Analog Input Protection

Internal protection diodes, which clamp the analog

50mV or be lower than GND by more than 50mV for

accurate conversions. 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 MXB7843 provides two conversion methods—dif-

ferential and single ended. The SER/DFR bit in the con-

trol word selects either mode. A logic 1 selects a

single-ended conversion, while a logic 0 selects a dif-

ferential conversion.

Differential vs. Single Ended

Changes in operating conditions can degrade the accu-

racy and repeatability of touch-screen measurements.

Therefore, the conversion results representing X and Y

coordinates may be incorrect. For example, in single-

ended 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

MXB7843 measures the position of the pointing device by

connecting X+ to IN+ of the ADC, enabling Y+ and Y- dri-

vers, and digitizing the voltage on X+. The ADC performs

a conversion with REF+ = REF and REF- = GND. In sin-

gle-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 200

Ω to 900Ω (depending

on the manufacturer), whereas the on-resistance of the X

and Y drivers is 8

Ω (typ). Limit the touch-screen current to

less than 50mA by using a touch screen with a resistance

higher than 100

Ω. 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-resis-

tance 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, respec-

tively. Differential mode uses the voltage at the Y+ pin

as the REF+ voltage and voltage at the Y- pin as REF-

voltage. 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 func-

tion. When used, PENIRQ requires a 10k

Ω to 100kΩ

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 MXB7843 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

value changes for different touch-screen parasitic

capacitance and resistance. The microprocessor

receives this interrupt and pulls CS low to initiate a con-

version. 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.

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.

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

Controller

10

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