MXB7846EEE+T Datasheet MXB7846EEE+T, MXB7846EEE+, MXB7846EUE+T | P19-P21

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 dis-

able 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 elimi-

nate this issue are described below.

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.

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.

Connection to Standard Interface

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.

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.

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.

MICROWIRE

I/O

SCK

MOSI

MASKABLE

INTERRUPT

MISO

DCLK

DIN

BUSY

DOUT

MXB7846

Figure 13. MICROWIRE Interface

QSPI/SPI

I/O

SCK

MOSI

MASKABLE

INTERRUPT

MISO

DCLK

DIN

BUSY

DOUT

MXB7846

Figure 14. QSPI/SPI Interface

TMS320LC3x

CLKX

FSR

CLKR

SCLK

DIN

BUSY

DOUT

MXB7846

Figure 15. TMS320 Serial Interface

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 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.

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.

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 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.

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.

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 mea-

sured using the end-point method.

MXB7846

DIN

BUSY

HIGH IMPEDANCE

DOUT

DCLK

START A2 A1 A0 MODE SER/DEF PD1 PD0

MSB B10 B1 B0

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

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 (t

) is the sample-to-sample variation in

the time between the samples.

Aperture Delay

Aperture delay (t

) 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

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

MXB7846

DCLK

DIN

PENIRQ

BUSY

DOUT

REF

Typical Application Circuit

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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New from this manufacturer.

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MXB7846EEE+T

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