MXB7846(2004) 查看數據表（PDF） - Maxim Integrated

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MXB7846
(Rev.:2004)

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

Maxim Integrated 

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

HIGH IMPEDANCE

DOUT

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

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.

MSB

B10

B1

B0

HIGH IMPEDANCE

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.

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