Datasheet MXB7846 Datasheet (MAXIM)

MXB7846

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

with Internal Reference and Temperature Sensor

________________________________________________________________

Maxim Integrated Products

1

Pin Configuration

19-2436; Rev 2; 1/08

For pricing delivery, and ordering information please contact Maxim Direct at 1-888-629-4642,
or visit Maxim’s website at www.maxim-ic.com.

General Description

The MXB7846 is an industry-standard 4-wire touch­screen controller. It contains a 12-bit sampling analog­to-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 measure­ments. In addition, this device has an on-chip tempera­ture 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.

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 shut­down 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.

Low-power operation makes the MXB7846 ideal for bat­tery-operated systems, such as personal digital assis­tants 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.

Applications

Personal Digital Assistants

Portable Instruments

Point-of-Sales Terminals

Pagers

Touch-Screen Monitors

Cellular Phones

Features

♦ 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
♦ SPI™/QSPI™, 3-Wire Serial Interface
♦ Programmable 8-/12-Bit Resolution
♦ Auxiliary Analog Input
♦ Automatic Shutdown Between Conversions
♦ Low Power (External Reference)

270µA at 125ksps
115µA at 50ksps
25µA at 10ksps
5µA at 1ksps
2µA Shutdown Current

Ordering Information

TRANSZORB is a trademark of Vishay Intertechnology, Inc.

SPI/QSPI are trademarks of Motorola, Inc.

Typical Application Circuit appears at end of data sheet.

PART TEMP RANGE

MXB7846EEE -40°C to +85°C 16 QSOP E16-6

MXB7846EUE -40°C to +85°C 16 TSSOP U16-1

PIN­PACKAGE

PKG

CODE

TOP VIEW

V

GND

BAT

AUX

DD

1

X+

2

Y+

3

4

X-

Y-

5

6

7

8

16

DCLK

15

CS

14

DIN

MXB7846

QSOP/TSSOP

13

BUSY

12

DOUT

PENIRQ

11

10

V

DD

REF

9

MXB7846

2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor

2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

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.

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

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

DD

+ 0.3V)

V

REF

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

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)

Continuous Power Dissipation (T

A

= +70°C)

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

ELECTRICAL CHARACTERISTICS

(VDD= 2.7V to 3.6V, V

REF

= 2.5V, f

DCLK

= 2MHz (50% duty cycle), f

SAMPLE

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

T

MIN

to T

MAX

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

DC ACCURACY (Note 1)

Resolution 12 Bits

No Missing Codes 11 12 Bits

Relative Accuracy INL (Note 2) ±1 ±2 LSB

Differential Nonlinearity DNL ±1 LSB

Offset Error ±6 LSB

Gain Error (Note 3) ±4 LSB

Noise Including internal reference 70 µV

CONVERSION RATE

Conversion Time t
Track/Hold Acquisition Time t

Throughput Rate f

Multiplexer Settling Time 500 ns

Aperture Delay 30 ns

Aperture Jitter 100 p s

Channel-to-Channel Isolation VIN = 2.5V

Serial Clock Frequency f

Duty Cycle 40 60 %

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

Input Voltage Range 0V

Input Capacitance 25 pF

Input Leakage Current On/off leakage, VIN = 0 to V

SWITCH DRIVERS

On-Resistance (Note 5)

INTERNAL REFERENCE

Reference Output Voltage V

REF Output Tempco TCV

REF Short-Circuit Current 18 mA

REF Output Impedance 250 Ω

PARAMETER SYM B O L CONDITIONS MIN TYP MAX UNITS

CONV

ACQ

SAMPLE

DCLK

REF

12 clock cycles (Note 4) 6 µs

3 clock cycles 1.5 µs

16 clock conversion 125 kHz

Y+, X+ 7

Y-, X- 9

VDD = 2.7V to 5.25V, TA = +25°C 2.45 2.50 2.55 V

REF

at 50kHz 100 dB

P-P

DD

0.1 2.0 MHz

REF

±0.1 ±1 µA

50 ppm°/C

RMS

V

Ω

MXB7846

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

with Internal Reference and Temperature Sensor

_______________________________________________________________________________________ 3

ELECTRICAL CHARACTERISTICS (continued)

(VDD= 2.7V to 3.6V, V

REF

= 2.5V, f

DCLK

= 2MHz (50% duty cycle), f

SAMPLE

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

T

MIN

to T

MAX

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

PARAMETER SYM B O L CONDITIONS MIN TYP MAX UNITS

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

Reference Input Voltage Range (Note 7) 1 V

DD

V

Input Resistance 1GΩ

Input Current

f

f

f

= 125kHz 13 40

SAMPLE

= 12.5kHz 2.5

SAMPLE

= 0 ±3

DCLK

µA

BATTERY MONITOR (BAT)

Input Voltage Range 06V

Input Resistance During acquisition 10 kΩ

V

= 2.5V ±2

Accuracy

REF

Internal reference ±3

%

TEMPERATURE MEASUREMENT

Resolution

Accuracy

Differential method (Note 8) 1.6 °C

Single-conversion method 0.3 °C

Differential method (Note 8) ±2°C

Single-conversion method ±3°C

DIGITAL INPUTS (DCLK, CS, DIN)

Input High Voltage V

Input Low Voltage V

Input Hysteresis V

Input Leakage Current I

Input Capacitance C

IH

IL

HYST

IN

IN

✕

V

0.7 V

DD

100 mV

15 pF

0.8 V

±1 µA

DIGITAL OUTPUT (DOUT, BUSY)

Output Voltage Low V

Output Voltage High V
PENIRQ Output Low Voltage V

Three-State Leakage Current I

Three-State Output Capacitance C

OUT

OL

OH

OL

L

I

= 250µA 0.4 V

SINK

I

50kΩ pullup to V

CS = V
CS = V

= 250µA V

SOURCE

DD

DD

DD

0.5 V

DD -

1 ±10 µA

15 pF

0.8 V

POWER REQUIREMENTS

Supply Voltage V

Supply Current I

Shutdown Supply Current I

Power-Supply Rejection Ratio P

DD

DD

SHDN

SRR

External reference 2.375 5.250

Internal reference 2.70 5.25

External
reference

Internal
reference

DCLK = CS = V

f

f

f

f

f

f

DD

= 125ksps 270 650

SAMPLE

= 12.5ksps 220

SAMPLE

= 0 150

SAMPLE

= 125ksps 780 950

SAMPLE

= 12.5ksps 720

SAMPLE

= 0 650

SAMPLE

3µA

VDD = 2.7V to 3.6V full scale 70 dB

V

µA

µA

MXB7846

2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor

4 _______________________________________________________________________________________

Note 1: Tested at VDD= 2.7V.
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

P-P

. An external reference below 2.5V can com-

promise the ADC performance.

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

TIMING CHARACTERISTICS (Figure 1)

(VDD= 2.7V to 3.6V, V

REF

= 2.5V, f

DCLK

= 2MHz (50% duty cycle), f

SAMPLE

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

T

MIN

to T

MAX

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

TIMING CHARACTERISTICS (Figure 1)

Acquisition Time t

DCLK Clock Period t

DCLK Pulse Width High t

DCLK Pulse Width Low t

DIN-to-DCLK Setup Time t

DIN-to-DCLK Hold Time t

CS Fall-to-DCLK Rise Setup Time t
CS Rise-to-DCLK Rise Ignore t

DCLK Falling-to-DOUT Valid t

CS Rise-to-DOUT Disable t
CS Fall-to-DOUT Enable t

DCLK Falling-to-BUSY Rising t

CS Falling-to-BUSY Enable t
CS Rise-to-BUSY Disable t

PARAMETER SYM B O L CONDITIONS MIN TYP MAX UNITS

ACQ

CP

CH

CL

DS

DH

CSS

CSH

DO

TR

DV

BD

BDV

BTR

C

C

C

LOAD

LOAD

LOAD

= 50pF 200 ns

= 50pF 200 ns

= 50pF 200 ns

1.5 µs

500 ns

200 ns

200 ns

100 ns

0ns

100 ns

0ns

200 ns

200 ns

200 ns

Typical Operating Characteristics

(VDD= 2.7V, V

REF

= 2.5V

EXTERNAL

, f

DCLK

= 2MHz, f

SAMPLE

= 125kHz, C

LOAD

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

otherwise noted.)

MXB7846

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

with Internal Reference and Temperature Sensor

_______________________________________________________________________________________

5

INTEGRAL NONLINEARITY

vs. DIGITAL OUTPUT CODE

0.5

0.4

0.3

0.2

0.1

0

INL (LSB)

-0.1

-0.2

-0.3

-0.4
0 4000

CHANGE IN OFFSET ERROR

1.0

0.5

OUTPUT CODE

vs. TEMPERATURE

350030002000 25001000 1500500

1.0

0.8

MXB7846 toc01

0.6

0.4

0.2

DNL (LSB)

-0.2

-0.4

-0.6

-0.8

-1.0

MXB7846 toc05

DIFFERENTIAL NONLINEARITY

vs. DIGITAL OUTPUT CODE

0

0 4000

OUTPUT CODE

CHANGE IN GAIN ERROR

vs. SUPPLY VOLTAGE

3

2

1

350030002000 25001000 1500500

MXB7846 toc02

OFFSET ERROR (LSB)

MXB7846 toc07

CHANGE IN OFFSET ERROR

vs. SUPPLY VOLTAGE

2.0

1.5

1.0

0.5

0

-0.5

-1.0

-1.5

-2.0

2.5 5.5
SUPPLY VOLTAGE (V)

CHANGE IN GAIN ERROR

vs. TEMPERATURE

1.0

0.5

0

MXB7846 toc04

5.04.53.0 3.5 4.0

MXB7846 toc08

0

-0.5

OFFSET ERROR FROM +25°C (LSB)

-1.0

-40 80

655035205-10-25

°

SWITCH ON-RESISTANCE vs. SUPPLY VOLTAGE

(X+, Y+ : +V

14

12

10

8

(Ω)

ON

R

6

4

2

0

2.5 5.5

TO PIN; X-, Y- : TO GND)

DD

X-

Y-

X+

Y+

SUPPLY VOLTAGE (V)

5.04.54.03.53.0

GAIN ERROR (LSB)

MXB7846 toc03

(Ω)

ON

R

0

-1

-2

-3

2.5 5.5
SUPPLY VOLTAGE (V)

5.04.54.03.53.0

SWITCH ON-RESISTANCE vs. TEMPERATURE

(X+, Y+ : +V

12
11
10

9
8
7

6
5

4
3

2
1
0

-40 50 65 80

TO PIN; X-, Y- : PIN TO GND)

DD

X-

X+

Y-

Y+

35205-10-25

TEMPERATURE (°C)

-0.5

-1.0

GAIN ERROR FROM +25°C (LSB)

-1.5

-2.0

2.6

2.5

MXB7846 toc06

2.4

2.3

2.2

INTERNAL REFERENCE (V)

2.1

2.0

-40 80

655035205-10-25

°

INTERNAL REFERENCE

vs. SUPPLY VOLTAGE

CL = 0.1μf

2.0 5.5
SUPPLY VOLTAGE (V)

5.04.54.03.53.02.5

MXB7846 toc09

Typical Operating Characteristics (continued)

(VDD= 2.7V, V

REF

= 2.5V

EXTERNAL

, f

DCLK

= 2MHz, f

SAMPLE

= 125kHz, C

LOAD

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

otherwise noted.)

MXB7846

2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor

6 _______________________________________________________________________________________

2.6

INTERNAL REFERENCE VOLTAGE

vs. TEMPERATURE

2.5

INTERNAL VOLTAGE REFERENCE

vs. TURN-ON TIME

INTERNAL VOLTAGE REFERENCE

vs. TURN-ON TIME

3.0

2.5

2.4

2.3

2.2

INTERNAL REFERENCE VOLTAGE (V)

2.1
VDD = 2.7V

= 0.1μF

C

L

2.0

-40

TEMPERATURE (°C)

REFERENCE CURRENT

vs. SUPPLY VOLTAGE

8.3

CL = 0.1μF

= 125kHz

f

SAMPLE

8.2

EXTERNAL REFERENCE

8.1

8.0

7.9

REFERENCE CURRENT (μA)

7.8

7.7

2.5 5.5
SUPPLY VOLTAGE (V)

TEMP DIODE VOLTAGE

vs. TEMPERATURE

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

TEMP DIODE VOLTAGE (V)

0.2

0.1

0

-40

TEMP2

TEMPERATURE (°C)

TEMP1

MXB7846 toc10

2.0

1.5

1.0

0.5

INTERNAL VOLTAGE REFERENCE (V)

80655035205-10-25

0

0

(1060μs) 12-BIT SETTLING

TURN-ON TIME (μs)

CL = 1μF

12001000800600400200

REFERENCE CURRENT vs. TEMPERATURE

8.3

8.2

MXB7846 toc12

8.1

8.0

7.9

REFERENCE CURRENT (μA)

VDD = 2.7V

= 0.1μF

C

L

7.8

5.04.54.03.53.0

7.7

= 125kHz

f

SAMPLE

EXTERNAL REFERENCE

-40 80

TEMPERATURE (°C)

655035205-10-25

TEMP0 DIODE VOLTAGE

vs. SUPPLY VOLTAGE

590

MXB7846 toc15

589

TEMP0

588

587

TEMP0 DIODE VOLTAGE (mV)

586

585

806535 50-10 5 20-25

2.7
SUPPLY VOLTAGE (V)

5.24.74.23.73.2

2.5

MXB7846 toc11a

2.0

1.5

1.0

INTERNAL VOLTAGE REFERENCE (V)

0.5

0

040

REFERENCE CURRENT vs. SAMPLE RATE

10

EXTERNAL REFERENCE

9

MXB7846 toc13

MXB7846 toc16

8

7

6

5

4

3

REFERENCE CURRENT (μA)

2

1

0

0 125

705

704

703

702

701

700

TEMP1 DIODE VOLTAGE (mV)

699

698

2.7

NO CAPACITOR

(30μs) 12-BIT SETTLING

TURN-ON TIME (μs)

SAMPLE RATE (kHz)

TEMP1 DIODE VOLTAGE

vs. SUPPLY VOLTAGE

TEMP1

SUPPLY VOLTAGE (V)

MXB7846 toc11b

3530252015105

MXB7846 toc14

100755025

MXB7846 toc17

5.24.74.23.73.2

Typical Operating Characteristics (continued)

(VDD= 2.7V, V

REF

= 2.5V

EXTERNAL

, f

DCLK

= 2MHz, f

SAMPLE

= 125kHz, C

LOAD

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

otherwise noted.)

MXB7846

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

with Internal Reference and Temperature Sensor

_______________________________________________________________________________________

7

SUPPLY CURRENT

vs. SUPPLY VOLTAGE

250

f

= 12.5kHz

SAMPLE

225

200

SUPPLY CURRENT (μA)

175

150

2.0 5.5
SUPPLY VOLTAGE (V)

SHUTDOWN CURRENT
vs. SUPPLY VOLTAGE

300

DCLK = CS = V

250

200

DD

5.04.54.03.53.02.5

MXB7846 toc18

SUPPLY CURRENT (μA)

MXB7846 toc21

290

285

280

275

270

265

260

255

250

-40 80

SHUTDOWN CURRENT vs. TEMPERATURE

120

110

100

90

SUPPLY CURRENT vs. TEMPERATURE

f

= 125kHz

SAMPLE

= 2.7V

V

DD

6550-25 -10 5 20 35

TEMPERATURE (°C)

DCLK = CS = VDD = 3V

MXB7846 toc19

MXB7846 toc22

SUPPLY CURRENT vs. SAMPLE RATE

250

VDD = 2.7V

= 2.5V

V

REF

225

200

175

150

SUPPLY CURRENT (μA)

125

100

0 125

SAMPLE RATE (kHz)

MAXIMUM SAMPLE RATE

vs. SUPPLY VOLTAGE

1000

100

MXB7846 toc20

100755025

MXB7846 toc23

150

SHUTDOWN CURRENT (nA)

100

50

2.7 5.2
SUPPLY VOLTAGE (V)

80

SAMPLE RATE (kHz)

70

SHUTDOWN CURRENT (nA)

60

4.74.23.73.2

50

-40 80

TEMPERATURE (°C)

655035205-10-25

10

1

2.0 5.5
SUPPLY VOLTAGE (V)

5.04.54.03.53.02.5

Pin Description

MXB7846

2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor

8 _______________________________________________________________________________________

Figure 1. Detailed Serial Interface Timing

PIN NAME FUNCTION

1VDDPositive Supply Voltage. Connect to pin 10.

2 X+ X+ Position Input, ADC Input Channel 1

3 Y+ Y+ Position Input, ADC Input Channel 2

4 X- X- Position Input

5 Y- Y- Position Input

6 GND Ground

7 BAT Battery Monitoring Inputs; ADC Input Channel 3

8 AUX Auxiliary Input to ADC; ADC Input Channel 4

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

9 REF

10 V

DD

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

11 PENIRQ Pen Interrupt Output. Open anode output. 10kΩ to 100kΩ pullup resistor required to VDD.

12 DOUT

13 BUSY

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

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

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

15 CS

16 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%).

CS

t

DCLK

DIN

DOUT

BUSY

t

CSS

t

DS

t

DH

t

DV

t

BDV

CH

t

CL

t

CP

t

DO

t

BD

t

CSH

t

TR

t

BTR

MXB7846

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

with Internal Reference and Temperature Sensor

_______________________________________________________________________________________ 9

Detailed Description

The MXB7846 uses a successive-approximation conver­sion technique to convert analog signals to a 12-bit digi­tal output. An SPI/QSPI/MICROWIRE™-compatible serial
interface provides easy communication to a micro­processor (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

).

Analog Inputs

Figure 2 shows a block diagram of the analog input sec­tion that includes the input multiplexer of the MXB7846,
the differential signal inputs of the ADC, and the differ­ential reference inputs of the ADC. The input multiplexer
switches between X+, X-, Y+, Y-, AUX, BAT, and the
internal temperature sensor.

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 refer­ence and switching matrix according to Tables 1 and 2.

During the acquisition interval, the selected channel
charges the sampling capacitance. The acquisition
interval starts on the fifth falling clock edge and ends
on the eighth falling clock edge.

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
(t

ACQ

) is the maximum time the device takes to acquire

the input signal to 12-bit accuracy. Calculate t

ACQ

with

the following equation:

where RIN= 2kΩ and RSis the source impedance of
the input signal.

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.

tRRpF

ACQ S IN

. =× +

()

×84 25

MICROWIRE is a trademark of National Semiconductor Corp.

Figure 2. Equivalent Input Circuit

PENIRQ

TEMP0

TEMP1

+V

V

DD

REF

X+
X-

Y+
Y-

V

AUX
GND

BAT

7.5kΩ

2.5kΩ

BATTERY

MXB7846

A2–A0

)

(SHOWN 001

B

2.5V

REFERENCE

ON

SER/DFR

(SHOWN HIGH)

REF ON/OFF

+IN

12-BIT ADC

-IN

REF+

REF-

MXB7846

2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor

10 ______________________________________________________________________________________

Functional Diagram

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

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

V

DD

PENIRQ

X+
X-

TEMPERATURE

SENSOR

DOUT

BUSY

PENIRQ

DCLK

DIN

CS

BAT
AUX

REF

Y+
Y-

BATTERY

MONITOR

6-TO-1

MUX

12-BIT ADC

SERIAL

DATA

INTERFACE

2.5V

REFERENCE

A2 A1 A0 MEASUREMENT ADC INPUT CONNECTION DRIVERS ON

0 0 0 Temp0 Temp0 —

0 0 1 Y position X+ Y+, Y-

0 1 0 BAT BAT —

0 1 1 Z1 X+ X-, Y+

1 0 0 Z2 Y- X-, Y+

1 0 1 X- position Y+ X-, X+

1 1 0 AUX AUX —

1 1 1 Temp1 Temp1 —

A2 A1 A0

ADC +REF

CONNECTION TO

0 0 1 Y+ Y- X+ Y position Y+, Y-

0 1 1 Y+ X- X+ Z1 position Y+, X-

1 0 0 Y+ X- Y- Z2 position Y+, X-

1 0 1 X+ X- Y+ X position X+, X-

ADC -REF

CONNECTION TO

ADC INPUT

CONNECTION TO

MEASUREMENT

PERFORMED

DRIVER ON

MXB7846

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

with Internal Reference and Temperature Sensor

______________________________________________________________________________________ 11

Input Bandwidth and Anti-Aliasing

The ADCs input tracking circuitry has a 25MHz small­signal bandwidth, so it is possible to digitize high­speed transient events. To avoid high-frequency sig­nals being aliased into the frequency band of interest,
anti-alias filtering is recommended.

Analog Input Protection

Internal protection diodes, which clamp the analog input
to VDDand GND, allow the analog input pins to swing
from GND - 0.3V to V

DD

+ 0.3V without damage. Analog
inputs must not exceed VDDby more than 50mV or be
lower than GND by more than 50mV for accurate con­version. 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—differ­ential and single ended. The SER/DFR bit in the control
word selects either mode. A logic 1 selects a single­ended conversion, while a logic 0 selects a differential
conversion.

Differential vs. Single Ended

Changes in operating conditions can degrade the accu­racy and repeatability of touch-screen measurements.
Therefore, the conversion results representing X and Y
coordinates may be incorrect. For example, in single­ended measurement mode, variation in the touch-screen
driver voltage drops results in incorrect input reading.
Differential mode minimizes these errors.

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

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 fur­ther measurement errors.

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, respec­tively. Differential mode uses the voltage at the Y+ pin
as the REF+ voltage and voltage at the Y- pin as REF­voltage. 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.

PEN Interrupt Request (PENIRQ)

Figure 5 shows the block diagram for the PENIRQ func­tion. When used, PENIRQ requires a 10kΩ to 100kΩ
pullup to +VDD. If enabled, PENIRQ goes low whenever
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.

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

TOUCH

. 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 con­version. 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.

Touch-Pressure Measurement

The MXB7846 provides two methods for measuring the
pressure applied to the touch screen (Figure 7). By
measuring R

TOUCH

, it is possible to differentiate
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 demon­strating the maximum precision of the MXB7846.

MXB7846

2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor

12 ______________________________________________________________________________________

Figure 3. Single-Ended Y-Coordinate Measurement

Figure 4. Ratiometric Y-Coordinate Measurement

Figure 5. PENIRQ Functional Block Diagram

V

DD

V

DD

Y+

X+

Y-

GND

+IN

-IN

REF

REF+

12-BIT ADC

REF-

Y+

REF+

X+

Y-

GND

+IN

12-BIT ADC

-IN

REF-

+V

DD

100kΩ

PENIRQ

TOUCH SCREEN

OPEN CIRCUIT

Y+

X+

Y-

ON

PENIRQ
ENABLE

MXB7846

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

with Internal Reference and Temperature Sensor

______________________________________________________________________________________ 13

The first method performs pressure measurements
using a known X-plate resistance. After completing
three conversions (X-position, Z1, and Z2), use the fol­lowing equation to calculate R

TOUCH

:

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.
Use the following equation to calculate R

TOUCH:

Internal Temperature Sensor

The MXB7846 provides two temperature measurement
options: single-ended conversion and differential con­version. Both temperature measurement techniques rely
on the semiconductor junction’s temperature character­istics. The forward diode voltage (VBE) vs. temperature
is a well-defined characteristic. The ambient tempera­ture can be calculated by knowing the value of VBEat a
fixed temperature and then monitoring the change in
that voltage as the temperature changes. The single
conversion method requires calibration at a known tem­perature, but only needs a single reading to calculate

R

R

Z

X

Z

R

Y

TOUCH

XPLATE POSITION

YPLATE

POSITION

=

⎛

⎝

⎜

⎞

⎠

⎟

×

⎛
⎝

⎜

⎞
⎠

⎟

×

⎛

⎝

⎜

⎞

⎠

⎟

⎡

⎣

⎢
⎢

⎧
⎨

⎪

⎩

⎪

×

⎛
⎝

⎜

⎞
⎠

⎟

⎧
⎨
⎩

⎫
⎬
⎭

−

−

11

4096

4096

1

4096

RR

XZ

Z

TOUCH XPLATE

POSITION

=

()

×

⎛
⎝

⎜

⎞
⎠

⎟

×

⎛

⎝

⎜

⎞

⎠

⎟

⎡

⎣

⎢
⎢

⎤

⎦

⎥
⎥

−

4096

1

2

1

Figure 7. Pressure Measurement Block Diagram

Figure 6. PENIRQ Timing Diagram

SCREEN TOUCHED HERE

PENIRQ

CS

DCLK

DIN

+

V

-

MEASURE Z1

1 2 3 4 5 6 7 8 1 2 3 1213141516

S A2 A1 A0 M S/D PD1 PD0

INTERRUPT PROCESSOR

t

TOUCH

X+

R

TOUCH

FORCED LINE

X- POSITION

X-

X+

OPEN CIRCUIT

Y+

NO RESPONSE TO TOUCH⎯MASK PENIRQ

MEASURE X- POSITION

Y+

SENSE LINE

Y-

PENIRQ ENABLED

R

SENSE LINE

X-

OPEN CIRCUIT

X+

TOUCH

OPEN CIRCUIT

Y-

Y+

+

V

-

FORCED LINE

R

TOUCH

+

V

-

X-

FORCED LINE

Y-

SENSE LINE

MEASURE Z2

MXB7846

2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor

14 ______________________________________________________________________________________

the ambient temperature. First, the PENIRQ diode for­ward bias voltage is measured by the ADC with an
address of A2 = 0, A1 = 0, and A0 = 0 at a known tem­perature. Subsequent diode measurements provide an
estimate of the ambient temperature through extrapola­tion. 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.

The differential conversion method uses two measure­ment points. The first measurement (Temp0) is per­formed 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 dif­ference between the first and second conversion is
proportional to the absolute temperature and is
expressed by the following formula:

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.

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 mon­itors battery voltages from 0 to 6V. An internal resistor
network divides down V

BAT

by 4 so that a 6.0V battery
voltage results in 1.5V at the ADC input. To minimize
power consumption, the divider is only enabled during
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 inter­nal reference by setting PD1 to a logic 0 when performing
ratiometric position measurements. The internal 2.5V ref­erence typically requires 10ms to settle (with no external
load). For optimum performance, connect a 0.1µF capac­itor from REF to GND. This internal reference can be over­driven with an external reference. For best performance,
the internal reference should be disabled when the exter­nal 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.)

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 cur­rent. 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.

Digital Interface

Initialization After Power-Up and Starting a

Conversion

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.

TC T T

VREF

() . ( )°= ×

⎛
⎝

⎜

⎞
⎠

⎟

×

⎡

⎣

⎢

⎤

⎦

⎥

−−260 1 0

4096

1000 273

Figure 8. Battery Measurement Functional Block Diagram

BATTERY
0 TO 6.0V

7.5kΩ

BAT

2.5kΩ

DC/DC

CONVERTER

0 TO 1.5V

+2.375V TO +5.25V

V

DD

12-BIT ADC

BATTERY
MEASUREMENT ON

MXB7846

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

with Internal Reference and Temperature Sensor

______________________________________________________________________________________ 15

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.

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 micro­controller: 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 8­bit transfer to configure the ADC, and two more 8-bit
transfers to read the conversion result; Figure 9).

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:

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).

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

3) Transmit TB and simultaneously receive a byte; call
it RB1.

4) Transmit a byte of all zeros ($00 hex) and simultane­ously receive byte RB2.

5) Transmit a byte of all zeros ($00 hex) and simultane­ously receive byte RB3.

6) Pull CS high.

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.

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 deci­sions 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.

The conversion must complete in 500µs or less; if not,
droop on the sample-and-hold capacitors can degrade
conversion results.

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 conver­sion starts on DCLK’s falling edge, after the eighth bit of
the control byte is clocked into DIN.

The first logic 1 clocked into DIN after bit 6 of a conver­sion in progress is clocked onto the DOUT pin and is
treated as a START bit (Figure 10).

Once a start bit has been recognized, the current con­version must be completed.

Table 3. Control Byte Format

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

START A2 A1 A0 MODE SER/DFR PD1 PD0

BIT NAME DESCRIPTION

7 START Start bit

6A2

5A1

4A0

3 MODE Conversion resolution: 1 = 8 bits, 0 = 12 bits
2 SER/DFR Conversion mode: 1 = single ended, 0 = differential

1 PD1

0 PD0

Address (Tables 1 and 2)

Power-down mode (Table 4)

MXB7846

2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor

16 ______________________________________________________________________________________

The fastest the MXB7846 can run with CS held continu­ously low is 15 clock conversions. Figure 10 shows the
serial-interface timing necessary to perform a conver­sion every 15 DCLK cycles. If CS is connected low and
DCLK is continuous, guarantee a start bit by first clock­ing 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 nec­essary to perform a conversion every 16 DCLK cycles.

8-Bit Conversion

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.

Table 4. Power-Mode Selection

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

SUPPLY CURRENT (typ) (µA)

PD1 PD0 PENIRQ STATUS

0 0 Enabled ADC is ON during conversion, OFF between conversion 200 1

0 1 Disabled ADC is always ON, reference is always OFF 200 200

1 0 Disabled ADC is always OFF, reference is always ON 400 400

1 1 Disabled ADC is always ON, reference is always ON 600 600

CS

T

B

t

ACQ

R

B2

DURING

CONVERSION

R

B3

AFTER

CONVERSION

DCLK

DIN

BUSY

DOUT

A/D STATE

1891216

SA2A1A0

(START)

DRIVERS 1 AND 2
(SER/DFR HIGH)

DRIVERS 1 AND 2
(SER/DFR LOW)

IDLE

SER/

MODE

PD1 PD0

DFR

ACQUIRE CONVERSION IDLE

RB1

(MSB) (LSB)

ACQUIRE

OFF

OFF OFFON

ON OFF

4567891011 3210

CONVERSION

20 244

IDLEIDLE

MXB7846

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

with Internal Reference and Temperature Sensor

______________________________________________________________________________________ 17

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.

Applications Information

Basic Operation of the MXB7846

The 4-wire touch-screen controller works by creating a
voltage gradient across the vertical or horizontal resis­tive network connected to the MXB7846, as shown in
the

Typical Application Circuit

. The touch screen is
biased through internal MOSFET switches that connect
each resistive layer to VDDand ground on an alternate
basis. For example, to measure the Y position when a
pointing device presses on the touch screen, the Y+

and Y- drivers are turned on, connecting one side of
the vertical resistive layer to V

DD

and the other side to
ground. In this case, the horizontal resistive layer func­tions 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.

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

Figure 11. 16-Clock/Conversion Timing

Figure 10. 15-Clock/Conversion Timing

CS

1 8 15 1 8 15 1

DCLK

DIN

S CONTROL BYTE 0 CONTROL BYTE 1 CONTROL BYTE 2SS

DOUT

BUSY

CS

1 8 16 1 8 16

DCLK

DIN

DOUT

BUSY

CONTROL BYTE 0 CONTROL BYTE 1SS

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

CONVERSION RESULT 0 CONVERSION RESULT 1

CONVERSION RESULT 0 CONVERSION RESULT 1

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

B11 B10 B9 B8 B7 B6B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0

. . .

. . .

. . .

. . .

. . .

MXB7846

2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor

18 ______________________________________________________________________________________

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

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 con­version takes place, DOUT shifts out zeros. On power­up, allow time for the reference to stabilize.

Power Modes

Save power by placing the converter in one of two low­current 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 con­version 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.

The power-up wait before conversion period is depen­dent on the power-down state. When exiting software
low-power modes, conversion can start immediately
when running at decreased clock rates. Upon power­on 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 ref­erence, allow enough time for reference to settle to 12­bit accuracy when exiting full power-down mode, as
shown in the

Typical Operating Characteristics

.

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 conver­sion completes.

PD1 = 0, PD0 = 0

In this mode, the MXB7846 powers down after the cur­rent 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 differen­tial conversions and single-ended measurement con­versions when using an external reference.

When operating at full speed and 16 clocks per conver­sion, 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.

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 conver­sion after the ADC returns to full power is valid.

PD1 = 0, PD0 = 1

This mode turns the internal reference off and leaves
the ADC on to perform conversions using an external
reference.

Figure 12. Ideal Input Voltages and Output Codes

OUTPUT CODE

11…111

11…110

11…101

00…011

00…010

00…001

00…000

123 FS0

INPUT VOLTAGE (LSB) = [(V

FULL-SCALE
TRANSITION

+IN

) - (V

FS-3/2LSB

)]

-IN

FS = (V

1LSB =

REF+

(V

REF+

4096

- V

)

REF-

- V

)

REF-

MXB7846

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

with Internal Reference and Temperature Sensor

______________________________________________________________________________________ 19

Hardware Power-Down

CS also places the MXB7846 into power-down. When
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 dis­able the internal reference, an additional command
byte is required before CS goes high (PD1 = 0). Set
PD1 = 0 before CS goes high.

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.

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 elimi­nate this issue are described below.

One option is to lengthen the acquisition time by stopping
or slowing down DCLK, allowing for the required touch­screen settling time. This method solves the settling time
problem for both single-ended and differential modes.

The second option is to operate the MXB7846 in the dif­ferential 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.

Connection to Standard Interface

MICROWIRE Interface

When using the MICROWIRE- (Figure 13) or SPI-com­patible 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 transi­tions 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 fol­lowed by 4 trailing zeros. DOUT then goes high imped­ance when CS goes high.

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 conver­sion on any analog input of the MXB7846.

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 connect­ed to the MXB7846 DCLK input.

Figure 13. MICROWIRE Interface

Figure 14. QSPI/SPI Interface

Figure 15. TMS320 Serial Interface

I/O

SCK

MISO

MICROWIRE

MOSI

MASKABLE
INTERRUPT

I/O

SCK

MISO

QSPI/SPI

MOSI

MASKABLE

INTERRUPT

XF

CLKX

CLKR

TMS320LC3x

DX

DR

FSR

CS

DCLK

DOUT

MXB7846

DIN

BUSY

CS

DCLK

DOUT

MXB7846

DIN

BUSY

CS

SCLK

MXB7846

DIN

DOUT

BUSY

MXB7846

2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor

20 ______________________________________________________________________________________

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 spe­cific 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.

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 recommend­ed. 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.

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.

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 paral­lel with a 0.1µF capacitor bypassed to GND. To maxi­mize 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 connect­ed in series as a lowpass filter.

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 convert­ed 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 single­ended measurement mode and at high data rates.

Definitions

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 mea­sured using the end-point method.

MXB7846

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

CS

DCLK

DIN

START A2 A1 A0 MODE SER/DEF PD1 PD0

BUSY

DOUT

HIGH IMPEDANCE

MSB B10 B1 B0

HIGH IMPEDANCE

MXB7846

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

with Internal Reference and Temperature Sensor

______________________________________________________________________________________ 21

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 Jitter

Aperture jitter (tAJ) is the sample-to-sample variation in
the time between the samples.

Aperture Delay

Aperture delay (tAD) is the time defined between the
falling edge of the sampling clock and the instant when
an actual sample is taken.

Chip Information

TRANSISTOR COUNT: 12,000
PROCESS: 0.6µm BiCMOS

16

15

14

13

12

11

10

9

1

2

3

4

5

6

7

8

+V

DD

SERIAL/CONVERSION CLOCK

CHIP SELECT

SERIAL DATA IN

CONVERTER STATUS

SERIAL DATA OUT

PEN INTERRUPT

50kΩ

X+

Y+

X-

Y-

GND

BAT

AUX

VOLTAGE

REGULATOR

AUXILIARY
INPUT

TO BATTERY

TOUCH
SCREEN

2.375V TO 5.5V

1μF TO 10μF

OPTIONAL

0.1μF

0.1μF

MXB7846

DCLK

CS

DIN

PENIRQ

BUSY

DOUT

+V

DD

REF

Typical Application Circuit

MXB7846

2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor

22 ______________________________________________________________________________________

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

.)

QSOP.EPS

MXB7846

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

with Internal Reference and Temperature Sensor

______________________________________________________________________________________ 23

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

.)

TSSOP4.40mm.EPS

PACKAGE OUTLINE, TSSOP 4.40mm BODY

21-0066

1

I

1

MXB7846

2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor

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.

24

____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600

© 2008 Maxim Integrated Products is a registered trademark of Maxim Integrated Products, Inc.

Revision History

REVISION

NUMBER

2 1/08 Changed input configuration, differential reference mode 10

REVISION

DATE

DESCRIPTION

PAGES

CHANGED