ATMXT540E-AT_技术文档

Features

• maXTouch^™Touchscreen

– Two touchscreens with true 12-bit multiple touch reporting and real-time XY

tracking for up to 10 concurrent touches per touchscreen

– Up to 8.9 inch diagonal screen size supported

• Number of Channels

– Electrode grid configurations of up to 18 X and 30 Y lines supported

– Touchscreens up to 540 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.9710FX

maXTouch^™

540-channel

Touchscreen

Controller

• 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

mXT540E-AT

Automotive

Version

– 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 >160Hz, subject to configuration

– Maximum 10 touches >130Hz, subject to configuration

– Configurable to allow power/speed optimization

– Programmable timeout for automatic transition from active to idle states

• Response Times

– Initial latency <25 ms for first touch from idle, subject to configuration

• Sensors

– Shieldless sensor compatible, including fully laminated sensors, touch-on-lens

stack-ups and on-cell designs

– Works with PET or glass sensors, including curved profiles

– Works with all proprietary sensor patterns recommended by Atmel^®

– Supports gloved touch with different materials up to 2.5 mm thickness, subject to

configuration

• Panel Thickness

– Supports different panel thickness

• Interfaces

– I²C-compatible slave mode, 400 kHz

• Dual-rail Power

– Digital 2.7V to 3.3V nominal

– Analog 2.7V to 3.3V nominal

• Package

– 100-pin TQFP 14 x 14 x 1 mm, 0.5 mm lead pitch

9710FX–AT42–01/15

1. Overview of the mXT540E-AT

1.1

Introduction

The Atmel^®maXTouch family of touch controllers has set a new industry benchmark for

capacitive touchscreens with their low current consumption, fast response time and high levels

of accuracy. The mXT540E-AT’s single-chip solution offers the benefits of the maXTouch

enhanced architecture on devices with touchscreens up to 8.9 in. diagonal:

• Patented capacitive sensing method – The mXT540E-AT uses a unique charge-transfer

acquisition engine to implement Atmel’s patented QMatrix^®capacitive sensing method.

This allows the measurement of up to 540 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 mXT540E-AT 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 mXT540E-AT 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 Concepts

If some of the concepts mentioned in this datasheet are unfamiliar, see the following sections

for more information:

• Appendix C on page 44 for a glossary of terms

• Appendix D on page 46 for QMatrix technology

2

mXT540E-AT

9710FX–AT42–01/15

mXT540E-AT

1.3

Resources

The following datasheet provides essential information on configuring the device:

• mXT540E-AT 2.1 Protocol Guide

The following documents may also be useful (available by contacting Atmel’s Touch

Technology 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

– Application Note QTAN0086 – Touchscreen Design for Gloved Operation

• 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

3

9710FX–AT42–01/15

2. Pinout and Schematic

2.1

Pinout Configuration

100

98 97 96 95 94 93 92 91 90 89 88 87 86

83

81 80

78 77 76

79

99

85 84

82

Y25

Y26

Y27

1

75

AVDD

2

74 Y4

73 Y3

3

Y28

Y29

4

5

72

71

Y2

Y1

6

AVDD

GND

VDD

70 Y0

69 GND

68 GND

7

8

9

AVDD

NC

X17

GND

Reserved

NC

Reserved

GND

67

66

65

64

63

10

11

12

mXT540E-AT

GPIO1/DBG_CLK/SNS0 13

14

15

16

17

18

19

20

21

22

23

24

25

GPIO2/SYNC/SNSK0

62 GND

Reserved

GPIO3

GPIO4/SNS1

61

60

59

X16

X15

X14

GPIO5/CHRG_IN/SNSK1

58 GND

57

56

55

NC

VDD

GND

AVDD

X13

X12

54 X11

53

RESET

VDD_INPUT

SDA

X10

52 X9

51 X8

SCL

26

28 29 30 31 32 33 34 35 36 37 38 39 40

43

45 46

48 49 50

47

27

41 42

44

4

mXT540E-AT

9710FX–AT42–01/15

mXT540E-AT

2.2

Pinout Descriptions

I

Table 2-1.

Pin Listing

Name

Y25

1

Type

Comments

If Unused, Connect To...

I

Y line connection

Analog power

Leave open

2

Y26

Leave open

3

Y27

I

Leave open

4

Y28

I

Leave open

5

Y29

I

Leave open

6

AVDD

GND

P

–

P

–

7

Ground

–

8

VDD

Digital Power

–

9

Reserved

NC

Always connect to GND

No connection

Always connect to GND

Ground

–

10

11

12

Leave open

Reserved

GND

–

GPIO1

DBG_CLK

SNS0

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

Debug clock

Self-capacitance sense pin

Make it available as a Test

13

14

I/O

GPIO2/

SYNC

SNSK0

General purpose I/O

External synchronization

Self-capacitance sense pin

Input: GND

Output: leave open

15

16

Reserved

GPIO3

–

Always connect to GND

General purpose I/O

–

Input: GND

Output: leave open

I/O

GPIO4

SNS1

General purpose I/O

Self-capacitance sense pin

Input: GND

Output: leave open

17

18

I/O

GPIO5

CHRG_IN

SNSK1

General purpose I/O

Charger present input

Self-capacitance sense pin

Input: GND

Output: leave open

19

20

21

22

23

24

25

26

27

28

29

NC

VDD

–

P

No connection

Leave open

Digital power

–

GND

P

Ground

–

RESET

VDD_INPUT

SDA

I

Reset low; has internal 20 k to 60 k pull-up resistor

For factory use only – connect to VDD

Serial Interface Data

Serial Interface Clock

State change interrupt

No connection

Vdd ⁽¹⁾

–

VDD

OD

–

SCL

–

CHG ⁽²⁾

Leave open

NC

–

NC

–

No connection

VDD

P

Digital power

5

9710FX–AT42–01/15

Table 2-1.

Pin Listing (Continued)

30

Name

GND

Type

Comments

If Unused, Connect To...

P

I

Ground

–

31

ADDSEL

I²C-compatible address select

GPIO0

DBG_DATA

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

Debug data

Make it available as a Test

32

33

34

I/O

P

VDD

Digital power

–

Digital core power. Must be connected as in schematics

(see Section 2.3 on page 9)

VDDCORE

P

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

GND

NC

X0

P

–

Ground

–

No connection

X matrix drive line

Ground

Leave open

–

O

P

O

P

X1

GND

X2

X matrix drive line

Analog power

Leave open

–

X3

X4

X5

X6

X7

AVDD

X line regulator bypass – see schematics in Section 2.3 on

page 9

50

XVDD

P

–

51

52

53

54

55

56

57

58

59

60

61

62

63

X8

X9

O

P

X matrix drive line

Analog power

Leave open

–

X10

X11

X12

X13

AVDD

GND

X14

X15

X16

GND

P

Ground

–

O

P

X matrix drive line

Ground

Leave open

–

P

Ground

–

6

mXT540E-AT

9710FX–AT42–01/15

mXT540E-AT

Table 2-1.

64

Pin Listing (Continued)

Name

X17

NC

Type

Comments

If Unused, Connect To...

Leave open

–

O

–

P

I

X matrix drive line

No connection

Analog power

65

66

NC

67

AVDD

GND

Y0

68

Ground

–

69

Ground

–

70

Y line connection

Analog power

Leave open

–

71

Y1

I

72

Y2

I

73

Y3

I

74

Y4

I

75

AVDD

NC

P

–

I

76

No connection

Y line connection

No connection

Y line connection

Ground

Leave open

–

77

Y5

78

Y6

I

79

Y7

I

80

Y8

I

81

Y9

I

82

Y10

Y11

Y12

NC

I

83

I

84

I

85

–

I

86

Y13

Y14

GND

AVDD

Y15

Y16

Y17

Y18

Y19

Y20

Y21

Y22

87

I

88

P

I

89

Analog power

–

90

Y line connection

Leave open

91

I

92

I

93

I

94

I

95

I

96

I

97

I

7

9710FX–AT42–01/15

Table 2-1.

Pin Listing (Continued)

98

Name

Y23

Y24

NC

Type

Comments

If Unused, Connect To...

Leave open

I

Y line connection

No connection

99

Leave open

100

–

Leave open

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.

I

Input only

OD

Open drain output

O

Output only, push-pull

P

Ground or power

8

mXT540E-AT

9710FX–AT42–01/15

mXT540E-AT

2.3

Schematics

2.3.1

I²C-compatible Mode

< 5 mm away from chip

S

ee No t e s

R

n

0R

AVDD

GND

C 1

100n

F

C

1

5

4

1

u

F

C 1

10

0

n

F

GND

AVDD

VDD

68

GND

R

E

S

E

T

22

RESET

40

41

43

44

45

46

47

48

51

52

53

54

55

56

59

60

61

64

X0

X1

mXT540E-AT

SDA

X2

S

D

A

24

25

26

31

X3

I²C-COMPATIBLE

X4

S

C

L

X5

SCL

X6

C

H

G

X7

CHG

X8

A

D

S

E

L

X9

ADDSEL

X10

X11

X12

X13

X14

X15

X16

X17

G

P

IO

0/

D

B

G

_

D

ATA

32

13

14

GPIO0/DBG_DATA

GPIO1/DBG_CLK/SNS0

GPIO2/SYNC/SNSK0

C

S

0

1

G

P

I O

1

/

D

BG

_

C LK

R

S

0

1

G

P

I O

2 /

S Y N C

P

R

O

X

I

M

I

T

Y

K

E

Y

0

5

Y29

Y28

Y27

Y26

Y25

Y24

Y23

Y22

Y21

Y20

Y19

Y18

Y17

Y16

Y15

Y14

Y13

Y12

Y11

Y10

Y9

4

G

P

I O

3

4

5

16

17

18

19

GPIO3

3

C

S

2

GPIO4/SNS1

1

R S

99

98

97

96

95

94

93

92

91

90

87

86

84

83

82

81

80

79

78

77

74

73

72

71

70

NOTES

P

R OX IM IT

K

Y

1

GPIO5/CHRG_IN/SNSK1

NC

E

Y

1. Capacitors C1 to C15 must be X7R or

X5R and placed <5 mm away from

the pins for which they act as bypass

capacitors. See also Section 4.8.1 on

page 17.

15

27

Reserved

NC

GND

2. Pin 9 and 11 must always be

connected to GND.

28

NC

3. Pin 23 (VDD_INPUT) should be

connected to VDD.

Y8

Y7

9

4. Pin 34 (VDDCORE) is a decoupling

connection for an internal circuit and

should not be powered.

Reserved

Y6

Y5

11

Y4

Y3

VDD

Y2

23

85

Y1

5. Pin 67 (AVDD) should be closely

connected to pin 68 (GND) by a

decoupling capacitor.

GND

VDD_INPUT

Y0

76

66

65

39

38

37

NC

100

10

NC

36

6. Pin 15 should be connected to GND.

7. For future enhancements, boards

should be laid out so that Rn and C1

can be removed.

GND

9

9710FX–AT42–01/15

3. Touchscreen Basics

3.1

Sensor Construction

A touchscreen is usually constructed from a number of transparent electrodes. These are typically on

a glass or plastic substrate. They can also be made using non-transparent electrodes, such as

copper 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. Thicker

ITO yields lower levels of resistance (perhaps tens to hundreds of /square) at the expense of

reduced optical clarity. Lower levels of resistance are generally more compatible with capacitive

sensing. 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 constants

formed between the resistance of the track and the capacitance of the electrode to ground can

inhibit the capacitive sensing function. In such cases, ITO tracks should be replaced by screen

printed 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 has

pioneered single-layer ITO capacitive touchscreens. For many applications these offer a near

optimum cost/performance balance. With a single layer screen, the electrodes are all connected

using ITO out to the edges of the sensor. From there the connection is picked up with printed silver

tracks. Sometimes two overprinted silver tracking layers are used to reduce the margins between

the 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 are

required. They are also an advantage where the capacitive sensing function needs to have a very

precise cut-off as a touch is moved to just off the active sensor area. With a two-layer design the

QMatrix transmitter electrodes are normally placed nearest the bottom and the receiver electrodes

nearest the top. The separation between layers can range from hundreds of nanometers to

hundreds of microns, with the right electrode design and considerations of the sensing environment.

3.2

Electrode Configuration

The specific electrode designs used in Atmel's touchscreens are the subject of various patents

and patent applications. Further information is available on request.

The device supports various configurations of electrodes as summarized below:

Touchscreens:

2 Touchscreens allowed

3X x 3Y minimum (depends on screen resolution)

18X x 30Y maximum (subject to other configurations)

Keys:

2 Key Arrays allowed

Each up to 32 keys (subject to other configurations)

10

mXT540E-AT

9710FX–AT42–01/15

mXT540E-AT

3.3

Scanning Sequence

All channels are scanned in sequence by the device. The channels are scanned by measuring

capacitive 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 intersections

between the next X line and the odd and even Y lines are scanned, and so on, until all X and Y

combinations have been measured.

The device can be configured in various ways. It is possible to disable some channels so that they

are not scanned at all. This can be used to improve overall scanning time.

3.4

Touchscreen Sensitivity

3.4.1

Adjustment

Sensitivity of touchscreens can vary across the extents of the electrode pattern due to natural

differences in the parasitics of the interconnections, control chip, and so on. An important

factor in the uniformity of sensitivity is the electrode design itself. It is a natural consequence of

a touchscreen pattern that the edges form a discontinuity and hence tend to have a different

sensitivity. The electrodes at the far edges do not have a neighboring electrode on one side

and this affects the electric field distribution in that region.

A sensitivity adjustment is available for the whole touchscreen. This adjustment is a basic

algorithmic threshold that defines when a channel is considered to have enough signal change

to qualify as being in detect.

The device supports mixed configurations of different touch objects, each having independent

threshold controls to allow fine tuning with mixed configurations.

3.4.2

Mechanical Stackup

The mechanical stackup refers to the arrangement of material layers that exist above and

below a touchscreen. The arrangement of the touchscreen in relation to other parts of the

mechanical stackup has an effect on the overall sensitivity of the screen. QMatrix technology

has 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 the

transmitting (X) and receiving (Y) electrodes than to the surface area of these electrodes. For

this 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 are

usually suitable up to about 1.2 mm, and glass up to about 2.5 mm (dependent upon the

screen size and layout). The thicker the front panel, the lower the signal-to-noise ratio of the

measured capacitive changes and hence the lower the resolution of the touchscreen. In

general, glass front panels are near optimal because they conduct electric fields almost twice

as easily as plastic panels.

11

9710FX–AT42–01/15

4. Detailed Operation

4.1

Power-up/Reset

There 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 power

supplies (Vdd and AVdd) are powering up. If a slope or slew is applied to the digital or analog

supplies, Vdd and AVdd must reach their nominal values before the RESET signal is

deasserted (that is, goes high). This is shown in Figure 4-1. See Section 7.2 on page 29 for

nominal values for Vdd and AVdd.

Figure 4-1. Power Sequencing on the mXT540E-AT

See note below

Nom

AVdd

Nom

Vdd

(Vdd)

RESET

> 90 ns

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.

Note that there are no specific power-up, or power-down sequences required for the

mXT540E-AT. This means that the digital or analog supplies can be applied independently

and in any order during power-up.

After power-up, the device takes ~47 ms before it is ready to start communications. Vdd must

drop to below 1.45V in order to effect a proper POR. See Section 7 for further specifications.

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. There

are 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.

12

mXT540E-AT

9710FX–AT42–01/15

mXT540E-AT

Figure 4-2. Power Sequencing on the mXT540E-AT – Late rise on AVDD

Nom

AVdd

Vdd

(Vdd)

RESET

Send command

The RESET pin can be used to reset the device whenever necessary. The RESET pin must

be asserted low for at least 90 ns to cause a reset. After releasing the RESET pin the device

takes ~46.5 ms before it is ready to start communications. It is recommended to connect the

RESET pin to a host controller to allow it to initiate a full hardware reset without requiring a

power-down.

Note that the voltage level on the RESET pin of the device must never exceed Vdd (digital

supply voltage).

A software reset command can be used to reset the chip (refer to the Command Processor

object in the mXT540E-AT 2.1 Protocol Guide). A software reset takes ~194 ms. After the chip

has finished it asserts the CHG line to signal to the host that a message is available. The reset

flag is set in the Message Processor object to indicate to the host that it has just completed a

reset cycle. This bit can be used by the host to detect any unexpected brownout events. This

allows 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 that

the settings have become corrupted. This is signaled to the host by setting the configuration

error bit in the message data for the Command Processor object (refer to the mXT540E-AT

2.1 Protocol Guide for more information).

Note that the CHG line is momentarily set (approximately 100 ms) as an input after power-up

or reset for diagnostic purposes. It is therefore particularly important that the line should be

allowed to float high via the CHG line pull-up resistors during this period. It should not be

driven by the host.

13

9710FX–AT42–01/15

4.2

Calibration

Calibration is the process by which a sensor chip assesses the background capacitance on

each 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 mXT540E-AT 2.1 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 mXT540E-AT 2.1 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 calibrated

together.

4.3

Operational Modes

The 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 (during

which measurements are taken) followed by an inactive sleep period. The difference between

these modes is the length of the cycles. Those in idle mode typically have longer sleep

periods. The cycle length is configured using the IDLEACQINT and ACTVACQINT settings in

the Power Configuration object. In addition, an Active to Idle timeout (ACTV2IDLETO) setting

is provided.

Refer to the mXT540E-AT 2.1 Protocol Guide for full information on how these modes operate,

and how to use the settings provided.

4.4

Touchscreen Layout

The 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. The

objects are disabled by default so only those that you wish to use need to be enabled. Refer to

the mXT540E-AT 2.1 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 Y

lines if the SlimSensor 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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4.5

Signal Processing

4.5.1

Adjacent Key Suppression Technology

Adjacent Key Suppression (AKS) technology is a patented method used to detect which touch

object is touched when objects are located close together. A touch in a group of AKS objects

is only indicated on the object in that group that is touched first. This is assumed to be the

intended object. Once an object in an AKS group is in detect, there can be no further

detections within that group until the object is released. Objects can be in more than one AKS

group.

Note that AKS technology works best when it operates in conjunction with a detect integration

setting of several acquisition cycles.

The device has two levels of AKS. The first level works between the touch objects (Multiple

Touch Touchscreen T9, Key Array T15, Proximity Key T52). The touch objects are assigned to

AKS groups. If a touch occurs within one of the touch objects in a group, then touches within

other objects inside that group are suppressed. For example, if a Touchscreen and Key Array

are placed in the same AKS group, then a touch in the Touchscreen will suppress touches in

the Key Array, and vice versa.

The second level of AKS is internal AKS within an individual Key Array object (note that

internal AKS is not present on other types of touch objects, only a Key Array). If internal AKS is

enabled, then when one key is touched, touches on all the other keys within the Key Array are

suppressed.

AKS is configured using the touch objects (Multiple Touch Touchscreen T9,or Key Array T15,

Proximity Key T52). Refer to the mXT540E-AT 2.1 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 Integrator

The device features a touch detection integration mechanism. This acts to confirm a detection

in a robust fashion. A counter is incremented each time a touch has exceeded its threshold

and has remained above the threshold for the current acquisition. When this counter reaches

a preset limit the sensor is finally declared to be touched. If, on any acquisition, the signal is

not seen to exceed the threshold level, the counter is cleared and the process has to start from

the beginning.

The detection integrator is configured using the appropriate touch objects (Multiple Touch

Touchscreen T9, Key Array T15). Refer to the mXT540E-AT 2.1 Protocol Guide for more

information.

4.5.3

Digital Filtering and Noise Suppression

The mXT540E-AT supports the on-chip filtering of the acquisition data received from the

sensor. Specifically, the Noise Suppression T48 object provides an algorithm to suppress the

effects of noise (for example, from a noisy charger plugged into the user’s product). This

algorithm can automatically adjust some of the acquisition parameters on-the-fly to filter the

analog-to-digital conversions (ADCs) received from the sensor. The algorithm can make use

of a Grass Cutter (which rejects any samples outside a predetermined limit) and a Median

Filter (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 is

turned on). A hardware trigger can be implemented using the CHRG_IN pin. Alternatively, the

host’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 is

in 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 improves

the signal-to-noise ratio (SNR) on a closely spaced X sensor matrix (when finger touches are

likely to cover more than one X line).

Refer to the mXT540E-AT 2.1 Protocol Guide for more information on the Noise Suppression

T48 object.

4.5.4

Stylus Support

The mXT540E-AT allows for the particular characteristics of stylus touches, whilst still allowing

conventional finger touches to be detected. Stylus touches are configured by the Stylus T47

object. There is one instance of the Stylus T47 object for each Multiple Touch Touchscreen T9

object 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 are

controls to distinguish a stylus touch from an unwanted approaching finger (such as on the

hand holding the stylus).

The touch sensitivity and threshold controls for stylus touches are configured separately from

those for conventional finger touches so that both types of touches can be accommodated.

4.5.5

Grip Suppression

The mXT540E-AT has a grip suppression mechanism to suppress false detections when the

user grips a handheld device.

Grip suppression works by specifying a boundary around a touchscreen, within which touches

can be suppressed whilst still allowing touches in the center of the touchscreen. This ensures

that 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 instance

of the Grip Suppression T40 object for each Multiple Touch Touchscreen T9 object present on

the device. Refer to the mXT540E-AT 2.1 Protocol Guide for more information.

4.5.6

Unintentional Touch Suppression

The Touch Suppression T42 object provides a mechanism to suppress false detections from

unintentional touches from a large body area, such as from a face, ear or palm. This

mechanism is enhanced by another mechanism (known as distance touch suppression) that

operates in conjunction with large object suppression to suppress false touches only if they are

less than a specified distance from a suppressed touch, whilst any touches greater than this

distance remain unsuppressed. The Touch Suppression T42 object also provides Maximum

Touch Suppression to suppress all touches if more than a specified number of touches has

been detected. There is one instance of the Touch Suppression T42 object for each Multiple

Touch Touchscreen T9 object present on the device. Refer to the mXT540E-AT 2.1 Protocol

Guide for more information.

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4.5.7

Gestures

The device supports the on-chip processing of touches so that specific gestures can be

detected. These may be a one-touch gesture (such as a tap or a drag) or they may be a

two-touch gesture (such as a pinch or a rotate).

Gestures are configured using the One-touch Gesture Processor T24 and the Two-touch

Gesture Processor T27 objects. There is one instance of the One-touch Gesture Processor

T24 and the Two-touch Gesture Processor T27 objects for each Multiple Touch Touchscreen

T9 object present on the device. Refer to the mXT540E-AT 2.1 Protocol Guide for more

information on gestures and their configuration.

4.6

4.7

GPIO Pins

The mXT540E-AT has six GPIO pins, which can be set as either input or output pins, as

required. Four of the GPIO pins can be set as PWM pins. The GPIO pins are configured using

the GPIO/PWM Configuration T19 object.

Self-capacitance Channels

The mXT540E-AT has two self-capacitance channels for the implementation of individual

touch keys or proximity sensors. These are separate from the touchscreen matrix and instead

use 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 a

large electrode size can be used, which is extremely effective in increasing the sensitivity of

the detector. The value of the Cs sampling capacitor (see Section 4.8.6 on page 18) will also

need to be increased to ensure improved sensitivity.

With the proper electrode and circuit design, the mXT540E-AT will project a touch or proximity

field to several centimeters through any dielectric.

4.8

Circuit Components

4.8.1

Bypass Capacitors

Each power supply (Vdd and AVdd) requires a 1 µF bypass capacitor. In addition, there

should be a 100 nF bypass capacitor on each power trace. The capacitors should be ceramic

X7R 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 exceed

5 mm in length. This limits any stray inductance that would reduce filtering effectiveness. See

also Section 7.5 on page 31.

4.8.2

Supply Quality

While the device has good Power Supply Rejection Ratio properties, poorly regulated and/or

noisy power can significantly reduce performance. See Section 7.5 on page 31.

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

sensitive analog stages in the device.

4.8.3

Supply Sequencing

Vdd 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 mXT540E-AT’s Vdd

pin, make sure that it is held low when Vdd is off. If this is not done, the RESET signal could

parasitically couple power via the mXT540E-AT’s RESET pin into the Vdd supply.

4.8.4

4.8.5

Oscillator

An oscillator is not required.

Decoupling Requirements

Certain 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 Capacitors

The Cs capacitors are the charge sensing sample capacitors for the two self-capacitance

channels. The optimal Cs values depend on the corresponding key electrode design, the

thickness of the panel and its dielectric constant, and the purpose of the design (touch key or

proximity sensor). Thicker panels require larger values of Cs. Larger values of Cs demand

higher stability and better dielectric to ensure reliable sensing. Values can be in the range

4.7 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 a

production unit or a prototype panel causes a reliable detection. The chosen Cs value should

never be so large that the key signals exceed ~1750, as reported by the chip in the debug

data.

The Cs capacitors should be a stable type, such as X7R ceramic or PPS film. For more

consistent sensing from unit to unit, 5 percent tolerance capacitors are recommended.

4.8.7

Rs Series Resistors

The Rs series resistors in the self-capacitance circuit are inline with the electrode connections

(close to the mXT540E-AT chip) and are used to limit electrostatic discharge (ESD) currents

and to suppress radio frequency (RF) interference. The value should be chosen so that the

sensor 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 of

Atmel’s website) for details.

Although these resistors may be omitted, the device may become susceptible to external

noise or radio frequency interference (RFI). For details on how to select these resistors refer to

QTAN0079, Buttons, Sliders and Wheels Sensor Design Guide.

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4.8.8

PCB Cleanliness

Modern no-clean-flux is generally compatible with capacitive sensing circuits.

CAUTION: If a PCB is reworked to correct soldering faults relating to any of the

devices, or to any associated traces or components, be sure that you fully

understand the nature of the flux used during the rework process. Leakage

currents from hygroscopic ionic residues can stop capacitive sensors from

functioning. If you have any doubts, a thorough cleaning after rework may be the

only safe option.

4.9

PCB Layout

See Appendix A on page 38 for general advice on PCB layout.

4.10 Debugging

The device provides a mechanism for obtaining raw data for development and testing

purposes by reading data from the Diagnostic Debug T37 object. Refer to the mXT540E-AT

2.1 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 the

low-level debug port. This can be accessed via the SPI interface. Refer to QTAN0050, Using

the 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 mXT540E-AT to find

hardware faults on the sense lines and the electrodes. Refer to the mXT540E-AT 2.1 Protocol

Guide and QTAN0059, Using the maXTouch Self Test Feature, for more information.

4.11 Communications

Communication with the host is achieved using the I²C-compatible interface (see Section 5 on

page 20).Connect the COMMSEL pin to GND to select the I²C-compatible interface.

4.12 Configuring the Device

The device has an object-based protocol that organizes the features of the device into objects

that can be controlled individually. This is configured using the Object Protocol common to

many of Atmel’s touch sensor devices. For more information on the Object Protocol and its

implementation on the device, refer to the mXT540E-AT 2.1 Protocol Guide.

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5. I²C-compatible Communications

5.1

Communications Protocol

The device can use an I²C-compatible interface for communication. See Appendix E on

page 48 for details of the I²C-compatible protocol.

The I²C-compatible interface is used in conjunction with the CHG line. The CHG line going

active signifies that a new data packet is available. This provides an interrupt-style interface

and allows the device to present data packets when internal changes have occurred.

5.2

I²C-compatible Addresses

5.2.1

I²C-compatible Addresses

The mXT540E-AT supports two I²C-compatible device addresses that are selected using the

ADDSEL 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+R

address when transmitted over the I²C-compatible interface, as shown in Table 5-1.

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

5.3

Writing To the Device

A WRITE cycle to the device consists of a START condition followed by the I²C-compatible

address of the device (SLA+W). The next two bytes are the address of the location into which

the writing starts. The first byte is the Least Significant Byte (LSByte) of the address, and the

second byte is the Most Significant Byte (MSByte). This address is then stored as the address

pointer.

Subsequent bytes in a multibyte transfer form the actual data. These are written to the location

of 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 STOP

condition is detected.

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

0x1234.

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

START SLA+W

0x34

0x12

0x96

0x9B

0xA0

0xA5

STOP

Write Address

(LSB, MSB)

Write Data

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5.4

I²C-compatible Writes in Checksum Mode

In I²C-compatible checksum mode an 8-bit CRC is added to all I²C-compatible writes. The

CRC is sent at the end of the data write as the last byte before the STOP condition. All the

bytes sent are included in the CRC, including the two address bytes. Any command or data

sent 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 of

the address is set to 1. For example, the I²C-compatible command shown in Figure 5-2 writes

a value of 150 (0x96) to address 0x1234 with a checksum. The address is changed to 0x9234

to indicate checksum mode.

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

Checksum

START SLA+W

0x34

0x92

0x96

STOP

Write Address

(LSB, MSB)

Write Data

5.5

Reading From the Device

Two I²C-compatible bus activities must take place to read from the device. The first activity is

an I²C-compatible write to set the address pointer (LSByte then MSByte). The second activity

is the actual I²C-compatible read to receive the data. The address pointer returns to its starting

value when the read cycle’s NACK is detected.

It is not necessary to set the address pointer before every read. The address pointer is

updated automatically after every read operation. The address pointer will be correct if the

reads occur in order. In particular, when reading multiple messages from the Message

Processor T5 object, the address pointer is automatically reset to allow continuous reads (see

Section 5.6).

The WRITE and READ cycles consist of a START condition followed by the I²C-compatible

address 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

Set Address Pointer

START SLA+W

0x34

0x12

STOP

Read Address

(LSB, MSB)

Read Data

START SLA+R

0x96

0x9B

0xA0

0xA5

STOP

Read Data

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5.6

Reading Status Messages with DMA

The device facilitates the easy reading of multiple messages using a single continuous read

operation. This allows the host hardware to use a direct memory access (DMA) controller for

the 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 mXT540E-AT 2.1 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 x

(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 messages

from the device without a checksum. Figure 5-5 on page 24 shows the same example with a

checksum.

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

Set Address Pointer

START SLA+W

LSB

MSB

STOP

Start Address of

Message Count Object

Read Message Count

Continuous

Read

START SLA+R Count = 3

Message Count Object

Read Message Data

(size – 1) bytes

Report ID

Data

Message Processor Object – Message # 1

Report ID

Data

Message Processor Object – Message # 2

Report ID

Data

STOP

Message Processor Object – Message # 3

Notes:

1.S

TOP 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

Set Address Pointer

MSB |

0x80

Checksum

START SLA+W

LSB

STOP

Start Address of

Message Count Object

Read Message Count

Continuous

Read

START SLA+R Count = 3

Message Count Object

Read Message Data

size bytes

Report ID

Checksum

Data

Message Processor Object – Message # 1

Report ID

Checksum

Data

Message Processor Object – Message # 2

Report ID

Checksum

Data

STOP

Message Processor Object – Message # 3

There are no checksums added on any other I²C-compatible reads. An 8-bit CRC can be

added, 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 Line

The CHG line is an active-low, open-drain output that is used to alert the host that a new

message is available in the Message Processor T5 object. This provides the host with an

interrupt-style interface with the potential for fast response times. It reduces the need for

wasteful I²C-compatible communications.

The CHG line remains low as long as there are messages to be read. The host should be

configured so that the CHG line is connected to an interrupt line that is level-triggered. The

host should not use an edge-triggered interrupt as this means adding extra software

precautions.

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The CHG line should be allowed to float during normal usage. This is particularly important

after 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 T18

object (refer to the mXT540E-AT 2.1 Protocol Guide).

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

Mode 0

I²C-compatible Interface

ACK

NACK

B0 B1 ... Bn

Message #m

START SLA-R

B0 B1 ... Bn

Message #1

B0 B1 ... Bn

...

STOP

Message #2

CHG Line

CHG line high or low; see text

Mode 1

I²C-compatible Interface

ACK

START SLA-R

B0 B1 ... Bn

...

B0 B1 ... Bn

Message #m

STOP

Message #1

Message #2

CHG Line

CHG line high or low; see text

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 report

ID of 255 (“invalid message”) is received. Alternatively the host ends the transfer by sending a

NACK after receiving the last byte of a message, followed by a STOP condition. If and when

there is another message present, the CHG line goes low, as in step 1. In this mode the state

of 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.

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Mode 1 allows the host to continually read the messages until the CHG line goes high, and the

state of the CHG line determines whether or not the host should continue receiving messages

from 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 the

CHG line. In addition to the CHG line operation modes described above, this object allows the

use of edge-based interrupts, as well as direct control over the state of the CHG line. Refer to

the mXT540E-AT 2.1 Protocol Guide for more information.

5.8

5.9

SDA, SCL

The I²C-compatible bus transmits data and clock with SDA and SCL, respectively. These are

open-drain. The device can only drive these lines low or leave them open. The termination

resistors (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 that

the rise times on SDA and SCL meet the I²C-compatible specifications (see Section 7.7 on

page 33).

Clock Stretching

The device supports clock stretching in accordance with the I²C specification. It may also

instigate a clock stretch if a communications event happens during a period when the device is

busy 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 if

SDA or SCL is stuck low for more than 200 ms. This means that if a prolonged clock stretch of

more than 200 ms is seen by the device, then any ongoing transfers with the device may be

corrupted. The bus monitor is enabled or disabled using the Communications Configuration

T18 object. Refer to the mXT540E-AT 2.1 Protocol Guide for more information.

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6. Getting Started With the mXT540E-AT

6.1

Establishing Contact

6.1.1

Communication with the Host

The host can use the I²C-compatible interface (see Section 5.1 on page 20) to communicate

with the device.

6.1.2

I²C-compatible Interface

On power-up, the CHG line goes low to indicate that there is new data to be read from the

Message Processor T5 object. If the CHG line does not go low, there is a problem with the

device.

The host should attempt to read any available messages to establish that the device is present

and running following power-up or a reset. Examples of messages include reset or calibration

messages. The host should also check that there are no configuration errors reported.

6.2

Using the Object Protocol

The device has an object-based protocol that is used to communicate with the device. Typical

communication includes configuring the device, sending commands to the device, and

receiving messages from the device. Refer to the mXT540E-AT 2.1 Protocol Guide for more

information.

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 Device

There are two mechanisms for writing to the device:

• Using an I²C-compatible write operation (see Section 5.3 on page 20).

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 mXT540E-AT 2.1 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 mXT540E-AT 2.1 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 to

receive any status information from the device. There are two mechanisms that provide an

interrupt-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 the

device 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 that

typically disables functionality. The objects must be configured before use and the settings

written to the nonvolatile memory using the Command Processor T6 object. Refer to the

mXT540E-AT 2.1 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 mXT540E-AT 2.1 Protocol Guide for more information on configuring the objects.

28

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7. Specifications

7.1

Absolute Maximum Specifications

Vdd

3.6 V

AVdd

3.6 V

Max continuous pin current, any control or drive pin

Voltage forced onto any pin

40 mA

–0.3 V to (Vdd or AVdd) + 0.3 V

10,000

Configuration parameters maximum writes

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.

7.2

Recommended Operating Conditions

Operating temp

Storage temp

–40°C to +85°C

–65°C to +150°C

2.7 V to 3.3 V ( 5ꢀ)

No sequencing required

See Section 7.5 on page 31

0.63 pF to 5 pF

Vdd

AVdd

Vdd vs AVdd power sequencing

Supply ripple

Cx transverse load capacitance per channel

GPIO maximum output current

2.7 mA

7.3

DC Specifications

7.3.1

Digital Power (Vdd)

Parameter

Vdd

Rate of rise ⁽¹⁾

Description

Min

2.57

Typ

Max

3.47

2.5

Units

V

Notes

Operating limits

0.002

V/µs

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

7.3.2

Analog Power (AVdd)

Parameter

Description

Min

Typ

Max

3.47

2.5

Units

V

Notes

AVdd

Operating limits

2.57

See Section 4.8.2 on page 17

Rate of rise⁽¹⁾

V/µs

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

29

9710FX–AT42–01/15

7.3.3

Input/Output

Parameter

Description

Min

Typ

Max

Units

Notes

Input (RESET, SDA, SCL, GPIO)

Vil

Vih

Iil

Low input logic level

–0.3

0.3 × Vdd

V

Vdd = 2.57 V to 3.47 V

Pull-up resistors disabled

High input logic level

0.7 × Vdd

Vdd + 0.3

Low input leakage current

High input leakage current

1

µA

Iih

Output (CHG, GPIO)

Vol

Voh

Iol

Low output voltage

0.2 × Vdd

I_OH= 2.7 mA

High output voltage

Low output current

High output current

0.8 × Vdd

V

2.7

mA

Ioh

7.4

Supply Current

IDLEADCSPERX=ACTVADCSPERX=16, SlimSensor T56 Enabled

7.4.1

Analog current

Parameter

Description

Min

Typ

7.5

Max

Units

mA

Notes

Active average supply current

Idle average supply current

Sleep average supply current

100 Hz, 1 Touch

16 Hz, No Touch

AIdd

1.18

0.1

mA

7.4.2

Digital Current

Parameter

Description

Min

Typ

9.07

1.03

0.1

Max

Units

mA

Notes

Active average supply current

Idle average supply current

Sleep average supply current

100 Hz, 1 Touch

16 Hz, No Touch

Idd

mA

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7.5

Power Consumption

I²C-Compatible Interface – 3.3 V Supply Current

XSIZE = 18, YSIZE = 30, CHRGTIME = 75 (2.5 µs), IDLESYNCSPERX/ACTSYNCSPERX = 16

Noise Suppression T48 disabled, SlimSensor T56 enabled, Optimal Integration and Noise cancellation enabled

18

AIdd No Touch

AIdd 1 Touch

DIdd No Touch

DIdd 1 Touch

16

14

12

10

8

6

4

2

0

Acquisition Interval (ms)

I²C-Compatible Interface – 3.3 V Supply Current

XSIZE = 18, YSIZE = 30, CHRGTIME = 75 (2.5 µs), IDLESYNCSPERX/ACTSYNCSPERX = 16

Noise Suppression T48 disabled, SlimSensor T56 disabled

18

16

14

12

10

8

AIdd No Touch

AIdd 1 Touch

DIdd No Touch

DIdd 1 Touch

6

4

2

0

Acquisition Interval (ms)

31

9710FX–AT42–01/15

7.6

Timing Specifications

7.6.1

Touch Latency

Touches = 1, XSIZE = 18, YSIZE = 30, CHRGTIME = 2.5 µs,, Room Temperature

Parameter

Description

Min

Typ

Max

Units

Notes

IDLEACQINT =

ACTVACQINT = 12

83 Hz

5.3

18.3

ms

IDLEACQINT =

ACTVACQINT = 10

Tlatency

100 Hz

143 Hz

5.3

16.4

13.5

ms

IDLEACQINT =

ACTVACQINT = 7

7.6.2

Speed

Acquisition Rate vs Number of Moving Touches

XSIZE = 18, YSIZE = 30, CHRGTIME = 75 (2.5 µs), IDLESYNCSPERX/ACTSYNCSPERX = 16

Noise Suppression T48 disabled, SlimSensor T56 enabled

220

210

200

190

180

170

160

150

140

130

120

Refresh Rate (Hz)

0

1

2

3

4

5

6

7

8

9

10

Number of Moving Touches

7.6.3

Reset Timings

Parameter

Min

Typ

47

Max

67

Units

ms

Notes

Power on to CHG line low

Hardware reset to CHG line low

Software reset to CHG line low

46.5

194

66

ms

276

ms

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7.7

I²C-compatible Specifications

Parameter

Operation

0x4C and 0x4D

400 kHz

Addresses

Maximum bus speed (SCL)

I²C specification

Version 2.1

7.8

Touch Accuracy and Repeatability

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

Parameter

Linearity

Min

Typ

0.5

1

Max

Units

mm

Notes

Accuracy

mm

Accuracy at edge

2

mm

X axis with 12-bit

resolution

Repeatability

0.25

ꢀ

7.9

Power Supply Ripple and Noise

Parameter

Min

Typ

Max

Units

Notes

Across frequency range

1Hz to 1 MHz

Vdd

50

mV

Across frequency range

1Hz to 1 MHz

AVdd

10

mV

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

33

9710FX–AT42–01/15

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

15V

Current

Buffer

50W

Coax Cable

3W

Signal

Generator

Vout

VDD/

AVDD

100 nF

1 mF

50W

Note: Bypass capacitors are <5 mm away from the chip.

7.10 ESD Information

1.5 k/100 pF/3 pulses

Parameter

Value

2000 V

250 V

Reference Standard

Electrostatic Discharge HBM

Electrostatic Discharge CDM

MIL– STD883 Method 3015.7

JEDEC-22A TEST METHOD C101-A

7.11 Thermal Packaging

7.11.1

Thermal Data

Parameter

Typ

49.7

14.2

Unit

Condition

Still air

Package

Junction to ambient thermal resistance

Junction to case thermal resistance

°C/W

TQFP 100, 14 X 14 mm

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7.11.2

Junction Temperature

The average chip junction temperature, T_Jin °C can be obtained from the following:

1. T_J= T_A+ (P_Dx _JA)

2. T_J= T_A+ (P_Dx (_HEATSINK+ _JC))

where:

• _JA= package thermal resistance, Junction to ambient (°C/W), provided in Section 7.11.1.

• _JC= package thermal resistance, Junction to case thermal resistance (°C/W), provided in

Section 7.11.1.

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

• P_D= device power consumption (W) estimated from data provided in Section 7.8 on

page 33.

• T_Ais the ambient temperature (°C).

From the first equation, the user can derive the estimated lifetime of the chip and decide if a

cooling device is necessary or not. If a cooling device is required, the second equation should

be used to calculate the resulting average chip-junction temperature T_Jin °C.

7.12 Soldering Profile

Profile Feature

Green Package

Average Ramp-up Rate (217°C to Peak)

Preheat Temperature 175°C 25°C

Time Maintained Above 217°C

Time within 5°C of Actual Peak Temperature

Peak Temperature Range

3°C/s max

150 – 200°C

60 – 150s

30s

260°C

Ramp down Rate

6°C/s max

8 minutes max

Time 25°C to Peak Temperature

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9710FX–AT42–01/15

7.13 Mechanical Dimensions

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7.14 Part Marking

Pin 1 ID

ATMEL

Abbreviation of

Part Number

MXT540E

-T

Date

(year and week)

Country Code

Die Revision

YYWW V CC

LOTCODE

Lot Code

(variable text)

7.15 Part Number

Part Number

Reference Number

Description

ATMXT540E-AT

(trays)

100-pin 14 x 14 x 1 mm TQFP

RoHS compliant

GBD

ATMXT540E-ATR

(tape and reels)

100-pin 14 x 14 x 1 mm TQFP

RoHS compliant

GBD

7.16 Moisture Sensitivity Level (MSL)

MSL Rating

Peak Body Temperature

Specifications

MSL3

260^oC

IPC/JEDEC J-STD-020

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9710FX–AT42–01/15

Appendix A. PCB Design Considerations

A.1 Introduction

The following sections give the design considerations that should be adhered to when

designing a PCB layout for use with the mXT540E-AT. Of these, power supply and ground

tracking considerations are the most critical.

By observing the following design rules, and with careful preparation for the PCB layout

exercise, designers will be assured of a far better chance of success and a correctly

functioning product.

A.2 Printed Circuit Board

Atmel recommends the use of a four layer printed circuit board for mXT540E-AT applications.

This, together with careful layout, will ensure that the board meets relevant EMC requirements

for both noise radiation and susceptibility, as laid down by the various national and

international standards agencies.

Standard component footprint, PCB/FPCB mask opening, pad finish, stencil design, solder

reflow parameters are suitable for assembly of the mXT540E-AT. The assembly provider

should be contacted for industry best practices when designing applications with the

mXT540E-AT.

A.3 Supply Rails and Ground Tracking

Power supply and clock distribution are the most critical parts of any board layout. Because of

this, it is advisable that these be completed before any other tracking is undertaken. After

these, 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 order

to reduce inductance.

The Power and Ground planes themselves can form a useful capacitor. Flood filling for either

or both of these supply rails, therefore, should be used where possible. It is important to

ensure that there are no floating copper areas remaining on the board: all such areas should

be connected to the 0 V plane. The flood filling should be done on the outside layers of the

board.

A.4 Power Supply Decoupling

As a rule, a suitable decoupling capacitor should be placed on each and every supply pin on

all digital devices. It is important that these capacitors are placed as close to the chip’s supply

pins as possible (less than 5mm away). The ground connection of these capacitors should be

tracked 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 100 nF,

should be used for this purpose.

In addition, at least one ‘bulk’ tantalum decoupling capacitor, with a minimum value of 4.7 µF

should 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 and

ESL. It is often possible to fit these decoupling capacitors underneath and on the opposite side

of the PCB to the digital ICs. This will provide the shortest tracking, and most effective

decoupling 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 Manufacturers

The AVdd supply stability is critical for the mXT540E-AT because this supply interacts directly

with the analog front end. Atmel therefore recommends that the supply for the analog section

of the board be supplied by a regulator that is separate from the logic supply regulator. This

reduces 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 the

analog supply input on the mXT540E-AT device. This, together with the regulator output

capacitor, and the capacitors at the DC input to the device, forms a simple filter on the supply

rail.

A low noise device should be chosen for the regulator. If possible this should have provision

for adding a capacitor across the internal reference for further noise reduction. Reference

should be made to the manufacturer’s datasheet.

The voltage regulators listed in Table 7-1 have been tested and found to work well with the

mXT540E-AT. They have compatible footprints and pin-out specifications, and are available in

the SOT-23 package.

Table 7-1.

Recommended Voltage Regulators

Manufacturer

Part Number

Linear Technology

Texas Instruments

LT1761ES5-BYP (variable)

TPS79301DBVR-Q1(variable)

TPS79328DBVR-Q1 (2.8 V)

TPS79330DBVR-Q1 (3.0 V)

TPS79333DBVR-Q1 (3.3 V)

Vdd only

AVdd and Vdd

Note some manufacturers claim that minimal or no capacitance is required for correct

regulator operation. However, in all cases, a minimum of a 1.0 µF ceramic, low ESR capacitor

at the input and output of these devices should be used. The manufacturers’ datasheets

should always be referred to when selecting capacitors for these devices and the typical

recommended values, types and dielectrics adhered to.

A.6 Analog I/O

In general, tracking for the analog I/O signals from the mXT540E-AT device should be kept as

short as possible. These normally go to a connector which interfaces directly to the

touchscreen.

Ensure that adequate ground-planes are used. An analog ground plane should be used in

addition to a digital one. Care should be taken to ensure that both ground planes are kept

separate and are connected together only at the point of entry for the power to the PCB. This

is usually at the input connector.

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9710FX–AT42–01/15

A.7 Component Placement

It is important to orient all devices so that the tracking for important signals (such as power and

clocks) are kept as short as possible. This simple point is often overlooked when initially

planning a PCB layout and can save hours of work at a later stage.

A.8 Digital Signals

In 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 that

there are no discontinuities in the ground return path.

A.9 EMC and Other Observations

The following recommendations are not mandatory, but may help in situations where

particularly 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 Considerations

If the two self-capacitance channels are to be used, the chip should be placed to minimize the

SNSK trace length to reduce low frequency pickup, and to reduce stray Cx which degrades

gain. The Cs and Rs resistors (see the schematics in Section 2.3 on page 9) should be placed

as close to the body of the chip as possible so that the trace between Rs and the SNSK pin is

very short. This reduces the antenna-like ability of this trace to pick up high frequency signals

and feed them directly into the chip.

Keep the electrode and its SNSK trace away from other signal, power, and ground traces. Any

adjacent 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 are

more than negated by the decreased gain from the circuit. Metal areas near the electrode will

also 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; for

example, when it is used as a proximity sensor. Sensitivity can often be increased by using a

larger electrode or reducing panel thickness. Increasing electrode size can have diminishing

returns, as high values of Cx will reduce sensor gain. Note that increasing the electrode's

surface area will not substantially increase touch sensitivity if its diameter is already much

larger 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 value

with the trade-off of slower response time and more power. Panel material can also be

changed to one having a higher dielectric constant, which will better help to propagate the

field.

41

9710FX–AT42–01/15

Appendix B. Reference Configuration

The values listed below are used in the reference unit to validate the interfaces and derive the

characterization 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 mXT540E-AT 2.1 Protocol Guide for information about the individual objects and their

fields.

The values for the user’s application will depend on the circumstances of that particular project

and will vary from those listed here. Further tuning will be required to achieve an optimal

performance..

Field

Value

Power Configuration T7 – GEN_POWERCONFIG_T7 (Instance 0)

IDLEACQINT

255

ACTVACQINT

Acquisition Configuration T8 – GEN_ACQUISITIONCONFIG_T8 (Instance 0)

CHRGTIME

TCHDRIFT

DRIFTST

75

20

1

ATCHCALSTHR

Multiple Touch Touchscreen T9 – TOUCH_MULTITOUCHSCREEN_T9 (Instance 0)

CTRL

143

XSIZE

24

32

192

50

16

10

YSIZE

BLEN

TCHTHR

NUMTOUCH

MRGHYST

MRGTHR

TCHHYST

CTE Configuration T46 – SPT_CTECONFIG_T46 (Instance 0)

IDLESYNCSPERX

16

ACTVSYNCSPERX

SlimSensor T56 – PROCI_SHIELDLESS_T56 (Instance 0)

CTRL

1

OPTINT

1

INTTIME

30

15

16

INTDELAY[0]

INTDELAY[1]

INTDELAY[2]

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Field

Value

16

15

16

15

16

15

16

24

1

INTDELAY[3]

INTDELAY[4]

INTDELAY[5]

INTDELAY[6]

INTDELAY[7]

INTDELAY[8]

INTDELAY[9]

INTDELAY[10]

INTDELAY[11]

INTDELAY[12]

INTDELAY[13]

INTDELAY[14]

INTDELAY[15]

INTDELAY[16]

INTDELAY[17]

INTDELAY[18]

INTDELAY[19]

INTDELAY[20]

INTDELAY[21]

INTDELAY[22]

INTDELAY[23]

NCNCL

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9710FX–AT42–01/15

Appendix C. Glossary of Terms

Channel

One of the capacitive measurement points at which the sensor controller can detect

capacitive change.

Jitter

The peak-to-peak variance in the reported location for an axis when a fixed touch is

applied. Typically jitter is random in nature and has a Gaussian ⁽¹⁾distribution, therefore

measurement of peak-to-peak jitter must be conducted over some period of time, typically

a 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 average

reported coordinate, the effects of linearity are ignored.

Linearity

The measurement of the peak-to-peak deviation of the reported touch coordinate in one

axis relative to the absolute position of touch on that axis. This is often referred to as the

nonlinearity. Nonlinearities in either X or Y axes manifest themselves as regions where the

perceived touch motion along that axis (alone) is not reflected correctly in the reported

coordinate giving the sense of moving too fast or too slow. Linearity is measured as a

percentage of the axis in question.

For each axis, a plot of the true coordinate versus the reported coordinate should be a

perfect straight line at 45°. A non linearity makes this plot deviate from this ideal line. It is

possible to correct modest nonlinearities using on-chip linearization tables, but this

correction trades linearity for resolution in regions where stronger corrections are needed

(because there is a stretching or compressing effect to correct the nonlinearity, so altering

the resolution in these regions). Linearity is typically measured using data that has been

sufficiently filtered to remove the effects of jitter. For example, a 100 mm slider with a

nonlinearity of 1 percent reports a position that is, at most, 1 mm away in either direction

from the true position.

Multitouch

The ability of a touchscreen to report multiple concurrent touches. The touches are reported as

separate sets of XY coordinates.

One-touch Gesture

A touch gesture that consists of a single touch. The combination of the duration of the touch

and 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 a

change in the reported coordinate for that axis. Resolution is normally expressed in bits

and 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 the

reported 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 XY

computation 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 two

touches in relation to each other characterizes a specific gesture. For example, a pinch gesture

is 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 Primer

D.1 Acquisition Technique

QMatrix capacitive acquisition uses a series of pulses to deposit charge into a sampling capacitor,

Cs. The pulses are driven on X lines from the controller. The rising edge of the pulse causes

current to flow in the mutual capacitance, Cx, formed between the X line and a neighboring receiver

electrode or Y line. While one X line is being pulsed, all others are grounded. This leads to excellent

isolation of the particular mutual capacitances being measured ⁽¹⁾, a feature that makes for good

inherent touchscreen performance.

After a fixed number of pulses (known as the burst length) the sampling capacitor's voltage is

measured to determine how much charge has accumulated. This charge is directly proportional to

Cx and therefore changes if Cx ⁽²⁾changes. The transmit-receive charge transfer process between

the X lines and Y lines causes an electric field to form that loops from X to Y. The field itself

emanates from X and terminates on Y. If the X and Y electrodes are fixed directly ⁽³⁾to a dielectric

material like plastic or glass, then this field tends to channel through the dielectric with very little

leakage of the field out into free-space (that is, above the panel). Some proportion of the field does

escape the surface of the dielectric, however, and so can be influenced during a touch.

When a finger is placed in close proximity (a few millimeters) or directly onto the dielectric's surface,

some of this stray field and some of the field that would otherwise have propagated via the dielectric

and terminated onto the Y electrode, is diverted into the finger and is conducted back to the

controller chip via the human body rather than via the Y line.

This means that less charge is accumulated in Cs, and hence the terminal voltage present on Cs,

after all the charge transfer pulses are complete, becomes less. In this way, the controller can

measure changes in Cx during touch. This means that the measured capacitance Cx goes down

during touch, because the coupled field is partly diverted by the touching object.

The spatial separation between the X and Y electrodes is significant to make the electric field to

propagate well in relation to the thickness of the dielectric panel.

D.2 Moisture Resistance

A useful side effect of the QMatrix acquisition method is that placing a floating conductive element

between the X and Y lines tends to increase the field coupling and so increases the capacitance

Cx. This is the opposite change direction to normal touch, and so can be quite easily ignored or

compensated for by the controller. An example of such floating conductive elements is the water

droplets caused by condensation.

As a result, QMatrix-based touchscreens tend not to go into false detect when they are covered in

small non-coalesced water droplets. Once the droplets start to merge, however, they can become

large enough to bridge the field across to nearby ground return paths (for example, other X lines

not currently driven, or ground paths in mechanical chassis components). When this happens, the

screen's behavior can become erratic.

1. A common problem with other types of capacitive acquisition technique when used for touchscreens, is that this

isolation is not so pronounced. This means that when touching one region of the screen, the capacitive signals also

tend to change slightly in nearby channels too, causing small but often significant errors in the reported touch

position.

2. To a first approximation.

3. Air gaps in front of QMatrix sensors massively reduce this field propagation and kill sensitivity. Normal optically clear

adhesives work well to attach QMatrix touchscreens to their dielectric front panel.

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mXT540E-AT

There are some measures used in these controllers to help with this situation, but in general there

comes a point where the screen is so contaminated by moisture that false detections become

inevitable. It should also be noted that uniform condensation soon becomes non-uniform once a

finger has spread it around. Finger grease renders the water highly conductive, making the situation

worse overall.

In general, QMatrix has industry-leading moisture tolerance but there comes a point when even the

best capacitive touchscreen suffers due to moisture on the dielectric surface.

D.3 Interference Sources

D.3.1

Power Supply

The device can tolerate short-term power supply fluctuations. If the power supply fluctuates

slowly with temperature, the device tracks and compensate for these changes automatically

with only minor changes in sensitivity. If the supply voltage drifts or shifts quickly, the drift

compensation mechanism is not able to keep up, causing sensitivity anomalies or false

detections.

The device itself uses the AVdd power supply as an analog reference, so the power should be

very clean and come from a separate regulator. A standard inexpensive Low Dropout (LDO)

type regulator should be used that is not also used to power other loads, such as LEDs,

relays, or other high current devices. Load shifts on the output of the LDO can cause AVdd to

fluctuate enough to cause false detection or sensitivity shifts. The digital Vdd supply is far

more tolerant to noise.

CAUTION: A regulator IC shared with other logic can result in erratic

operation and is not advised.

Noise on AVdd can appear directly in the measurement results. Vdd should be checked to

ensure that it stays within specification in terms of noise, across a whole range of product

operating conditions.

Ceramic bypass capacitors on AVdd and Vdd, placed very close (<5 mm) to the chip are

recommended. A bulk capacitor of at least 1 µF and a higher frequency capacitor of around

10 nF to 100 nF in parallel are recommended; both must be X7R or X5R dielectric capacitors.

D.3.2

Other Noise Sources

Refer to QTAN0079, Buttons, Sliders and Wheels Sensor Design Guide, for information

(downloadable from the Touch Technology area of the Atmel website).

47

9710FX–AT42–01/15

Appendix E. I²C Basics (I²C-compatible Operation)

7.17 Interface Bus

The device communicates with the host over an I²C-compatible bus. The following sections

give an overview of the bus; more detailed information is available from www.i2C-bus.org.

Devices are connected to the I²C-compatible bus as shown in Figure E-1. Both bus lines are

connected to Vdd via pull-up resistors. The bus drivers of all I²C-compatible devices must be

open-drain type. This implements a wired AND function that allows any and all devices to drive

the bus, one at a time. A low level on the bus is generated when a device outputs a zero.

Figure E-1. I²C-compatible Interface Bus

Vdd

Device 1

Device 2

Device 3

Device n

R1

R2

SDA

SCL

E.1 Transferring Data Bits

Each data bit transferred on the bus is accompanied by a pulse on the clock line. The level of

the data line must be stable when the clock line is high; the only exception to this rule is for

generating START and STOP conditions.

Figure E-2. Data Transfer

SDA

SCL

Data Stable

Data Change

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mXT540E-AT

E.2 START and STOP Conditions

The host initiates and terminates a data transmission. The transmission is initiated when the

host issues a START condition on the bus, and is terminated when the host issues a STOP

condition. Between the START and STOP conditions, the bus is considered busy. As shown in

Figure E-3, START and STOP conditions are signaled by changing the level of the SDA line

when the SCL line is high.

Figure E-3. START and STOP Conditions

SDA

SCL

START

STOP

E.3 Address Byte Format

All address bytes are 9 bits long, consisting of 7 address bits, one READ/WRITE control bit

and an acknowledge bit. If the READ/WRITE bit is set, a read operation is performed,

otherwise a write operation is performed. When the device recognizes that it is being

addressed, it will acknowledge by pulling SDA low in the ninth SCL (ACK) cycle. An address

byte consisting of a slave address and a READ or a WRITE bit is called SLA+R or SLA+W,

respectively.

The most significant bit of the address byte is transmitted first. The address sent by the host

must be consistent with that selected with the option jumpers.

Figure E-4. Address Byte Format

Addr MSB

Addr LSB

ACK

R/W

SDA

SCL

1

2

7

8

9

START

E.4 Data Byte Format

All data bytes are 9 bits long, consisting of 8 data bits and an acknowledge bit. During a data

transfer, the host generates the clock and the START and STOP conditions, while the receiver

is responsible for acknowledging the reception. An acknowledge (ACK) is signaled by the

receiver pulling the SDA line low during the ninth SCL cycle. If the receiver leaves the SDA

line high, a NACK is signaled.

49

9710FX–AT42–01/15

Figure E-5. Data Byte Format

Data LSB

Data MSB

ACK

Aggregate

SDA

SDA from

Transmitter

SDA from

Receiver

SCL from

Master

1

2

7

8

9

SLA+R/W

Data Byte

Stop or Next

Data Byte

E.5 Combining Address and Data Bytes into a Transmission

A transmission consists of a START condition, an SLA+R/W, one or more data bytes and a

STOP condition. The wired “ANDing” of the SCL line is used to implement handshaking

between the host and the device. The device extends the SCL low period by pulling the SCL

line low whenever it needs extra time for processing between the data transmissions.

Note: Each write or read cycle must end with a stop condition. The device may not respond

correctly if a cycle is terminated by a new start condition.

Figure E-6 shows a typical data transmission. Note that several data bytes can be transmitted

between the SLA+R/W and the STOP.

Figure E-6. Byte Transmission

Addr MSB

Addr LSB

Data MSB

Data LSB

ACK

R/W

SDA

SCL

1

2

7

8

9

1

2

7

8

9

START

SLA+RW

Data Byte

STOP

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mXT540E-AT

Table of Contents

Features ................................................................................................. 1

Overview of the mXT540E-AT ............................................................. 2

1

2

3

1.1

1.2

1.3

Introduction ...................................................................................................2

Understanding Unfamiliar Concepts ..............................................................2

Resources .....................................................................................................3

Pinout and Schematic .......................................................................... 4

2.1

2.2

2.3

Pinout Configuration ......................................................................................4

Pinout Descriptions .......................................................................................5

Schematics ....................................................................................................9

Touchscreen Basics .......................................................................... 10

3.1

3.2

3.3

3.4

Sensor Construction ....................................................................................10

Electrode Configuration ...............................................................................10

Scanning Sequence ....................................................................................11

Touchscreen Sensitivity ..............................................................................11

4

Detailed Operation ............................................................................. 12

4.1

Power-up/Reset ..........................................................................................12

Calibration ...................................................................................................14

Operational Modes ......................................................................................14

Touchscreen Layout ....................................................................................14

Signal Processing .......................................................................................15

GPIO Pins ...................................................................................................17

Self-capacitance Channels .........................................................................17

Circuit Components .....................................................................................17

PCB Layout .................................................................................................19

Debugging ...................................................................................................19

Communications .........................................................................................19

Configuring the Device ................................................................................19

4.2

4.3

4.4

4.5

4.6

4.7

4.8

4.9

4.10

4.11

4.12

5

I2C-compatible Communications ..................................................... 20

5.1

5.2

5.3

5.4

5.5

Communications Protocol ...........................................................................20

I2C-compatible Addresses ..........................................................................20

Writing To the Device ..................................................................................20

I²C-compatible Writes in Checksum Mode ..................................................21

Reading From the Device ...........................................................................21

51

9710FX–AT42–01/15

5.6

5.7

5.8

5.9

Reading Status Messages with DMA ..........................................................22

CHG Line ....................................................................................................24

SDA, SCL ....................................................................................................26

Clock Stretching ..........................................................................................26

6

7

Getting Started With the mXT540E-AT ............................................. 27

6.1

6.2

6.3

6.4

6.5

Establishing Contact ...................................................................................27

Using the Object Protocol ...........................................................................27

Writing to the Device ...................................................................................27

Reading from the Device .............................................................................27

Configuring the Device ................................................................................28

Specifications ..................................................................................... 29

7.1

Absolute Maximum Specifications ...............................................................29

Recommended Operating Conditions .........................................................29

DC Specifications ........................................................................................29

Supply Current ............................................................................................30

Power Consumption ....................................................................................31

Timing Specifications ..................................................................................32

I2C-compatible Specifications .....................................................................33

Touch Accuracy and Repeatability ..............................................................33

Power Supply Ripple and Noise ..................................................................33

ESD Information ..........................................................................................34

Thermal Packaging .....................................................................................34

Soldering Profile ..........................................................................................35

Mechanical Dimensions ..............................................................................36

Part Marking ................................................................................................37

Part Number ................................................................................................37

Moisture Sensitivity Level (MSL) .................................................................37

7.2

7.3

7.4

7.5

7.6

7.7

7.8

7.9

7.10

7.11

7.12

7.13

7.14

7.15

7.16

Appendix A

PCB Design Considerations ........................................ 38

Introduction .................................................................................................38

Printed Circuit Board ...................................................................................38

Supply Rails and Ground Tracking ..............................................................38

Power Supply Decoupling ...........................................................................38

Suggested Voltage Regulator Manufacturers .............................................39

Analog I/O ...................................................................................................39

Component Placement ................................................................................40

A.1

A.2

A.3

A.4

A.5

A.6

A.7

52

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mXT540E-AT

A.8

Digital Signals .............................................................................................40

EMC and Other Observations .....................................................................40

Self-capacitance Layout Considerations .....................................................40

A.9

A.10

Appendix B

Appendix C

Reference Configuration .............................................. 42

Glossary of Terms ........................................................ 44

Appendix D

QMatrix Primer .............................................................. 46

Acquisition Technique .................................................................................46

Moisture Resistance ....................................................................................46

Interference Sources ...................................................................................47

D.1

D.2

D.3

Appendix E

I2C Basics (I2C-compatible Operation) ...................... 48

Interface Bus ...............................................................................................48

Transferring Data Bits .................................................................................48

START and STOP Conditions .....................................................................49

Address Byte Format ..................................................................................49

Data Byte Format ........................................................................................49

Combining Address and Data Bytes into a Transmission ...........................50

7.17

E.1

E.2

E.3

E.4

E.5

Table of Contents................................................................................ 51

Revision History.................................................................................. 54

53

9710FX–AT42–01/15

Revision History

Revision Number

History

Revision AX – May 2012

Revision BX – June 2012

Revision CX – August 2012

Revision DX – October 2012

Revision EX – February 2013

Revision FX – January 2015

Initial release for firmware revision 2.1

Updated Specifications for firmware revision 2.1

Corrected Pin number 86

Updated with Characterization data

Minor correction to the analog and digital voltage ranges

Minor correction to part marking

54

mXT540E-AT

9710FX–AT42–01/15

mXT540E-AT

Notes

55

9710FX–AT42–01/15

Headquarters

International

Atmel Corporation

1600 Technology Drive

San Jose, CA 95110

USA

Tel: (+1) (408) 441-0311

Fax: (+1) (408) 436-4314

Atmel Asia

Atmel Munich GmbH

Business Campus

Parkring 4

D- 85748 Garching b.

MUNICH

Atmel Japan

16F Shin-Osaki Kangyo Building

1-6-4 Osaki

Shinagawa-ku, Tokyo 141-0032

JAPAN

Tel: (+81) (3) 6417-0300

Fax: (+81) (3) 6417-0370

Unit 01-05 & 16, 19F

BEA Tower, Millennium City 5

418 Kwun Tong Road

Kwun Tong

Kowloon

HONG KONG

Tel: (+49) 89-31970-111

Fax: (+49) 89-3194621

Tel: (+852) 2245-6100

Fax: (+852) 2722-1369

Touch Technology Division

1560 Parkway

Solent Business Park

Whiteley

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Hampshire

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UNITED KINGDOM

Tel: (+44) (0) 844 894 1920

Fax: (+44) (0) 1489 557 066

Product Contact

Web Site

Technical Support

Sales Contact

www.atmel.com

touch@atmel.com

www.atmel.com/contacts

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intellectual property right is granted by this document or in connection with the sale of Atmel products. EXCEPT AS SET FORTH IN ATMEL’S TERMS AND

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contained herein. Unless specifically provided otherwise, Atmel products are not suitable for, and shall not be used in, automotive applications. Atmel’s products are

not intended, authorized, or warranted for use as components in applications intended to support or sustain life.

QMatrix^®, QTouch^®, and others are registered trademarks, maXTouch^™and others are trademarks of Atmel Corporation or its subsidiaries.

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names may be registered trademarks or trademarks of others.

9710FX–AT42–01/15


型号：	ATMXT540E-AT
厂家：	MICROCHIP
描述：	Analog Circuit
文件：	总56页 (文件大小：1334K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

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