MXB7846EEE_技术文档

19-2436; Rev 1; 5/04

2.375V to 5.25V, 4-Wire Touch-Screen Controller

with Internal Reference and Temperature Sensor

General Description

Features

The MXB7846 is an industry-standard 4-wire touch-

screen controller. It contains a 12-bit sampling analog-

to-digital converter (ADC) with a synchronous serial

interface and low on-resistance switches for driving

resistive touch screens. The MXB7846 uses an internal

+2.5V reference or an external reference. The

MXB7846 can make absolute or ratiometric measure-

ments. In addition, this device has an on-chip tempera-

ture sensor, a battery-monitoring channel, and has the

ability to perform touch-pressure measurements without

external components. The MXB7846 has one auxiliary

ADC input. All analog inputs are fully ESD protected,

eliminating the need for external TransZorb™ devices.

♦ ESD-Protected ADC Inputs

±15kV IEC 61000-4-2 Air-Gap Discharge

±8kV IEC 61000-4-2 Contact Discharge

♦ Pin Compatible with MXB7843

♦ +2.375V to +5.25V Single Supply

♦ Internal +2.5V Reference

♦ Direct Battery Measurement (0 to 6V)

♦ On-Chip Temperature Measurement

♦ Touch-Pressure Measurement

♦ 4-Wire Touch-Screen Interface

♦ Ratiometric Conversion

The MXB7846 is guaranteed to operate with a supply

voltage down to +2.375V when used with an external

reference or +2.7V with an internal reference. In shut-

down mode, the typical power consumption is reduced

to under 0.5µW, while the typical power consumption at

125ksps throughput and a +2.7V supply is 650µW.

♦ SPI™/QSPI™, 3-Wire Serial Interface

♦ Programmable 8-/12-Bit Resolution

♦ Auxiliary Analog Input

♦ Automatic Shutdown Between Conversions

Low-power operation makes the MXB7846 ideal for bat-

tery-operated systems, such as personal digital assis-

tants with resistive touch screens and other portable

equipment. The MXB7846 is available in 16-pin QSOP

and TSSOP packages, and is guaranteed over the

-40°C to +85°C temperature range.

♦ Low Power (External Reference)

270µA at 125ksps

115µA at 50ksps

25µA at 10ksps

5µA at 1ksps

2µA Shutdown Current

Applications

Personal Digital Assistants

Portable Instruments

Point-of-Sales Terminals

Pagers

Ordering Information

PART

TEMP RANGE

-40°C to +85°C

PIN-PACKAGE

16 QSOP

MXB7846EEE

MXB7846EUE

16 TSSOP

Touch-Screen Monitors

Cellular Phones

Pin Configuration

Typical Application Circuit appears at end of data sheet.

TOP VIEW

V

DD

X+

Y+

X-

Y-

1

2

3

4

5

6

7

8

16 DCLK

15 CS

TransZorb is a trademark of Vishay Intertechnology, Inc.

SPI/QSPI are trademarks of Motorola, Inc.

14 DIN

MXB7846

13 BUSY

12 DOUT

11 PENIRQ

GND

BAT

AUX

10

9

V

DD

REF

QSOP/TSSOP

________________________________________________________________ Maxim Integrated Products

1

For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at

1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.

2.375V to 5.25V, 4-Wire Touch-Screen Controller

with Internal Reference and Temperature Sensor

ABSOLUTE MAXIMUM RATINGS

Continuous Power Dissipation (T = +70°C)

A

V

DD

, VBAT, DIN, CS, DCLK to GND ........................-0.3V to +6V

16-Pin QSOP (derate 8.30mW/°C above +70°C).........667mW

16-Pin TSSOP (derate 5.70mW/°C above +70°C) .......456mW

Operating Temperature Range ...........................-40°C to +85°C

Junction Temperature......................................................+150°C

Storage Temperature Range.............................-65°C to +150°C

Lead Temperature (soldering, 10s) .................................+300°C

Digital Outputs to GND...............................-0.3V to (V

+ 0.3V)

DD

V

, X+, X-, Y+, Y-, AUX to GND..............-0.3V to (V

REF

Maximum Current into Any Pin ......................................... 50mA

Maximum ESD per IEC-61000-4-2 (per MIL STD-883 HBM)

X+, X-, Y+, Y-, VBAT, AUX......................................15kV (4kV)

All Other Pins ..........................................................2kV (500V)

Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional

operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to

absolute maximum rating conditions for extended periods may affect device reliability.

ELECTRICAL CHARACTERISTICS

(V

= 2.7V to 3.6V, V

= 2.5V, f

= 2MHz (50% duty cycle), f

= 125kHz, 12-bit mode, 0.1µF capacitor at REF, T =

SAMPLE A

DD

REF

DCLK

T

MIN

to T

, unless otherwise noted. Typical values are at T = +25°C.)

MAX A

PARAMETER

SYMBOL

CONDITIONS

MIN

TYP

MAX

12

UNITS

DC ACCURACY (Note 1)

Resolution

Bits

LSB

No Missing Codes

Relative Accuracy

Differential Nonlinearity

Offset Error

11

12

1

INL

(Note 2)

(Note 3)

2

DNL

1

6

4

Gain Error

Noise

Including internal reference

70

µV

RMS

CONVERSION RATE

Conversion Time

Track/Hold Acquisition Time

Throughput Rate

Multiplexer Settling Time

Aperture Delay

t

12 clock cycles (Note 4)

3 clock cycles

6

µs

CONV

t

1.5

ACQ

f

16 clock conversion

125

kHz

ns

SAMPLE

500

30

ns

Aperture Jitter

100

ps

Channel-to-Channel Isolation

Serial Clock Frequency

Duty Cycle

V

= 2.5V

at 50kHz

dB

MHz

%

IN

P-P

f

0.1

40

2.0

60

DCLK

ANALOG INPUT (X+, X-, Y+, Y-, AUX)

Input Voltage Range

0

V

REF

1

Input Capacitance

25

pF

µA

Input Leakage Current

SWITCH DRIVERS

On/off leakage, V = 0 to V

IN

0.1

DD

Y+, X+

Y-, X-

7

9

On-Resistance (Note 5)

Ω

INTERNAL REFERENCE

Reference Output Voltage

V

= 2.7V to 5.25V, T = +25°C

2.45

2.50

50

2.55

V

REF

DD

A

REF Output Tempco

TCV

ppm°/C

mA

REF

REF Short-Circuit Current

REF Output Impedance

18

250

Ω

2

_______________________________________________________________________________________

2.375V to 5.25V, 4-Wire Touch-Screen Controller

with Internal Reference and Temperature Sensor

ELECTRICAL CHARACTERISTICS (continued)

(V

= 2.7V to 3.6V, V

= 2.5V, f

= 2MHz (50% duty cycle), f

= 125kHz, 12-bit mode, 0.1µF capacitor at REF, T =

SAMPLE A

DD

REF

DCLK

T

MIN

to T

, unless otherwise noted. Typical values are at T = +25°C.)

MAX A

PARAMETER

SYMBOL

CONDITIONS

MIN

TYP

MAX

UNITS

EXTERNAL REFERENCE (Internal reference disabled, reference applied to REF)

Reference Input Voltage Range

Input Resistance

(Note 7)

1

V

DD

1

GΩ

f

= 125kHz

= 12.5kHz

13

2.5

40

3

SAMPLE

µA

Input Current

= 0

DCLK

BATTERY MONITOR (BAT)

Input Voltage Range

Input Resistance

0

6

V

During acquisition

= 2.5V

10

kΩ

V

±2

±3

REF

Accuracy

%

Internal reference

TEMPERATURE MEASUREMENT

Resolution

Differential method (Note 8)

Single-conversion method

Differential method (Note 8)

Single-conversion method

1.6

0.3

±2

°C

Accuracy

±3

DIGITAL INPUTS (DCLK, CS, DIN)

Input High Voltage

♦

V

0.7

V

IH

DD

Input Low Voltage

V

0.8

1

IL

Input Hysteresis

V

100

15

mV

µA

pF

HYST

Input Leakage Current

Input Capacitance

I

IN

C

IN

DIGITAL OUTPUT (DOUT, BUSY)

Output Voltage Low

V

I

= 250µA

0.4

V

OL

SINK

Output Voltage High

V

= 250µA

V

- 0.5

DD

OH

SOURCE

PENIRQ Output Low Voltage

Three-State Leakage Current

Three-State Output Capacitance

POWER REQUIREMENTS

V

50kΩ pullup to V

0.8

10

V

OL

DD

I

CS = V

1

µA

pF

L

DD

C

15

OUT

External reference

2.375

2.70

5.250

5.25

650

Supply Voltage

V

DD

Internal reference

f

= 125ksps

= 12.5ksps

= 0

270

220

150

780

720

650

SAMPLE

External

reference

µA

f

Supply Current

I

= 125ksps

= 12.5ksps

= 0

950

3

Internal

reference

f

µA

f

Shutdown Supply Current

I

DCLK = CS = V

µA

dB

SHDN

DD

Power-Supply Rejection Ratio

P

V

= 2.7V to 3.6V full scale

DD

70

SRR

_______________________________________________________________________________________

3

2.375V to 5.25V, 4-Wire Touch-Screen Controller

with Internal Reference and Temperature Sensor

TIMING CHARACTERISTICS (Figure 1)

(V

= 2.7V to 3.6V, V

= 2.5V, f

= 2MHz (50% duty cycle), f

= 125kHz, 12-bit mode, 0.1µF capacitor at REF, T =

SAMPLE A

DD

REF

DCLK

T

MIN

to T

, unless otherwise noted. Typical values are at T = +25°C.)

MAX A

PARAMETER

SYMBOL

CONDITIONS

MIN

TYP

MAX

UNITS

TIMING CHARACTERISTICS (Figure 1)

Acquisition Time

t

1.5

500

200

100

0

µs

ns

ACQ

DCLK Clock Period

t

CP

CH

DCLK Pulse Width High

DCLK Pulse Width Low

DIN-to-DCLK Setup Time

DIN-to-DCLK Hold Time

CS Fall-to-DCLK Rise Setup Time

CS Rise-to-DCLK Rise Ignore

DCLK Falling-to-DOUT Valid

CS Rise-to-DOUT Disable

CS Fall-to-DOUT Enable

DCLK Falling-to-BUSY Rising

CS Falling-to-BUSY Enable

CS Rise-to-BUSY Disable

t

CL

DS

DH

t

100

0

CSS

CSH

t

C

= 50pF

200

DO

LOAD

t

TR

DV

BD

t

BDV

t

BTR

Note 1: Tested at V

= 2.7V.

DD

Note 2: Relative accuracy is the deviation of the analog value at any code from its theoretical value after the full-scale range has

been calibrated.

Note 3: Offset nulled.

Note 4: Conversion time is defined as the number of clock cycles multiplied by the clock period; clock has 50% duty cycle.

Note 5: Resistance measured from the source to drain of the switch.

Note 6: External load should not change during conversion for specified accuracy.

Note 7: ADC performance is limited by the conversion noise floor, typically 300µV . An external reference below 2.5V can com-

P-P

promise the ADC performance.

Note 8: Difference between Temp0 and Temp1. No calibration necessary.

4

_______________________________________________________________________________________

2.375V to 5.25V, 4-Wire Touch-Screen Controller

with Internal Reference and Temperature Sensor

Typical Operating Characteristics

(V

= 2.7V, V

= 2.5V

, f

= 2MHz, f

= 125kHz, C

= 50pF, 0.1µF capacitor at REF, T = +25°C, unless

DD

REF

EXTERNAL DCLK

SAMPLE

LOAD A

otherwise noted.)

INTEGRAL NONLINEARITY

vs. DIGITAL OUTPUT CODE

DIFFERENTIAL NONLINEARITY

vs. DIGITAL OUTPUT CODE

CHANGE IN OFFSET ERROR

vs. SUPPLY VOLTAGE

0.5

0.4

0.3

0.2

0.1

0

1.0

0.8

2.0

1.5

1.0

0.5

0

0.6

0.4

0.2

0

-0.2

-0.5

-1.0

-1.5

-2.0

-0.1

-0.2

-0.3

-0.4

-0.6

-0.8

-1.0

0

500 1000 1500 2000 2500 3000 3500 4000

OUTPUT CODE

500 1000 1500 2000 2500 3000 3500 4000

OUTPUT CODE

2.5

3.0

3.5

4.0

4.5

5.0

5.5

SUPPLY VOLTAGE (V)

CHANGE IN GAIN ERROR

vs. SUPPLY VOLTAGE

CHANGE IN GAIN ERROR

vs. TEMPERATURE

CHANGE IN OFFSET ERROR

vs. TEMPERATURE

1.0

0.5

1.0

0.5

0

3

2

0

1

-0.5

-1.0

-1.5

-2.0

0

-1

-2

-3

-0.5

-1.0

-40 -25 -10

5

20 35 50 65 80

-40 -25 -10

5

20 35 50 65 80

2.5

3.0

3.5

4.0

4.5

5.0

5.5

TEMPERATURE (°C)

SUPPLY VOLTAGE (V)

INTERNAL REFERENCE

vs. SUPPLY VOLTAGE

SWITCH ON-RESISTANCE vs. SUPPLY VOLTAGE

SWITCH ON-RESISTANCE vs. TEMPERATURE

(X+, Y+ : +V TO PIN; X-, Y- : PIN TO GND)

(X+, Y+ : +V TO PIN; X-, Y- : TO GND)

DD

2.6

2.5

2.4

2.3

2.2

2.1

2.0

14

12

10

8

12

C = 0.1µf

L

11

10

9

X-

Y-

X-

X+

8

7

Y+

Y-

X+

Y+

6

5

4

3

2

1

0

2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5

SUPPLY VOLTAGE (V)

2.5

3.0

3.5

4.0

4.5

5.0

5.5

-40 -25 -10

5

20 35 50 65 80

SUPPLY VOLTAGE (V)

TEMPERATURE (°C)

_______________________________________________________________________________________

5

2.375V to 5.25V, 4-Wire Touch-Screen Controller

with Internal Reference and Temperature Sensor

Typical Operating Characteristics (continued)

(V

= 2.7V, V

= 2.5V

, f

= 2MHz, f

= 125kHz, C

= 50pF, 0.1µF capacitor at REF, T = +25°C, unless

DD

REF

EXTERNAL DCLK

SAMPLE

LOAD A

otherwise noted.)

INTERNAL REFERENCE VOLTAGE

vs. TEMPERATURE

INTERNAL VOLTAGE REFERENCE

vs. TURN-ON TIME

INTERNAL VOLTAGE REFERENCE

vs. TURN-ON TIME

2.6

2.5

2.4

2.3

2.2

2.1

2.0

2.5

2.0

1.5

1.0

0.5

3.0

2.5

2.0

1.5

1.0

0.5

0

C = 1µF

V

= 2.7V

L

DD

NO CAPACITOR

(30µs) 12-BIT SETTLING

(1060µs) 12-BIT SETTLING

C = 0.1µF

L

0

-40 -25 -10

5

20 35 50 65 80

200 400 600 800 1000 1200

0

5

10 15 20 25 30 35 40

TEMPERATURE (°C)

TURN-ON TIME (µs)

REFERENCE CURRENT

vs. SUPPLY VOLTAGE

REFERENCE CURRENT vs. TEMPERATURE

REFERENCE CURRENT vs. SAMPLE RATE

8.3

8.2

8.1

8.0

7.9

7.8

7.7

8.3

8.2

8.1

8.0

7.9

7.8

7.7

10

9

8

7

6

5

4

3

2

1

0

EXTERNAL REFERENCE

C = 0.1µF

= 125kHz

EXTERNAL REFERENCE

L

f

SAMPLE

V

= 2.7V

DD

C = 0.1µF

L

f

= 125kHz

SAMPLE

EXTERNAL REFERENCE

2.5

3.0

3.5

4.0

4.5

5.0

5.5

-40 -25 -10

5

20 35 50 65 80

0

25

50

75

100

125

SUPPLY VOLTAGE (V)

TEMPERATURE (°C)

SAMPLE RATE (kHz)

TEMP1 DIODE VOLTAGE

vs. SUPPLY VOLTAGE

TEMP0 DIODE VOLTAGE

vs. SUPPLY VOLTAGE

TEMP DIODE VOLTAGE

vs. TEMPERATURE

590

589

588

587

586

585

705

704

703

702

701

700

699

698

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

TEMP1

TEMP0

TEMP1

TEMP2

2.7

3.2

3.7

4.2

4.7

5.2

2.7

3.2

3.7

4.2

4.7

5.2

-40 -25 -10

5

20 35 50 65 80

SUPPLY VOLTAGE (V)

TEMPERATURE (°C)

6

_______________________________________________________________________________________

2.375V to 5.25V, 4-Wire Touch-Screen Controller

with Internal Reference and Temperature Sensor

Typical Operating Characteristics (continued)

(V

= 2.7V, V

= 2.5V

, f

= 2MHz, f

= 125kHz, C

= 50pF, 0.1µF capacitor at REF, T = +25°C, unless

DD

REF

EXTERNAL DCLK

SAMPLE

LOAD A

otherwise noted.)

SUPPLY CURRENT

vs. SUPPLY VOLTAGE

SUPPLY CURRENT vs. TEMPERATURE

SUPPLY CURRENT vs. SAMPLE RATE

290

285

280

275

270

265

260

255

250

225

200

175

150

125

100

250

225

200

175

150

V

= 2.7V

= 2.5V

DD

REF

f

V

= 125kHz

= 2.7V

f

= 12.5kHz

SAMPLE

DD

SAMPLE

-40 -25 -10

5

20 35 50 65 80

0

25

50

75

100

125

2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5

SUPPLY VOLTAGE (V)

TEMPERATURE (°C)

SAMPLE RATE (kHz)

MAXIMUM SAMPLE RATE

vs. SUPPLY VOLTAGE

SHUTDOWN CURRENT

vs. SUPPLY VOLTAGE

SHUTDOWN CURRENT vs. TEMPERATURE

300

250

200

150

100

50

1000

100

10

120

110

100

90

DCLK = CS = V = 3V

DD

DCLK = CS = V

DD

80

70

60

1

50

2.7

3.2

3.7

4.2

4.7

5.2

2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5

SUPPLY VOLTAGE (V)

-40 -25 -10

5

20 35 50 65 80

SUPPLY VOLTAGE (V)

TEMPERATURE (°C)

_______________________________________________________________________________________

7

2.375V to 5.25V, 4-Wire Touch-Screen Controller

with Internal Reference and Temperature Sensor

Pin Description

1

NAME

FUNCTION

V

Positive Supply Voltage. Connect to pin 10.

X+ Position Input, ADC Input Channel 1

Y+ Position Input, ADC Input Channel 2

X- Position Input

DD

2

X+

3

Y+

X-

4

5

Y-

Y- Position Input

6

GND

BAT

AUX

Ground

7

Battery Monitoring Inputs; ADC Input Channel 3

Auxiliary Input to ADC; ADC Input Channel 4

8

Voltage Reference Output/Input. Reference voltage for analog-to-digital conversion. In internal

reference mode, the reference buffer provides a 2.50V nominal output. In external reference mode,

9

REF

apply a reference voltage between 1V and V . Bypass REF to GND with a 0.1µF capacitor.

DD

Positive Supply Voltage, +2.375V (2.70V) to +5.25V. External (internal) reference. Bypass with a 1µF

capacitor. Connect to pin 1.

10

11

12

V

DD

PENIRQ

Pen Interrupt Output. Open anode output. 10kΩ to 100kΩ pullup resistor required to V

.

DD

Serial Data Output. Data changes state on the falling edge of DCLK. High impedance when CS is

HIGH.

DOUT

Busy Output. BUSY pulses high for one clock period before the MSB decision. High impedance when

CS is HIGH.

13

14

15

BUSY

DIN

CS

Serial Data Input. Data clocked in on the rising edge of DCLK.

Active-Low Chip Select. Data is only clocked into DIN when CS is low. When CS is HIGH, DOUT and

BUSY are high impedance.

Serial Clock Input. Clocks data in and out of the serial interface and sets the conversion speed (duty

cycle must be 40% to 60%).

16

DCLK

CS

t

CH

t

CP

t

CSS

CSH

t

CL

DCLK

t

DO

t

DS

t

DH

DIN

t

TR

t

DV

DOUT

t

BDV

BTR

BUSY

t

BD

Figure 1. Detailed Serial Interface Timing

_______________________________________________________________________________________

8

2.375V to 5.25V, 4-Wire Touch-Screen Controller

with Internal Reference and Temperature Sensor

During the acquisition interval, the selected channel

Detailed Description

charges the sampling capacitance. The acquisition

The MXB7846 uses a successive-approximation conver-

interval starts on the fifth falling clock edge and ends

on the eighth falling clock edge.

sion technique to convert analog signals to a 12-bit digi-

tal output. An SPI/QSPI/MICROWIRE™-compatible serial

interface provides easy communication to a micro-

processor (µP). It features an internal 2.5V reference, an

on-chip temperature sensor, a battery monitor, and a

4-wire touch-screen interface (Functional Diagram).

The time required for the T/H to acquire an input signal

is a function of how quickly its input capacitance is

charged. If the input signal’s source impedance is high,

the acquisition time lengthens, and more time must be

allowed between conversions. The acquisition time

Analog Inputs

Figure 2 shows a block diagram of the analog input sec-

tion that includes the input multiplexer of the MXB7846,

the differential signal inputs of the ADC, and the differ-

ential reference inputs of the ADC. The input multiplexer

switches between X+, X-, Y+, Y-, AUX, BAT, and the

internal temperature sensor.

(t

) is the maximum time the device takes to acquire

ACQ

the input signal to 12-bit accuracy. Calculate t

the following equation:

with

ACQ

t

= 8.4 × R + R

× 25pF

(

)

ACQ

S

IN

where R = 2kΩ and R is the source impedance of

IN

S

the input signal.

In single-ended mode, conversions are performed using

REF as the reference. In differential mode, ratiometric

conversions are performed with REF+ connected to X+ or

Y+, and REF- connected to X- or Y-. Configure the refer-

ence and switching matrix according to Tables 1 and 2.

Source impedances below 1kΩ do not significantly affect

the ADC’s performance. Accommodate higher source

impedances by either slowing down DCLK or by placing

a 1µF capacitor between the analog input and GND.

+V

DD

V

REF

PENIRQ

TEMP1

TEMP0

MXB7846

A2–A0

(SHOWN 001 )

SER/DFR

(SHOWN HIGH)

B

X+

X-

REF ON/OFF

Y+

Y-

REF+

12-BIT ADC

-IN

+IN

REF-

2.5V

REFERENCE

7.5kΩ

V

BAT

2.5kΩ

BATTERY

ON

AUX

GND

Figure 2. Equivalent Input Circuit

MICROWIRE is a trademark of National Semiconductor Corp.

_______________________________________________________________________________________

9

2.375V to 5.25V, 4-Wire Touch-Screen Controller

with Internal Reference and Temperature Sensor

Functional Diagram

V

DD

PENIRQ

X+

X-

TEMPERATURE

SENSOR

DOUT

BUSY

Y+

Y-

PENIRQ

6-TO-1

MUX

SERIAL

DATA

INTERFACE

12-BIT ADC

DCLK

DIN

BATTERY

MONITOR

BAT

CS

AUX

2.5V

REFERENCE

REF

Table 1. Input Configuration, Single-Ended Reference Mode (SER/DFR HIGH)

A2

A1

A0

MEASUREMENT

Temp0

Y position

BAT

ADC INPUT CONNECTION

DRIVERS ON

0

Temp0

X+

—

Y+, Y-

—

0

1

0

1

0

BAT

X+

0

1

Z1

X-, Y+

X-, X+

—

1

0

Z2

Y-

1

0

1

X- position

AUX

Y+

1

0

AUX

Temp1

1

Temp1

—

Table 2. Input Configuration, Differential Reference Mode (SER/DFR LOW)

ADC +REF

ADC -REF

ADC INPUT

MEASUREMENT

PERFORMED

A2

A1

A0

DRIVER ON

CONNECTION TO CONNECTION TO CONNECTION TO

0

1

0

1

0

1

0

1

Y+

X+

Y-

X-

X+

Y-

Y position

Z1 position

Z2 position

X position

Y+, Y-

Y+, X-

X+, X-

Y+

10 ______________________________________________________________________________________

2.375V to 5.25V, 4-Wire Touch-Screen Controller

with Internal Reference and Temperature Sensor

resistive-divider created by the touch screen and the

on-resistance of the X and Y drivers result in both an

offset and a gain shift. Also, the on-resistance of the X

and Y drivers does not track the resistance of the touch

screen over temperature and supply. This results in fur-

ther measurement errors.

Input Bandwidth and Anti-Aliasing

The ADCs input tracking circuitry has a 25MHz small-

signal bandwidth, so it is possible to digitize high-

speed transient events. To avoid high-frequency sig-

nals being aliased into the frequency band of interest,

anti-alias filtering is recommended.

Differential Measurement Mode

Figure 4 shows the switching matrix configuration for

Y-coordinate measurement. The REF+ and REF- inputs

are connected directly to the Y+ and Y- pins, respec-

tively. Differential mode uses the voltage at the Y+ pin

as the REF+ voltage and voltage at the Y- pin as REF-

voltage. This conversion is ratiometric and independent

of the voltage drop across the drivers and variation in

the touch-screen resistance. In differential mode, the

touch screen remains biased during the acquisition and

conversion process. This results in additional supply

current and power dissipation during conversion when

compared to the absolute measurement mode.

Analog Input Protection

Internal protection diodes, which clamp the analog input

to V

and GND, allow the analog input pins to swing

DD

from GND - 0.3V to V

+ 0.3V without damage. Analog

DD

inputs must not exceed V

by more than 50mV or be

DD

lower than GND by more than 50mV for accurate con-

version. If an off-channel analog input voltage exceeds

the supplies, limit the input current to 50mA. The analog

input pins are ESD protected to 8kV using the Contact

Discharge method and 15kV using the Air-Gap

method specified in IEC 61000-4-2.

Touch-Screen Conversion

The MXB7846 provides two conversion methods—differ-

ential and single ended. The SER/DFR bit in the control

word selects either mode. A logic 1 selects a single-

ended conversion, while a logic 0 selects a differential

conversion.

PEN Interrupt Request (PENIRQ)

Figure 5 shows the block diagram for the PENIRQ func-

tion. When used, PENIRQ requires a 10kΩ to 100kΩ

pullup to +V . If enabled, PENIRQ goes low whenever

DD

the touch screen is touched. The PENIRQ output can

be used to initiate an interrupt to the microprocessor,

which can write a control word to the MXB7846 to start

a conversion.

Differential vs. Single Ended

Changes in operating conditions can degrade the accu-

racy and repeatability of touch-screen measurements.

Therefore, the conversion results representing X and Y

coordinates may be incorrect. For example, in single-

ended measurement mode, variation in the touch-screen

driver voltage drops results in incorrect input reading.

Differential mode minimizes these errors.

Figure 6 shows the timing diagram for the PENIRQ pin

function. The diagram shows that once the screen is

touched while CS is high, the PENIRQ output goes low

after a time period indicated by t

. The t

TOUCH

value changes for different touch-screen parasitic

capacitance and resistance. The microprocessor

receives this interrupt and pulls CS low to initiate a con-

version. At this instant, the PENIRQ pin should be

masked, as transitions can occur due to a selected

input channel or the conversion mode. The PENIRQ pin

functionality becomes valid when either the last data bit

is clocked out, or CS is pulled high.

Single-Ended Mode

Figure 3 shows the switching matrix configuration for

Y-coordinate measurement in single-ended mode. The

MXB7846 measures the position of the pointing device

by connecting X+ to IN+ of the ADC, enabling Y+ and

Y- drivers, and digitizing the voltage on X+. The ADC

performs a conversion with REF+ = REF and REF- =

GND. In single-ended measurement mode, the bias to

the touch screen can be turned off after the acquisition

to save power. The on-resistance of the X and Y drivers

results in a gain error in single-ended measurement

mode. Touch-screen resistance ranges from 200Ω to

900Ω (depending on the manufacturer), whereas the

on-resistance of the X and Y drivers is 8Ω (typ). Limit

the touch-screen current to less than 50mA by using a

touch screen with a resistance higher than 100Ω. The

Touch-Pressure Measurement

The MXB7846 provides two methods for measuring the

pressure applied to the touch screen (Figure 7). By

measuring R

, it is possible to differentiate

TOUCH

between a finger or stylus in contact with the touch

screen. Although 8-bit resolution is typically sufficient,

the following calculations use 12-bit resolution demon-

strating the maximum precision of the MXB7846.

______________________________________________________________________________________ 11

2.375V to 5.25V, 4-Wire Touch-Screen Controller

with Internal Reference and Temperature Sensor

V

DD

V

DD

Y+

X+

Y-

REF

Y+

X+

Y-

+IN

-IN

REF+

+IN

-IN

REF+

12-BIT ADC

REF-

GND

Figure 3. Single-Ended Y-Coordinate Measurement

Figure 4. Ratiometric Y-Coordinate Measurement

+V

DD

100kΩ

OPEN CIRCUIT

Y+

PENIRQ

TOUCH SCREEN

X+

Y-

ON

PENIRQ

ENABLE

Figure 5. PENIRQ Functional Block Diagram

12 ______________________________________________________________________________________

2.375V to 5.25V, 4-Wire Touch-Screen Controller

with Internal Reference and Temperature Sensor

SCREEN TOUCHED HERE

PENIRQ

CS

DCLK

DIN

1

2

3

4

5

6

7

8

1

2

3

12

13

14

15

16

S

A2

A1

A0

M

S/D PD1 PD0

INTERRUPT PROCESSOR

NO RESPONSE TO TOUCHMASK PENIRQ

PENIRQ ENABLED

t

TOUCH

Figure 6. PENIRQ Timing Diagram

The first method performs pressure measurements

using a known X-plate resistance. After completing

three conversions (X-position, Z1, and Z2), use the fol-

MEASURE X- POSITION

X+

Y+

lowing equation to calculate R

:

TOUCH





+

-

R





Z

TOUCH



X



POSITION

4096

2

R

= R

(

×



 ⁻¹



)









V

TOUCH

XPLATE



Z









1





X- POSITION

The second method requires knowing both the X-plate

and Y-plate resistance. Three conversions are required in

this method: the X-position, Y-position, and Z1-position.

X-

Y-

OPEN CIRCUIT

MEASURE Z1

Use the following equation to calculate R

TOUCH:

X+

Y+















R

X

4096

Z

1









XPLATE

POSITION

4096

+







 ⁻¹



R

=

×









TOUCH

R

TOUCH

Z











1





V

-





Y





POSITION

4096

⁻^R









×

YPLATE





X-

Y-

OPEN CIRCUIT

Internal Temperature Sensor

OPEN CIRCUIT

X+

The MXB7846 provides two temperature measurement

options: single-ended conversion and differential con-

version. Both temperature measurement techniques rely

on the semiconductor junction’s temperature character-

Y+

+

R

TOUCH

V

-

istics. The forward diode voltage (V ) vs. temperature

BE

is a well-defined characteristic. The ambient tempera-

ture can be calculated by knowing the value of V at a

BE

X-

Y-

fixed temperature and then monitoring the change in

that voltage as the temperature changes. The single

conversion method requires calibration at a known tem-

perature, but only needs a single reading to calculate

the ambient temperature. First, the PENIRQ diode for-

SENSE LINE

MEASURE Z2

Figure 7. Pressure Measurement Block Diagram

______________________________________________________________________________________ 13

2.375V to 5.25V, 4-Wire Touch-Screen Controller

with Internal Reference and Temperature Sensor

ward bias voltage is measured by the ADC with an

address of A2 = 0, A1 = 0, and A0 = 0 at a known tem-

perature. Subsequent diode measurements provide an

estimate of the ambient temperature through extrapola-

tion. This assumes a temperature coefficient of

-2.1mV/°C. The single conversion method results in a

resolution of 0.3°C/LSB and a typical accuracy of 3°C.

Battery Voltage Monitor

A dedicated analog input (BAT) allows the MXB7846 to

monitor the system battery voltage. Figure 8 shows the

battery voltage monitoring circuitry. The MXB7846 mon-

itors battery voltages from 0 to 6V. An internal resistor

network divides down V

by 4 so that a 6.0V battery

BAT

voltage results in 1.5V at the ADC input. To minimize

power consumption, the divider is only enabled during

The differential conversion method uses two measure-

ment points. The first measurement (Temp0) is per-

formed with a fixed bias current into the PENIRQ diode.

The second measurement (Temp1) is performed with a

fixed multiple of the original bias current with an

address of A2 = 1, A1 = 1, and A0 = 1. The voltage dif-

ference between the first and second conversion is

proportional to the absolute temperature and is

expressed by the following formula:

the sampling of V

.

BAT

Internal Reference

Enable the internal 2.5V reference by setting PD1 in the

control byte to a logic 1 (see Tables 3 and 4). The

MXB7846 uses the internal reference for single-ended

measurement mode, battery monitoring, temperature

measurement, and for measurement on the auxiliary

input. To minimize power consumption, disable the inter-

nal reference by setting PD1 to a logic 0 when performing

ratiometric position measurements. The internal 2.5V ref-

erence typically requires 10ms to settle (with no external

load). For optimum performance, connect a 0.1µF capac-

itor from REF to GND. This internal reference can be over-

driven with an external reference. For best performance,

the internal reference should be disabled when the exter-

nal reference is applied. The internal reference of the

MXB7846 must also be disabled to maintain compatibility

with the MXB7843. To disable the internal reference of the

MXB7846 after power-up, a control byte with PD1 = 0 is

required. (See Typical Operating Characteristics for

power-up time of the reference from power down.)





VREF

4096





T(°C)= 2.60 × (T1 − T0)

× 1000











 ^{− 273}





where T0 (Temp0) and T1 (Temp1) are the conversion

results.

This differential conversion method can provide much

improved absolute temperature measurement; however,

the resolution is reduced to 1.6°C/LSB.

External Reference

Although the internal reference may be overdriven with

an external reference, the internal reference should be

disabled (PD1 = 0) for best performance when using

an external reference. During conversion, an external

reference at REF must deliver up to 40µA DC load cur-

rent. If the reference has a higher output impedance or

is noisy, bypass it close to the REF pin with a 0.1µF and

a 4.7µF capacitor. Temperature measurements are

always performed using the internal reference.

DC/DC

CONVERTER

+2.375V TO +5.25V

BATTERY

0 TO 6.0V

V

DD

BAT

0 TO 1.5V

12-BIT ADC

7.5kΩ

Digital Interface

Initialization After Power-Up and Starting a

Conversion

2.5kΩ

The digital interface consists of three inputs, DIN, DCLK,

CS, and one output, DOUT. A logic-high on CS disables

the MXB7846 digital interface and places DOUT in a

high-impedance state. Pulling CS low enables the

MXB7846 digital interface.

BATTERY

MEASUREMENT ON

Figure 8. Battery Measurement Functional Block Diagram

14 ______________________________________________________________________________________

2.375V to 5.25V, 4-Wire Touch-Screen Controller

with Internal Reference and Temperature Sensor

Table 3. Control Byte Format

BIT 7

BIT 6

A2

BIT 5

A1

BIT 4

A0

BIT 3

BIT 2

BIT 1

PD1

BIT 0

PD0

START

MODE

SER/DFR

BIT

7

NAME

START

A2

DESCRIPTION

Start bit

Address (Tables 1 and 2)

6

5

A1

4

A0

3

MODE

SER/DFR

PD1

Conversion resolution: 1 = 8 bits, 0 = 12 bits

2

Conversion mode: 1 = single ended, 0 = differential

1

Power-down mode (Table 4)

0

PD0

Start a conversion by clocking a control byte into DIN

(Table 3) with CS low. Each rising edge on DCLK

clocks a bit from DIN into the MXB7846’s internal shift

register. After CS falls, the first arriving logic 1 bit

defines the control byte’s START bit. Until the START bit

arrives, any number of logic 0 bits can be clocked into

DIN with no effect.

Figure 9 shows the timing for this sequence. Byte RB2

and RB3 contain the result of the conversion, padded

with four trailing zeros. The total conversion time is a

function of the serial-clock frequency and the amount of

idle timing between 8-bit transfers.

Digital Output

The MXB7846 outputs data in straight binary format. Data

is clocked out on the falling edge of the DCLK MSB first.

The MXB7846 is compatible with SPI/QSPI/MICROWIRE

devices. For SPI, select the correct clock polarity and

sampling edge in the SPI control registers of the micro-

controller: set CPOL = 0 and CPHA = 0. MICROWIRE,

SPI, and QSPI all transmit a byte and receive a byte at

the same time. The simplest software interface requires

only three 8-bit transfers to perform a conversion (one 8-

bit transfer to configure the ADC, and two more 8-bit

transfers to read the conversion result; Figure 9).

Serial Clock

The external clock not only shifts data in and out, but it

also drives the analog-to-digital conversion steps.

BUSY pulses high for one clock period after the last bit

of the control byte. Successive-approximation bit deci-

sions are made and appear at DOUT on each of the

next 12 DCLK falling edges. BUSY and DOUT go into a

high-impedance state when CS goes high.

Simple Software Interface

Make sure the CPU’s serial interface runs in master

mode so the CPU generates the serial clock. Choose a

clock frequency from 500kHz to 2MHz:

The conversion must complete in 500µs or less; if not,

droop on the sample-and-hold capacitors can degrade

conversion results.

1) Set up the control byte and call it TB. TB should be

in the format: 1XXXXXXX binary, where X denotes

the particular channel, selected conversion mode,

and power mode (Tables 3, 4).

Data Framing

The falling edge of CS does not start a conversion. The

first logic high clocked into DIN is interpreted as a start

bit and defines the first bit of the control byte. A conver-

sion starts on DCLK’s falling edge, after the eighth bit of

the control byte is clocked into DIN.

2) Use a general-purpose I/O line on the CPU to pull

CS low.

The first logic 1 clocked into DIN after bit 6 of a conver-

sion in progress is clocked onto the DOUT pin and is

treated as a START bit (Figure 10).

3) Transmit TB and simultaneously receive a byte; call

it RB1.

4) Transmit a byte of all zeros ($00 hex) and simultane-

ously receive byte RB2.

Once a start bit has been recognized, the current con-

version must be completed.

5) Transmit a byte of all zeros ($00 hex) and simultane-

ously receive byte RB3.

6) Pull CS high.

______________________________________________________________________________________ 15

2.375V to 5.25V, 4-Wire Touch-Screen Controller

with Internal Reference and Temperature Sensor

Table 4. Power-Mode Selection

SUPPLY CURRENT (typ) (µA)

PD1

PD0

PENIRQ

STATUS

DURING

CONVERSION

200

AFTER

CONVERSION

1

0

1

0

1

0

1

Enabled

Disabled

ADC is ON during conversion, OFF between conversion

ADC is always ON, reference is always OFF

ADC is always OFF, reference is always ON

ADC is always ON, reference is always ON

200

400

600

200

400

600

CS

T

R

B3

B

B2

t

ACQ

DCLK

DIN

1

4

8

9

12

16

20

24

SER/

DFR

S

A2

IDLE

A1

A0 MODE

PD1 PD0

ACQUIRE

CONVERSION

IDLE

(START)

BUSY

DOUT

RB1

11

10

9

8

7

6

5

4

3

2

1

0

(MSB)

(LSB)

A/D STATE

IDLE

CONVERSION

IDLE

ACQUIRE

DRIVERS 1 AND 2

(SER/DFR HIGH)

OFF

ON

OFF

ON

DRIVERS 1 AND 2

(SER/DFR LOW)

OFF

Figure 9. Conversion Timing, 24-Clock per Conversion, 8-Bit Bus Interface

8-Bit Conversion

The fastest the MXB7846 can run with CS held continu-

The MXB7846 provides an 8-bit conversion mode

selected by setting the MODE bit in the control byte

high. In the 8-bit mode, conversions complete four

clock cycles earlier than in the 12-bit output mode,

resulting in 25% faster throughput. This can be used in

conjunction with serial interfaces that provide 12-bit

transfers, or two conversions could be accomplished

with three 8-bit transfers. Not only does this shorten each

conversion by 4 bits, but each conversion can also

occur at a faster clock rate since settling to better than 8

bits is all that is required. The clock rate can be as much

as 25% faster. The faster clock rate and fewer clock

cycles combine to increase the conversion rate.

ously low is 15 clock conversions. Figure 10 shows the

serial-interface timing necessary to perform a conver-

sion every 15 DCLK cycles. If CS is connected low and

DCLK is continuous, guarantee a start bit by first clock-

ing in 16 zeros.

Most microcontrollers (µCs) require that data transfers

occur in multiples of eight DCLK cycles; 16 clocks per

conversion is typically the fastest that a µC can drive the

MXB7846. Figure 11 shows the serial interface timing nec-

essary to perform a conversion every 16 DCLK cycles.

16 ______________________________________________________________________________________

2.375V to 5.25V, 4-Wire Touch-Screen Controller

with Internal Reference and Temperature Sensor

and Y- drivers are turned on, connecting one side of

the vertical resistive layer to V and the other side to

Data Format

The MXB7846 output data is in straight binary format as

shown in Figure 12. This figure shows the ideal output

code for the given input voltage and does not include

the effects of offset, gain, or noise.

DD

ground. In this case, the horizontal resistive layer func-

tions as a sense line. One side of this resistive layer

gets connected to the X+ input, while the other side is

left open or floating. The point where the touch screen

is pressed brings the two resistive layers in contact and

forms a voltage-divider at that point. The data converter

senses the voltage at the point of contact through the

X+ input and digitizes it. The horizontal layer resistance

does not introduce any error in the conversion because

no DC current is drawn.

Applications Information

Basic Operation of the MXB7846

The 4-wire touch-screen controller works by creating a

voltage gradient across the vertical or horizontal resis-

tive network connected to the MXB7846, as shown in

the Typical Application Circuit. The touch screen is

biased through internal MOSFET switches that connect

The conversion process of the analog input voltage to

digital output is controlled through the serial interface

between the A/D converter and the µP. The processor

controls the MXB7846 configuration through a control

byte (see Tables 3 and 4). Once the processor instructs

each resistive layer to V

and ground on an alternate

DD

basis. For example, to measure the Y position when a

pointing device presses on the touch screen, the Y+

CS

1

8

15

1

8

15

1

DCLK

DIN

S

CONTROL BYTE 0

S

CONTROL BYTE 1

S

CONTROL BYTE 2

B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0

CONVERSION RESULT 1

B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0

CONVERSION RESULT 0

DOUT

BUSY

Figure 10. 15-Clock/Conversion Timing

. . .

CS

1

8

16

1

8

16

DCLK

DIN

S

CONTROL BYTE 0

S

CONTROL BYTE 1

_B6. . .

. . .

B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0

CONVERSION RESULT 0

B11 B10 B9 B8 B7

CONVERSION RESULT 1

DOUT

BUSY

Figure 11. 16-Clock/Conversion Timing

______________________________________________________________________________________ 17

2.375V to 5.25V, 4-Wire Touch-Screen Controller

with Internal Reference and Temperature Sensor

the MXB7846 to initiate a conversion, the MXB7846

biases the touch screen through the internal switches at

the beginning of the acquisition period. The voltage

transient at the touch screen needs to settle down to a

stable voltage before the acquisition period is over.

After the acquisition period is over, the A/D converter

goes into a conversion period with all internal switches

turned off if the device is in single-ended mode. If the

device is in differential mode, the internal switches

remain on from the start of the acquisition period to the

end of the conversion period.

The power-up wait before conversion period is depen-

dent on the power-down state. When exiting software

low-power modes, conversion can start immediately

when running at decreased clock rates. Upon power-

on reset, the MXB7846 is in power-down mode with

PD1 = 0 and PD0 = 0. When exiting software shutdown,

the MXB7846 is ready to perform a conversion in 10µs

with an external reference. When using the internal ref-

erence, allow enough time for reference to settle to 12-

bit accuracy when exiting full power-down mode, as

shown in the Typical Operating Characteristics.

Power-On Reset

When power is first applied, internal power-on circuitry

resets the MXB7846. Allow 10µs for the first conversion

after the power supplies stabilize. If CS is low, the first

logic 1 on DIN is interpreted as a start bit. Until a con-

version takes place, DOUT shifts out zeros. On power-

up, allow time for the reference to stabilize.

PD1 = 1, PD0 = 1

In this mode, the MXB7846 is always powered up and

both the reference and the ADC are always on. The

device remains fully powered after the current conver-

sion completes.

PD1 = 0, PD0 = 0

In this mode, the MXB7846 powers down after the cur-

rent conversion completes or on the next rising edge of

CS, whichever occurs first. The next control byte

received on DIN powers up the MXB7846. At the start

of a new conversion, it instantly powers up. When each

conversion is finished, the part enters power-down

mode, unless otherwise indicated. The first conversion

after the ADC returns to full power is valid for differen-

tial conversions and single-ended measurement con-

versions when using an external reference.

Power Modes

Save power by placing the converter in one of two low-

current operating modes or in full power down between

conversions. Select the power-down mode through

PD1 and PD0 of the control byte (Tables 3 and 4).

The software power-down modes take effect after the

conversion is completed. The serial interface remains

active while waiting for a new control byte to start a con-

version and switches to full-power mode. After complet-

ing its conversion, the MXB7846 enters the programmed

power mode until a new control byte is received.

When operating at full speed and 16 clocks per conver-

sion, the difference in power consumption between

PD1 = 0, PD0 = 1, and PD1 = 0, PD0 = 0 is negligible.

Also, in the case where the conversion rate is

decreased by slowing the frequency of the DCLK input,

the power consumption between these two modes is

not very different. When the DCLK frequency is kept at

the maximum rate during a conversion, conversions are

done less often. There is a significant difference in

power consumption between these two modes.

OUTPUT CODE

FULL-SCALE

TRANSITION

11…111

11…110

11…101

FS = (V

- V )

REF-

REF+

PD1 = 1, PD0 = 0

In this mode, the MXB7846 is powered down. This

mode becomes active after the current conversion

completes or on the next rising edge of CS, whichever

occurs first. The next command byte received on the

DIN returns the MXB7846 to full power. The first conver-

sion after the ADC returns to full power is valid.

(V

- V

)

REF+

REF-

1LSB =

4096

00…011

00…010

00…001

PD1 = 0, PD0 = 1

This mode turns the internal reference off and leaves

the ADC on to perform conversions using an external

reference.

00…000

0

1

2

3

FS

FS-3/2LSB

INPUT VOLTAGE (LSB) = [(V ) - (V )]

+IN -IN

Figure 12. Ideal Input Voltages and Output Codes

18 ______________________________________________________________________________________

2.375V to 5.25V, 4-Wire Touch-Screen Controller

with Internal Reference and Temperature Sensor

Hardware Power-Down

CS also places the MXB7846 into power-down. When

I/O

SCK

CS

CS goes HIGH, the MXB7846 immediately powers

down and aborts the current conversion. The internal

reference does not turn off when CS goes high. To dis-

able the internal reference, an additional command

byte is required before CS goes high (PD1 = 0). Set

PD1 = 0 before CS goes high.

DCLK

DOUT

MISO

MICROWIRE

MXB7846

MOSI

DIN

MASKABLE

INTERRUPT

BUSY

Touch-Screen Settling

There are two key touch-screen characteristics that can

degrade accuracy. First, the parasitic capacitance

between the top and bottom layers of the touch screen

can result in electrical ringing. Second, vibration of the

top layer of the touch screen can cause mechanical

contact bouncing.

Figure 13. MICROWIRE Interface

I/O

SCK

CS

External filter capacitors may be required across the

touch screen to filter noise induced by the LCD panel

or backlight circuitry, etc. These capacitors lengthen

the settling time required when the panel is touched

and can result in a gain error, as the input signal may

not settle to its final steady-state value before the ADC

samples the inputs. Two methods to minimize or elimi-

nate this issue are described below.

DCLK

DOUT

MISO

QSPI/SPI

MXB7846

MOSI

DIN

MASKABLE

INTERRUPT

BUSY

One option is to lengthen the acquisition time by stopping

or slowing down DCLK, allowing for the required touch-

screen settling time. This method solves the settling time

problem for both single-ended and differential modes.

Figure 14. QSPI/SPI Interface

The second option is to operate the MXB7846 in the dif-

ferential mode only for the touch screen, and perform

additional conversions with the same address until the

input signal settles. The MXB7846 can then be placed

in the power-down state on the last measurement.

XF

CLKX

CLKR

CS

SCLK

TMS320LC3x

MXB7846

DIN

DX

DR

Connection to Standard Interface

DOUT

BUSY

MICROWIRE Interface

When using the MICROWIRE- (Figure 13) or SPI-com-

patible interface (Figure 14), set the CPOL = CPHA = 0.

Two consecutive 8-bit readings are necessary to obtain

the entire 12-bit result from the ADC. DOUT data transi-

tions occur on the serial clock’s falling edge and are

clocked into the µP on the DCLK’s rising edge. The first

8-bit data stream contains the first 8 bits of the current

conversion, starting with the MSB. The second 8-bit

data stream contains the remaining 4 result bits fol-

lowed by 4 trailing zeros. DOUT then goes high imped-

ance when CS goes high.

FSR

Figure 15. TMS320 Serial Interface

TMS320LC3x Interface

Figure 15 shows an example circuit to interface the

MXB7846 to the TMS320. The timing diagram for this

interface circuit is shown in Figure 16.

Use the following steps to initiate a conversion in the

MXB7846 and to read the results:

1) The TMS320 should be configured with CLKX (trans-

mit clock) as an active-high output clock and CLKR

(TMS320 receive clock) as an active-high input

clock. CLKX and CLKR on the TMS320 are connect-

ed to the MXB7846 DCLK input.

QSPI/SPI Interface

The MXB7846 can be used with the QSPI/SPI interface

using the circuit in Figure 14 with CPOL = 0 and CPHA

= 0. This interface can be programmed to do a conver-

sion on any analog input of the MXB7846.

______________________________________________________________________________________ 19

2.375V to 5.25V, 4-Wire Touch-Screen Controller

with Internal Reference and Temperature Sensor

CS

DCLK

DIN

START

A2

A1

A0

MODE

SER/DEF

PD1

PD0

BUSY

DOUT

HIGH IMPEDANCE

MSB

B10

B1

B0

Figure 16. MXB7846-to-TMS320 Serial Interface Timing Diagram

Power-supply decoupling is also crucial for optimal

device performance. A good way to decouple analog

supplies is to place a 10µF tantalum capacitor in paral-

lel with a 0.1µF capacitor bypassed to GND. To maxi-

mize performance, place these capacitors as close as

possible to the supply pin of the device. Minimize

capacitor lead length for best supply-noise rejection. If

the supply is very noisy, a 10Ω resistor can be connect-

ed in series as a lowpass filter.

2) The MXB7846’s CS pin is driven low by the TMS320’s

XF I/O port to enable data to be clocked into the

MXB7846’s DIN pin.

3) An 8-bit word (1XXXXXXX) should be written to the

MXB7846 to initiate a conversion and place the

device into normal operating mode. See Table 3 to

select the proper XXXXXXX bit values for your spe-

cific applications.

4) The MXB7846’s BUSY output is monitored through

the TMS320’s FSR input. A falling edge on the BUSY

output indicates that the conversion is in progress

and data is ready to be received from the device.

While using the MXB7846, the interconnection between

the converter and the touch screen should be as short

as possible. Since touch screens have low resistance,

longer or loose connections may introduce error. Noise

can also be a major source of error in touch-screen

applications (e.g., applications that require a backlight

LCD panel). EMI noise coupled through the LCD panel

to the touch screen may cause flickering of the convert-

ed data. Utilizing a touch screen with a bottom-side

metal layer connected to ground decouples the noise

to ground. In addition, the filter capacitors from Y+, Y-,

X+, and X- inputs to ground also help further reduce

the noise. Caution should be observed for settling time

of the touch screen, especially operating in the single-

ended measurement mode and at high data rates.

5) The TMS320 reads in 1 data bit on each of the next

16 rising edges of DCLK. These bits represent the

12-bit conversion result followed by 4 trailing bits.

6) Pull CS high to disable the MXB7846 until the next

conversion is initiated.

Layout, Grounding, and Bypassing

For best performance, use printed circuit (PC) boards

with good layouts; wire-wrap boards are not recommend-

ed. Board layout should ensure that digital and analog

signal lines are separated from each other. Do not run

analog and digital (especially clock) lines parallel to one

another, or digital lines underneath the ADC package.

Definitions

Establish a single-point analog ground (star ground

point) at GND. Connect all analog grounds to the star

ground. Connect the digital system ground to the star

ground at this point only. For lowest noise operation,

minimize the length of the ground return to the star

ground’s power supply.

Integral Nonlinearity

Integral nonlinearity (INL) is the deviation of the values

on an actual transfer function from a straight line. This

straight line can be either a best-straight-line fit or a line

drawn between the endpoints of the transfer function,

once offset and gain errors have been nullified. The

static linearity parameters for the MXB7846 are mea-

sured using the end-point method.

20 ______________________________________________________________________________________

2.375V to 5.25V, 4-Wire Touch-Screen Controller

with Internal Reference and Temperature Sensor

Differential Nonlinearity

Differential nonlinearity (DNL) is the difference between

an actual step width and the ideal value of 1LSB. A

DNL error specification of less than 1LSB guarantees

no missing codes and a monotonic transfer function.

Aperture Delay

Aperture delay (t ) is the time defined between the

AD

falling edge of the sampling clock and the instant when

an actual sample is taken.

Chip Information

TRANSISTOR COUNT: 12,000

Aperture Jitter

Aperture jitter (t ) is the sample-to-sample variation in

AJ

PROCESS: 0.6µm BiCMOS

the time between the samples.

Typical Application Circuit

2.375V TO 5.5V

1µF TO 10µF

0.1µF

OPTIONAL

1

2

3

4

+V

X+

Y+

X-

Y-

DCLK 16

CS 15

SERIAL/CONVERSION CLOCK

CHIP SELECT

DD

SERIAL DATA IN

DIN 14

CONVERTER STATUS

SERIAL DATA OUT

PEN INTERRUPT

MXB7846 BUSY 13

DOUT 12

TOUCH

5

6

7

8

SCREEN

TO BATTERY

GND

BAT

AUX

PENIRQ 11

+V

10

9

DD

50kΩ

AUXILIARY

INPUT

REF

0.1µF

VOLTAGE

REGULATOR

______________________________________________________________________________________ 21

2.375V to 5.25V, 4-Wire Touch-Screen Controller

with Internal Reference and Temperature Sensor

Package Information

(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information,

go to www.maxim-ic.com/packages.)

PACKAGE OUTLINE, QSOP .150", .025" LEAD PITCH

1

21-0055

E

1

22 ______________________________________________________________________________________

2.375V to 5.25V, 4-Wire Touch-Screen Controller

with Internal Reference and Temperature Sensor

Package Information (continued)

(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information,

go to www.maxim-ic.com/packages.)

Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are

implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.

Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 23

Printed USA

is a registered trademark of Maxim Integrated Products.


型号：	MXB7846EEE
厂家：	MAXIM INTEGRATED PRODUCTS
描述：	2.375V to 5.25V, 4-Wire Touch-Screen Controller with Internal Reference and Temperature Sensor 2.375V至5.25V ， 4线触摸屏控制器，内置电压基准及温度传感器转换器模数转换器传感器温度传感器光电二极管信息通信管理控制器
文件：	总23页 (文件大小：964K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

MXB7846EEE [MAXIM]

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