Datasheet

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 (Z
1
and Z
2
) 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 Z
1
. 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
AcquireIdle 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)
A2S
On
On
Off Off
Off Off
DIN A1 A0 MODE
SER/
DFR
PD1 PD0
1098765 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 = 0
B
. (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–