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
TOUCH
=
R
X
– plate •
X – Position
Z
2
Z – 1
4096
1
Measure X-Position
X+
Touch
Y+
X-Position
X–
Y–
Measure Z
1
-Position
X+
Touch
Y+
(2)
Z
1
-Position
X–
Y–
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:
X+
Touch
Y+
R
TOUCH
R
−
plate • X
−
Position
4096
=
X
Z – 1
4096
1
Y Position
– R
Y
−
plate
•
1–
4096
(3)
X–
Z
2
-Position
Y–
Measure Z
2
-Position
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.
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
CS
t
ACQ
DCLK
DIN
1
SER/
8
1
8
1
8
S
A2
A1
Idle
A0 MODE
DFR
PD1 PD0
Acquire
Conversion
Idle
(START)
BUSY
DOUT
DRIVERS 1 AND
(SER/DFR High)
DRIVERS 1 AND 2
(1, 2)
(SER/DFR Low)
2
(1)
11
(MSB)
Off
On
Off
10
9
8
7
6
5
4
3
2
1
0
(LSB)
Zero Filled...
Off
On
Off
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.
FIGURE 9. Conversion Timing, 24 Clocks-per-Conversion, 8-bit Bus Interface. No DCLK delay required with dedicated
serial port.
12
ADS7846
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