MXB7846EEE+T Datasheet MXB7846EEE+T, MXB7846EEE+, MXB7846EUE+T | P13-P15

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 fol-

lowing 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 con-

version. Both temperature measurement techniques rely

on the semiconductor junction’s temperature character-

istics. The forward diode voltage (V

) vs. temperature

is a well-defined characteristic. The ambient tempera-

ture can be calculated by knowing the value of V

at a

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

TOUCH

XPLATE POSITION

YPLATE

POSITION

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⎟

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⎜

⎞

⎠

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⎜

⎞

⎠

⎟

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⎢

⎧

⎨

⎪

⎩

⎪

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⎟

⎧

⎨

⎩

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−

4096

TOUCH XPLATE

POSITION

()

⎛

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−

4096

TOUCH

OPEN CIRCUIT

FORCED LINE

SENSE LINE

MEASURE Z2

MEASURE Z1

X+

X-

Y+

Y-

TOUCH

OPEN CIRCUIT

FORCED LINE

SENSE LINE

X+

X-

Y+

Y-

MEASURE X- POSITION

X- POSITION

TOUCH

OPEN CIRCUIT

FORCED LINE

SENSE LINE

X+

X-

Y+

Y-

Figure 7. Pressure Measurement Block Diagram

NO RESPONSE TO TOUCH⎯MASK PENIRQ

PENIRQ ENABLED

TOUCH

SCREEN TOUCHED HERE

INTERRUPT PROCESSOR

S A2 A1 A0 M S/D PD1 PD0

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

PENIRQ

DIN

DCLK

Figure 6. PENIRQ Timing Diagram

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 for-

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.

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:

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 mon-

itors 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 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.)

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.

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

() . ( )°= ×

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⎜

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⎠

⎟

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⎤

⎦

⎥

−−260 1 0

4096

1000 273

DC/DC

CONVERTER

12-BIT ADC

BATTERY

0 TO 6.0V

0 TO 1.5V

2.5kΩ

7.5kΩ

+2.375V TO +5.25V

BAT

BATTERY

MEASUREMENT ON

Figure 8. Battery Measurement Functional Block Diagram

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

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 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).

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 simultane-

ously receive byte RB2.

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

ously 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 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.

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 conver-

sion 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 conver-

sion 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 con-

version must be completed.

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

Address (Tables 1 and 2)

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

2 SER/DFR Conversion mode: 1 = single ended, 0 = differential

1 PD1

0 PD0

Power-down mode (Table 4)

Table 3. Control Byte Format

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P21 P22-P24

MXB7846EEE+T

Mfr. #:

Buy MXB7846EEE+T

Manufacturer:

Maxim Integrated

Description:

Touch Screen Controllers 2.375-5.25V 4-Wire Touch-Screen Ctlr

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MXB7846EEE+T

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