PRESSURE MEASUREMENT
Measure X-Position
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:
X+
Y+
Y–
Touch
X-Position
X–
Measure Z1-Position
Y+
X+
Touch
X –Position Z2
Z1-Position
X–
RTOUCH = RX – plate•
–1
(2)
4096
Z1
Y–
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:
Y+
X+
Touch
RX − plate • X − Position
4096
Z1
RTOUCH
=
– 1
4096
Z2-Position
(3)
Y Position
–RY − plate • 1–
X–
Y–
Measure Z2-Position
4096
FIGURE 8. Pressure Measurement Block Diagrams.
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.
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
tACQ
DCLK
DIN
1
8
1
8
1
8
SER/
DFR
S
A2 A1 A0 MODE
Idle
PD1 PD0
Acquire
(START)
Conversion
Idle
BUSY
DOUT
11 10
(MSB)
9
8
7
6
5
4
3
2
1
0
Zero Filled...
(LSB)
DRIVERS 1 AND 2(1)
(SER/DFR High)
Off
Off
On
Off
DRIVERS 1 AND 2(1, 2)
(SER/DFR Low)
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 = 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.
FIGURE 9. Conversion Timing, 24 Clocks-per-Conversion, 8-bit Bus Interface. No DCLK delay required with dedicated
serial port.
ADS7846
12
SBAS125H
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