ADS7846

12

SBAS125H

www.ti.com

The first eight clock cycles are used to provide the control

byte via the DIN pin. When the converter has enough

information about the following conversion to set the input

multiplexer and reference inputs appropriately, the converter

enters the acquisition (sample) mode and, if needed,

the touch panel drivers are turned on. After three more

clock cycles, the control byte is complete and the converter

enters the conversion mode. At this point, the input

FIGURE 9. Conversion Timing, 24 Clocks-per-Conversion, 8-bit Bus Interface. No DCLK delay required with dedicated

serial port.

PRESSURE MEASUREMENT

Measuring touch pressure can also be done with the ADS7846.

To determine pen or finger touch, the pressure of the touch

needs to be determined. Generally, it is not necessary to have

very high performance for this test; therefore, the 8-bit resolu-

tion mode is recommended (however, calculations will be

shown here are in 12-bit resolution mode). There are several

different ways of performing this measurement. The ADS7846

supports two methods. The first method requires knowing the

X-plate resistance, measurement of the X-Position, and two

additional cross-panel measurements (Z1 and Z2) of the touch

screen, as shown in Figure 8. Using Equation 2 calculates the

touch resistance:

R

R

plate

X

Position Z

Z

TOUCH

X

=









–•

–

–

4096

1

2

1

(2)

The second method requires knowing both the X-plate and

Y-plate resistance, measurement of X-Position and Y-Posi-

tion, and Z1. Using Equation 3 also calculates the touch

resistance:

R

R

plate • X

Position

4096

4096

Z

– 1

– R

plate

Y Position

4096

TOUCH

X

1

Y

=

−−









−• 







1–

(3)

DIGITAL INTERFACE

Figure 9 shows the typical operation of the ADS7846 digital

interface. This diagram assumes that the source of the

digital signals is a microcontroller or digital signal processor

with a basic serial interface. Each communication between

the processor and the converter, such as SPI/SSI or

Microwire™ synchronous serial interface, consists of eight

clock cycles. One complete conversion can be accom-

plished with three serial communications for a total of 24

clock cycles on the DCLK input.

FIGURE 8. Pressure Measurement Block Diagrams.

t

ACQ

Acquire

Idle

Conversion

Idle

1

DCLK

CS

81

11

DOUT

BUSY

DRIVERS 1 AND 2(1)

(SER/DFR High)

DRIVERS 1 AND 2(1, 2)

(SER/DFR Low)

(MSB)

(START)

(LSB)

A2

S

On

On

Off

Off

Off

Off

DIN

A1

A0 MODE

SER/

DFR

PD1 PD0

10

98765

4

3210

Zero Filled...

81

8

NOTES: (1) For Y-Position, Driver 1 is on, X+ is selected, and Driver 2 is off. For X-Position, Driver 1 is off, Y+ is selected, and

Driver 2 is on. Y– will turn on when power-down mode is entered and PD0 = 0B. (2) Drivers will remain on if PD0 = 1 (no power

down) until selected input channel, reference mode, or power-down mode is changed, or CS is HIGH.

X-Position

Measure X-Position

Measure Z

1-Position

Touch

X+

Y+

X–

Y–

Z

1-Position

Touch

X+

Y+

Y–

X–

Measure Z

2-Position

Z

2-Position

Touch

X+

Y+

Y–

X–