applications.This, together with careful layout, will ensure that the board meets relevant EMC requirementsfor both noise radiation and susceptibility, as laid down by the various national andinternational standards agencies.

Standard component footprint, PCB/FPCB mask opening, pad finish, stencil design, solderreflow parameters are suitable for assembly of the mXT768E-AB. The assembly providershould be contacted for industry best practices when designing applications with themXT768E-AB

A.3 Supply Rails and Ground TrackingPower supply and clock distribution are the most critical parts of any board layout. Because ofthis, it is advisable that these be completed before any other tracking is undertaken. Afterthese, supply decoupling, and analog and high speed digital signals should be addressed.Track widths for all signals, especially power rails should be kept as wide as possible in orderto reduce inductance.

The Power and Ground planes themselves can form a useful capacitor. Flood filling for eitheror both of these supply rails, therefore, should be used where possible. It is important toensure that there are no floating copper areas remaining on the board: all such areas shouldbe connected to the 0V plane. The flood filling should be done on the outside layers of theboard.

A.4 Power Supply DecouplingAs a rule, a suitable decoupling capacitor should be placed on each and every supply pin onall digital devices. It is important that these capacitors are placed as close to the chip’s supplypins as possible (less than 5mm away). The ground connection of these capacitors should betracked to 0V by the shortest, heaviest traces possible.

Capacitors with a Type II dielectric, such as X5R or X7R and with a value of at least 100nF,should be used for this purpose.

In addition, at least one ‘bulk’ tantalum decoupling capacitor, with a minimum value of 4.7 µFshould be placed on each power rail, close to where the supply enters the board.

Surface mounting capacitors are preferred to wire leaded types due to their lower ESR andESL. It is often possible to fit these decoupling capacitors underneath and on the opposite sideof the PCB to the digital ICs. This will provide the shortest tracking, and most effectivedecoupling possible.

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Refer to the application note Selecting Decoupling Capacitors for Atmel’s PLDs (doc0484.pdf;available on Atmel’s website) for further general information on decoupling capacitors.

A.5 Suggested Voltage Regulator ManufacturersThe AVdd supply stability is critical for the mXT768E-AB because this supply interacts directlywith the analog front end. Atmel therefore recommends that the supply for the analog sectionof the board be supplied by a regulator that is separate from the logic supply regulator. Thisreduces the amount of noise injected into the sensitive, low signal level parts of the design.

A single low value series resistor (around 1) is required from the regulator output to theanalog supply input on the mXT768E-AB device. This, together with the regulator outputcapacitor, and the capacitors at the DC input to the device, forms a simple filter on the supplyrail.

A low noise device should be chosen for the regulator. If possible this should have provisionfor adding a capacitor across the internal reference for further noise reduction. Referenceshould be made to the manufacturer’s datasheet.

The voltage regulators listed in Table 7-1 have been tested and found to work well with themXT768E-AB. They have compatible footprints and pin-out specifications, and are available inthe SOT-23 package.

Note some manufacturers claim that minimal or no capacitance is required for correctregulator operation. However, in all cases, a minimum of a 1.0 µF ceramic, low ESR capacitorat the input and output of these devices should be used. The manufacturers’ datasheetsshould always be referred to when selecting capacitors for these devices and the typicalrecommended values, types and dielectrics adhered to.

A.6 Analog I/OIn general, tracking for the analog I/O signals from the mXT768E-AB device should be kept asshort as possible. These normally go to a connector which interfaces directly to thetouchscreen.

Ensure that adequate ground-planes are used. An analog ground plane should be used inaddition to a digital one. Care should be taken to ensure that both ground planes are keptseparate and are connected together only at the point of entry for the power to the PCB. Thisis usually at the input connector.

Table 7-1. Recommended Voltage Regulators

Manufacturer Part Number Pin

Linear Technology LT1761ES5-BYP Vdd only

Texas Instruments TPS79301DBVR-Q1 AVdd and Vdd

Texas Instruments TPS79328DBVR-Q1 (2.8 V) AVdd and Vdd



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A.7 Component PlacementIt is important to orient all devices so that the tracking for important signals (such as power andclocks) are kept as short as possible. This simple point is often overlooked when initiallyplanning a PCB layout and can save hours of work at a later stage.

A.8 Digital SignalsIn general, when tracking digital signals, it is advisable to avoid sharp directional changes,sensitive signal tracks (such as analog I/O) and any clock or crystal tracking.

A good ground return path for all signals should be provided, where possible, to ensure thatthere are no discontinuities in the ground return path.

A.9 EMC and Other ObservationsThe following recommendations are not mandatory, but may help in situations whereparticularly difficult EMC or other problems are present:

• Try to keep as many signals as possible on the inside layers of the board. If suitable ground flood fills are used on the top and bottom layers, these will provide a good level of screening for noisy signals, both into and out of the PCB.

• Ensure that the on-board regulators have sufficient tracking around and underneath the devices to act as a heatsink. This heatsink will normally be connected to the 0V or ground supply pin. Increasing the width of the copper tracking to any of the device pins will aid in removing heat. There should be no solder mask over the copper track underneath the body of the regulators.

• Ensure that the decoupling capacitors, especially tantalum, or high capacity ceramic types, have the requisite low ESR, ESL and good stability/temperature properties. Refer to the regulator manufacturer’s datasheet for more information.

A.10 Self-capacitance Layout ConsiderationsIf the two self-capacitance channels are to be used, the chip should be placed to minimize theSNSK trace length to reduce low frequency pickup, and to reduce stray Cx which degradesgain. The Cs and Rs resistors (see the schematics in Section 2.3 on page 9) should be placedas close to the body of the chip as possible so that the trace between Rs and the SNSK pin isvery short. This reduces the antenna-like ability of this trace to pick up high frequency signalsand feed them directly into the chip.

Keep the electrode and its SNSK trace away from other signal, power, and ground traces. Anyadjacent trace or ground plane will cause an increase in Cx load and desensitize the device.The possible signal-to-noise ratio benefits of a ground plane around or under the electrode aremore than negated by the decreased gain from the circuit. Metal areas near the electrode willalso reduce the field strength and increase Cx loading and should be avoided, if possible.

For best EMC performance the circuit should be made entirely with SMT components.

In some cases it may be desirable to increase sensitivity of a self-capacitance channel; forexample, when it is used as a proximity sensor. Sensitivity can often be increased by using alarger electrode or reducing panel thickness. Increasing electrode size can have diminishingreturns, as high values of Cx will reduce sensor gain. Note that increasing the electrode'ssurface area will not substantially increase touch sensitivity if its diameter is already muchlarger in surface area than the object being detected.

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The value of Cs also has a dramatic effect on sensitivity, and this can be increased in valuewith the trade-off of slower response time and more power. Panel material can also bechanged to one having a higher dielectric constant, which will better help to propagate thefield.

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Appendix B. Reference ConfigurationThe values listed below are used in the reference unit to validate the interfaces and derive thecharacterization data provided in Section 7.

The fields that are not listed have their values set to 0 (which is replaced by a default value).See mXT768E-AB 2.2 Protocol Guide for information about the individual objects and theirfields.

The values for the user’s application will depend on the circumstances of that particular projectand will vary from those listed here. Further tuning will be required to achieve an optimalperformance..

Field Value

User Data T38 – SPT_USERDATA_T38 (Instance 0)

DATA[0] 144

Power Configuration T7 – GEN_POWERCONFIG_T7 (Instance 0)

ACTV2IDLETO 10

Acquisition Configuration T8 – GEN_ACQUISITIONCONFIG_T8 (Instance 0)

CHRGTIME 60

TCHDRIFT 10

DRIFTST 10

Multiple Touch Touchscreen T9 – TOUCH_MULTITOUCHSCREEN_T9 (Instance 0)

CTRL 143

XSIZE 22

YSIZE 32

BLEN 160

TCHTHR 40

TCHDI 2

ORIENT 5

MOVHYSTI 10

MOVHYSTN 2

NUMTOUCH 10

AMPHYST 10

JUMPLIMIT 15

TCHHYST 5

Multiple Touch Touchscreen T9 – TOUCH_MULTITOUCHSCREEN_T9 (Instance 1)

CTRL 128

YORIGIN 16

XSIZE 24

YSIZE 16

BLEN 160

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TCHTHR 50

TCHDI 4

ORIENT 7

NUMTOUCH 10

XLOCLIP 11

XHICLIP 11

YLOCLIP 18

YHICLIP 18

XEDGECTRL 64

XEDGEDIST 34

JUMPLIMIT 25

Self Test T25 – SPT_SELFTEST_T25 (Instance 0)

SIGLIM[0].UPSIGLIM 27284

SIGLIM[0].LOSIGLIM 24384

CTE Configuration T46 – SPT_CTECONFIG_T46 (Instance 0)

IDLESYNCSPERX 16

ACTVSYNCSPERX 16

Noise Suppression T48 – PROCG_NOISESUPPRESSION_T48 (Instance 0)

CTRL 3

SlimSensor T56 – PROCI_SHIELDLESS_T56 (Instance 0)

NCNCL 1

Field Value

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Appendix C. Glossary of TermsChannel

One of the capacitive measurement points at which the sensor controller can detectcapacitive change.

Jitter

The peak-to-peak variance in the reported location for an axis when a fixed touch isapplied. Typically jitter is random in nature and has a Gaussian (1) distribution, thereforemeasurement of peak-to-peak jitter must be conducted over some period of time, typicallya few seconds. Jitter is typically measured as a percentage of the axis in question.

For example a 100 x 100 mm touchscreen that shows ±0.5 percent jitter in X and±1 percent jitter in Y would show a peak deviation from the average reported coordinate of±0.5 mm in X and ±1 mm in Y. Note that by defining the jitter relative to the averagereported coordinate, the effects of linearity are ignored.

Linearity

The measurement of the peak-to-peak deviation of the reported touch coordinate in oneaxis relative to the absolute position of touch on that axis. This is often referred to as thenonlinearity. Nonlinearities in either X or Y axes manifest themselves as regions where theperceived touch motion along that axis (alone) is not reflected correctly in the reportedcoordinate giving the sense of moving too fast or too slow. Linearity is measured as apercentage of the axis in question.

For each axis, a plot of the true coordinate versus the reported coordinate should be aperfect straight line at 45°. A non linearity makes this plot deviate from this ideal line. It ispossible to correct modest nonlinearities using on-chip linearization tables, but thiscorrection trades linearity for resolution in regions where stronger corrections are needed(because there is a stretching or compressing effect to correct the nonlinearity, so alteringthe resolution in these regions). Linearity is typically measured using data that has beensufficiently filtered to remove the effects of jitter. For example, a 100 mm slider with anonlinearity of ±1 percent reports a position that is, at most, 1 mm away in either directionfrom the true position.

Multitouch

The ability of a touchscreen to report multiple concurrent touches. The touches are reported asseparate sets of XY coordinates.

One-touch Gesture

A touch gesture that consists of a single touch. The combination of the duration of the touchand any change in position (that is, movement) of the touch characterizes a specific gesture.For example, a tap gesture is characterized by a short-duration touch followed by a release,and no significant movement.

Resolution

The measure of the smallest movement on a slider or touchscreen in an axis that causes achange in the reported coordinate for that axis. Resolution is normally expressed in bitsand tends to refer to resolution across the whole axis in question. For example, a resolution

1. Sometimes called Bell-shaped or Normal distribution.

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of 10 bits can resolve a movement of 0.0977 mm on a slider 100 mm long. Jitter in thereported position degrades usable resolution.

Touchscreen

A two-dimensional arrangement of electrodes whose capacitance changes when touched,allowing the location of touch to be computed in both X and Y axes. The output from the XYcomputation is a pair of numbers, typically 12-bits each, ranging from 0 to 4095,representing the extents of the touchscreen active region.

Two-touch Gesture

A touch gesture that consists of two simultaneous touches. The change in position of the twotouches in relation to each other characterizes a specific gesture. For example, a pinch gestureis characterized by two long-duration touches that have a decreasing distance between them(that is, they are moving closer together).

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Appendix D. QMatrix Pri

maxtouch 768-channel touchscreen controller mxt768e-ab...

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