pointing device presses on the touch screen, the Y+

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

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.

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 MXB7843 configuration through a control

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

MXB7843 to initiate a conversion, the MXB7843 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 volt-

age before the acquisition period is over. After the acqui-

sition 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 con-

version period.

Power-On Reset

When power is first applied, internal power-on circuitry

resets the MXB7843. 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.

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 MXB7843 enters the programmed

power mode until a new control byte is received.

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 MXB7843 is in power-down mode with

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

the MXB7843 is ready to perform a conversion in 10µs.

PD1 = 1, PD0 = 1

In this mode, the MXB7843 is always powered. The

device remains fully powered after the current conver-

sion completes.

PD1 = 0, PD0 = 0

In this mode, the MXB7843 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 MXB7843. At the start of a new con-

version, it instantly powers up. When each conversion is

finished, the part enters power-down mode, unless other-

wise indicated. The first conversion after the ADC returns

to full power is valid for differential conversions and sin-

gle-ended measurement conversions.

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

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

Controller

16

______________________________________________________________________________________

OUTPUT CODE

FS-3/2LSB

FULL-SCALE

TRANSITION

12

3

FS

0

11…111

11…110

11…101

00…011

00…010

00…001

00…000

1LSB =

4096

Figure 10. Ideal Input Voltages and Output Codes