BATTERY MEASUREMENT
An added feature of the TSC2046 is the ability to monitor
the battery voltage on the other side of the voltage regulator
(DC/DC converter), as shown in Figure 7. The battery voltage
can vary from 0V to 6V, while maintaining the voltage to the
TSC2046 at 2.7V, 3.3V, etc. The input voltage (VBAT) is
divided down by 4 so that a 5.5V battery voltage is repre-
sented as 1.375V to the ADC. This simplifies the multiplexer
and control logic. In order to minimize the power consump-
tion, the divider is only on during the sampling period when
A2 = 0, A1 = 1, and A0 = 0 (see Table I for the relationship
between the control bits and configuration of the TSC2046).
Measure X-Position
X+
Y+
Y–
Touch
X-Position
X–
Measure Z1-Position
Y+
Y–
X+
Touch
2.7V
DC/DC
Converter
Battery
Z1-Position
X–
+
0.5V
to
5.5V
+VCC
Y+
X+
X–
Touch
0.125V to 1.375V
VBAT
ADC
Z2-Position
7.5kΩ
2.5kΩ
Y–
Measure Z2-Position
FIGURE 8. Pressure Measurement Block Diagrams.
DIGITAL INTERFACE
FIGURE 7. Battery Measurement Functional Block Diagram.
See Figure 9 for the typical operation of the TSC2046
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 communica-
tion 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
accomplished with three serial communications for a total
of 24 clock cycles on the DCLK input.
PRESSURE MEASUREMENT
Measuring touch pressure can also be done with the TSC2046.
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 in the 12-bit resolution mode). There are several
different ways of performing this measurement. The TSC2046
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:
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 sample-and-
hold goes into the hold mode and the touch panel drivers turn
off (in single-ended mode). The next 12 clock cycles accom-
plish the actual analog-to-digital conversion. If the conver-
sion is ratiometric (SER/DFR = 0), the drivers are on during
the conversion and a 13th clock cycle is needed for the last
bit of the conversion result. Three more clock cycles are
needed to complete the last byte (DOUT will be low), which
are ignored by the converter.
X – Position Z2
RTOUCH = RX– plate
•
– 1
(2)
4096
Z
1
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:
RX−plate • X −Position
4096
RTOUCH
=
– 1
4096
Z1
(3)
Y − Position
–RY−plate 1−
4096
Microwire is a registered trademark of National Semiconductor.
TSC2046
12
SBAS265C
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