XPT2046
Touch Screen Controller
XPT2046 Data Sheet
2007.5
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XPT2046
Touch Screen Controller
CONTENTS
GENERAL DESCRIPTION .............................................................................................................3
FEATURES.........................................................................................................................................3
APPLICATIONS................................................................................................................................3
BLOCK DIAGRAM ..........................................................................................................................4
ABSOLUTE MAXIMUM RATINGS ..............................................................................................4
ELECTRICAL CHARACTERISTICS ...........................................................................................5
PIN CONFIGURATION ...................................................................................................................8
PIN LAYOUT ......................................................................................................................................8
PIN DESCRIPTION...............................................................................................................................9
TYPICAL CHARACTERISTICS ..................................................................................................10
THEORY OF OPRATION..............................................................................................................13
BASIC OPERATION OF THE XPT2046...............................................................................................13
ANALOG INPUT ................................................................................................................................14
INTERNAL REFERENCE.....................................................................................................................15
REFERENCE INPUT ...........................................................................................................................16
SIMPLIFIED DIAGRAM OF SINGLE-ENDED REFERENCE ....................................................................16
SIMPLIFIED DIAGRAM OF DIFFERENTIAL REFERENCE ......................................................................17
TOUCH SCREEN SETTLING ...............................................................................................................17
TEMPERATURE MEASUREMENT .......................................................................................................18
BATTERY MEASUREMENT ...............................................................................................................19
PRESSURE MEASUREMEN ................................................................................................................20
DIGITAL INTERFACE...................................................................................................................21
PENIRQ OUTPUT ..........................................................................................................................24
PER-CONVERSION .......................................................................................................................26
16 CLOCKS-PER-CONVERSION .........................................................................................................26
DIGITAL TIMING ..............................................................................................................................26
15 CLOCKS-PER-CONVERSION .........................................................................................................28
DATA FORMAT ................................................................................................................................28
8-BIT CONVERSION .........................................................................................................................29
POWER DISSIPATION ..................................................................................................................29
DEMO ...............................................................................................................................................30
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XPT2046
Touch Screen Controller
General Description
The XPT2046 is a 4-wire resistive touch screen controller that incorporates a 12-bit 125 kHz sampling SAR type
A/D converter.
The XPT2046 operates down to 2.2V supply voltage and supports digital I/O interface voltage from 1.5V to VCC
in order to connect low voltage uP.
The XPT2046 can detect the pressed screen location by performing two A/D conversions. In addition to location,
the XPT2046 also measures touch screen pressure.On-chip VREF can be utilized for analog auxiliary input,
temperature measurement and battery monitoring withthe ability to measure voltage from 0V to 5V.
The XPT2046 also has an on-chip temperature sensor
The XPT2046 is available in 16pin QFN thin package(0.75mm in height) and has the operating temperature range
of -40°C to +85°C
Features
12 bit SAR type A/D converter with S/H circuit
Low voltage operation (VCC = 2.2V ∼ 3.6V)
Low voltage digital I/F (1.5V ∼ VCC)
4-wire I/F
Sampling frequency: 125 kHz (max)
On-Chip voltage reference (2.5V)
Pen pressure measurement
On-chip thermo sensor
Direct battery masurement
Low power consumption (260μA)
Package 16pin QFN
Applications
Personal digital assistants
Portable instruments
Point -of-sale terminals
Pagers
Touch screen monitors
Cellular phones
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XPT2046
Touch Screen Controller
Block Diagram
Figure 1. Block Diagram
Absolute Maximum Ratings
+VCC and IOVDD to GND
−0.3V to +6V
Analog Inputs to GND
−0.3V to +VCC + 0.3V
Digital Inputs to GND
−0.3V to IOVDD + 0.3V
Power Dissipation
. 250mW
Maximum Junction Temperature
+150°C
Operating Temperature Range
. −40°C to +85°C
Storage Temperature Range
−65°C to +150°C
Lead Temperature (soldering, 10s)
+300°C
Table 1. Absolute Maximum Ratings
WARNING: Stresses above these ratings may cause permanent damage.Exposure to absolute maximum conditions
for xtended periods may degrade device reliability. These are stress ratings only, and functional operation of the
device at these or any other conditions beyond those specified is not implied.
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XPT2046
Touch Screen Controller
Electrical Characteristics: VS = +2.7V to +5.5V
At TA = −40°C to +85°C, +VCC = +2.7V, VREF = 2.5V internal voltage, fSAMPLE = 125kHz, fCLK = 16 • fSAMPLE = 2MHz,
12-bit mode, digital inputs = GND or IOVDD, and +VCC must be • IOVDD.
PARAMETER
CONDITION
XPT2046
MIN
TYP
MAX
UNITS
ANALOG INPUT
Full-Scale Input Span
Positive Input−Negative Input
0
VREF
V
Absolute Input Range
Positive Input
-0.2
+VCC+0.2
V
Negative Input
-0.2
+0.2
V
Capacitance
25
pF
Leakage Current
0.1
µA
12
Bits
SYSTEM PERFORMANCE
Resolution
Bits
11
No Missing Codes
Integral Linearity Error
±2
LSB1
Offset Error
±6
LSB
±4
LSB
Gain Error
Noise
External VREF
Including Internal VREF
Power-Supply Rejection
70
µVrms
70
dB
SAMPLING DYNAMICS
12
Conversion Time
Cycles
3
Acquisition Time
CLK
125
Throughput Rate
CLK
Multiplexer Settling Time
500
Cycles
Aperture Delay
30
KHz
Aperture Jitter
100
ns
100
ns
Channel-to-Channel Isolation
VIN＝2.5Vpp，fs＝50KHz
ps
dB
SWITCH DRIVERS
On-Resistance
YP、XP
5
Ω
YN、XN
6
Ω
Drive Current(2)
50
mA
2.55
V
Duration 100ms
REFERENCE OUTPUT
Internal Reference Voltage
2.45
2.50
Internal Reference Drift
15
ppm/℃
Quiescent Current
500
µA
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XPT2046
Touch Screen Controller
REFERENCE INPUT
Input Impedance
VCC
1.0
Range
SER/——
DFR＝0，PD1＝0
Internal Reference Off
V
1
GΩ
250
Ω
Internal Reference On
BATTERY MONITOR
Input Voltage Range
6.0
0.5
V
Input Impedance
Sampling Battery
10
KΩ
Battery Monitor Off
1
GΩ
Accuracy
VBAT=0.5V~5.5V, ExternalVREF=2.5V
-2
+2
%
VBAT=0.5V~5.5V, Internal Reference
-3
+3
%
-40
+85
℃
TEMPERATURE ASUREMENT
Temperature Range
Resolution
Accuracy
Differential Method(3)
1.6
℃
TEMP0(4)
0.3
℃
Differential Method(3)
±2
℃
TEMP0(4)
±3
℃
DIGITAL INPUT/OUTPUT
CMOS
Logic Family
Capacitance
VIH
15
pF
IOVDD*0.7
IOVDD+0.3
V
0.3*IOVDD
V
5
All Digital Control Input Pins
| IIH |≤+5µA
VIL
| IIL |≤+5µA
-0.3
VOH
IOH=-250µA
IOVDD*0.8
VOL
IOL=250µA
V
0.4
V
Straight
Data Format
Binary
POWER-SUPPLYREQUIREMENTS
+VCC (5)
Specified Performance
2.7
3.6
V
Operating Range
2.2
5.25
V
1.5
VCC
V
650
µA
IOVDD (6)
Quiescent Current (7)
Internal Reference Off
280
Internal Reference On
780
µA
fSAMPLE = 12.5kHz
220
µA
Power-Down Mode with
3
µA
1.8
mW
(CS=DCLK=DIN=IOVDD)
Power Dissipation
VCC=+2.7V
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XPT2046
Touch Screen Controller
-40
TEMPERATURE RANGE
+85
℃
Specified Performance
Table 2. Electrical Characteristics
(1) LSB means Least Significant Bit. With VREF = +2.5V, one LSB is 610 V.
(2) Assured by design, but not tested. Exceeding 50mA source current may result in device degradation.
(3) Difference between TEMP0 and TEMP1 measurement, no calibration necessary.
(4) Temperature drift is −2.1mV/ C.
(5) XPT2046 operates down to 2.2V.
(6) IOVDD must be − (+VCC).
(7) Combined supply current from +VCC and IOVDD. Typical values obtained from conversions on AUX input
with PD0 = 0.
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XPT2046
Touch Screen Controller
Pin Configuration
Pin Layout
QFN-16
TSSOP－16
VFBGA-16
Figure 2. Pin Layout
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XPT2046
Touch Screen Controller
Pin Description
QFN PIN #
TSSOP PIN#
NAME
DESCRIPTION
1
13
A5
BUSY
Busy Output. This output is high
impedance when CS is high.
2
14
A4
DIN
Serial Data Input. If CS is low, data is
latched on the rising edge of DCLK.
3
15
A3
——
Chip Select Input. Controls conversion
timing and enables the serial input/output
4
16
A2
DCLK
External Clock Input. This clock runs the
SAR conversion process and synchronizes
5
1
B1和C1
VCC
6
2
D1
XP
7
3
E1
YP
8
4
G2
XN
9
5
G3
YN
10
6
G4和G5
GND
11
7
G6
VBAT
12
8
E7
AUX
13
9
D7
VREF
14
10
C7
IOVDD
15
11
B7
16
12
VFBGA PIN #
A6
CS
Power Supply
XP Position Input
YP Position Input
XN Position Input
YN Position Input
Ground
Battery Monitor Input
Auxiliary Input to ADC
Voltage Reference Input/Output
Digital I/O Power Supply
PENIRQ
Pen Interrupt
DOUT
Serial Data Output. Data is shifted on the
falling edge of DCLK. This output is high
Table 3. Pin Description
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XPT2046
Touch Screen Controller
Typical Characteristics
At TA = +25 C, +VCC = +2.7V, IOVDD = +1.8V, VREF = External +2.5V, 12-bit mode, PD0 = 0, fSAMPLE
= 125kHz, and fCLK = 16 fSAMPLE = 2MHz,
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XPT2046
Touch Screen Controller
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XPT2046
Touch Screen Controller
Figure 3. Typical Characteristics
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XPT2046
Touch Screen Controller
Theory Of Opration
The XPT2046 is a classic successive approximation register (SAR) analog-to-digital converter (ADC). The
architecture is based on capacitive redistribution, which inherently includes a sample-and-hold function. The
converter is fabricated on a 0.6μm CMOS process. The basic operation of the XPT2046 is shown in Figure 4
The device features an internal 2.5V reference and uses an external clock. Operation is maintained from a single
supply of 2.7V to 5.25V. The internal reference can be overdriven with an external, low-impedance source
between 1V and +VCC. The value of the reference voltage directly sets the input range of the converter.
The analog input (X-, Y-, and Z-Position coordinates, auxiliary input, battery voltage, and chip temperature)
to the converter is provided via a multiplexer. A unique configuration of low on-resistance touch panel driver
switches allows an unselected ADC input channel to provide power and the accompanying pin to provide
ground for an external device, such as a touch screen. By maintaining a differential input to the converter and
a differential reference architecture, it is possible to negate the error from each touch panel driver switch’s
on-resistance (if this is a source of error for theparticular measurement).
Basic Operation of the XPT2046
Figure 4. Basic Operation
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XPT2046
Touch Screen Controller
Analog Input
Figure 5 hows a block diagram of the input multiplexer on the XPT2046, the differential input of the ADC, andt
he differential reference of the converter. Table 4 and Table 5 show the relationship between the A2, A1, A0, and
SER/DFR control bits and the configuration of the XPT2046.The control bits are provided serially via the DIN
pin—see theDigital Interface section of this data sheet for more details.
Figure 5. Simplified Diagram of Analog Input
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XPT2046
A2
A1
A0
0
0
0
0
0
1
0
1
0
0
1
1
1
0
0
1
0
1
1
1
0
1
1
1
VBAT
AUXIN
TEMP
YN
XP
Touch Screen Controller
YP
Y-
X-
Z1-
Z2-
X-
Y-
POSITIO
POSITION
POSITION
POSITION
DRIVERS
DRIVERS
off
off
Off
On
Off
Off
XN, On
YP, On
XN, On
YP, On
On
Off
Off
Off
Off
Off
+IN
(TEMP0)
+IN
M
+IN
+IN
M
+IN
M
+IN
M
+IN
+IN
Table 4.Input Configuration (DIN), Differential Reference Mode (SER/DFR low)
A2
A1
A0
+REF
−REF
0
0
0
YN
XP
1
YP
YN
+IN
1
1
YP
XN
+IN
1
0
0
YP
XN
1
0
1
XP
XN
YP
Y-POSITION
X-POSITION
Z1-POSITION
Z2-POSITION
YP,
M
YN
YP,
M
+IN
M
+IN
DRIVERS
YP,
XP,
M
Table 5.Input Configuration (DIN), Differential Reference Mode (SER/DFR low)
Internal Reference
The XPT2046 has an internal 2.5V voltage reference that can be turned on or off with the control bit, PD1 (see
Table 8 and Figure 6. Typically, the internal reference voltage is onlyused in the single-ended mode for battery
monitoring, temperature measurement, and for using the auxiliary input.Optimal touch screen performance is
achieved when using the differential mode. The internal reference voltage of the XPT2046 must be commanded to
be off to maintain compatibility with the ADS7843. Therefore, after power-up,a write of PD1 = 0 is required to
insure the reference is off (see the Typical Characteristics for power-up time of the reference from power-down).
Figure 6. Simplified Diagram of the Internal Reference
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XPT2046
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Reference Input
The voltage difference between +REF and –REF (see Figure 5 sets the analog input range. The XPT2046 perates
with a reference in the range of 1V to +VCC. There are several critical items concerning the reference input and
its wide voltage range. As the reference voltage is reduced, the analog voltage weight of each digital output code
is also reduced.
This is often referred to as the LSB (least significant bit) size and is equal to the reference voltage divided by 4096
in 12-bit mode. Any offset or gain error inherent in the ADC appears to increase, in terms of LSB size, as the
reference voltage is reduced. With a ower reference voltage, more care mustbe taken to provide a clean layout
including adequate bypassing, a clean (low-noise, low-ripple) power supply, alow-noise reference (if an external
reference is used), and a low-noise input signal.The voltage into the VREF input directly drives the capacitor
digital-to-analog converter (CDAC) portion of the XPT2046. Therefore, the input current is very low (typically<
13 A).
Simplified Diagram of Single-Ended Reference
There is also a critical item regarding the reference when making measurements while the switch drivers are ON.
For this discussion, it is useful to consider the basic operation of the XPT2046 (see Figure 4. This particular
application shows the device being used to digitize a resistive touch screen. A measurement of the current
Y-Position of the pointing device is made by connecting the X+ input to the ADC, turning on the Y+ and Y–
drivers, and digitizing the voltage on X+ (Figure 7 hows a block diagram). For this measurement, the resistance in
the X+ lead does not affect the conversion (it does affect the settling time, but the resistance is usually small
enough that this is not a concern). However, since the resistance between Y+ and Y– is fairly low, the
on-resistance of the Y drivers does make a small difference. Under the situation outlined so far, it is not possible
to achieve a 0V input or a full-scale input regardless of where the pointing device is on the touch screen because
some voltage is lost across the internal switches. In addition,the internal switch resistance is unlikely to track the
resistance of the touch screen, providing an additional source of error.
Figure 7. Simplified Diagram of Single-Ended Reference (SER/DFR high,
Y switches enabled,X+ is analog input)
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XPT2046
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Simplified Diagram of Differential Reference
This situation can be remedied as shown in Figure 8 By setting the SER/DFR bit low, the +REF and –REF inputs
are connected directly to Y+ and Y–, respectively, which makes the analog-to-digital conversion ratiometric. The
result of the conversion is always a percentage of the external resistance, regardless of how it changes in relation
to the on-resistance of the internal switches. Note that there is an important consideration regarding power
dissipation when using the ratiometric mode of operation (see the Power Dissipation section for more details).
As a final note about the differential reference mode, it must be used with +VCC as the source of the +REF
voltage and cannot be used with VREF. It is possible to use a high-precision reference on VREF and single-ended
reference mode for measurements which do not need to be ratiometric. In some cases, it is possible to power the
converter directly from a precision reference. Most references can provide enough power for the XPT2046,but
might not be able to supply enough current for the external load (such as a resistive touch screen).
VCC
YP
XP
IN +
REF +
Converter
IN-
REF -
YN
GND
Figure 8. Simplified Diagram of Differential Reference (SER/DFR low,
Y switches enabled,X+ is analog input)
Touch Screen Settling
In some applications, external capacitors may be required across the touch screen for filtering noise picked up by
the touch screen (e.g., noise generated by the LCD panel or backlight circuitry). These capacitors provide a
low-pass filter to reduce the noise, but cause a settling time requirement when the panel is touched that typically
shows up as a gain error. There are several methods for minimizing or eliminating this issue. The problem is that
the input and/or reference has not settled to the final steady-state value prior to the ADC sampling the input(s)and
providing the digital output. Additionally, the reference voltage may still be changing during the measurement
cycle. Option 1 is to stop or slow down the XPT2046 DCLK for the required touch screen settling time. This
allows the input and reference to have stable values for the Acquire period (3 clock cycles of the XPT2046; see
Figure 12). This works for both the single-ended and the differential modes.Option 2 is to operate the XPT2046 in
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XPT2046
Touch Screen Controller
the differential mode only for the touch screen measurements and command the XPT2046 to remain on (touch
screen drivers ON) and not go into power-down (PD0 = 1). Several conversions are made depending on thes
ettling time required and the XPT2046 data rate. Once the required number of conversions have been made, the
processor commands the XPT2046 to go into its power-down state on the last measurement. This process isr
equired for X-Position,Y-Position, and Z-Position measurements. Option 3 is to operate in the 15
Clock-per-Conversion mode, which overlaps the analog-to-digital conversions and maintains the touch screen
drivers on until commanded to stop by the processor (see Figure 16).
Temperature Measurement
In some applications, such as battery recharging, a measurement of ambient temperature is required. The
temperature measurement technique used in the XPT2046 relies on the characteristics of a semiconductor
junction operating at a fixed current level. The forward diode voltage (VBE) has a well-defined characteristic
versus temperature. The ambient temperature can be predicted in applications by knowing the +25 C value of
the VBE voltage and then monitoring the delta of that voltage as the temperature changes. The XPT2046 offers
two modes of operation. The first mode requires calibration at a known temperature, but only requires a single
reading to predict the ambient temperature. A diode is used (turned on) during this measurement cycle. The
voltage across the diode is connected through the MUX for digitizing the forward bias voltage by the ADC with
an address of A2 = 0, A1 = 0, and A0 = 0 (see Table 1 and Figure 6 for details). This voltage is typically 600mV
at +25 C with a 20 A current through the diode. The absolute value of this diode voltage can vary a few
millivolts.However, the TC of this voltage is very consistent at –2.1mV/ C. During the final test of the end
product, the diode voltage would be stored at a known room temperature, in memory, for calibration purposes by
the user. The result is an equivalent temperature measurement resolution of 0.3 C/LSB (in 12-bit mode).
VCC
TEMP1
TEMP0
MUX
ADC
Figure 9. Functional Block Diagram of Temperature Measurement
The second mode does not require a test temperature calibration, but uses a two-measurement method to eliminate
the need for absolute temperature calibration and for achieving 2 C accuracy. This mode requires a second
conversion with an address of A2 = 1, A1 = 1, and A0 = 1, with a 91 times larger current. The voltage difference
between the first and second conversion using 91 times the bias current is represented by Equation (1):
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XPT2046
Touch Screen Controller
△V ＝
kT
• ln(N )
q
… … … … … … … … … … （ 1）
where:
N is the current ratio = 91.
k = Boltzmann’s constant (1.38054 • 10−23 electron volts/ degrees Kelvin).
q = the electron charge (1.602189 • 10–19 C).
T = the temperature in degrees Kelvin.
This method can provide improved absolute temperature measurement over the first mode at the cost of less
resolution (1.6°C/LSB). The equation for solving for °K is:
°K = q •
ΔV
…………………………（2）
(k • ln( N ))
where:
ΔV = V (I91) – V (I1) (in mV)
°K = 2.573 °K/mV • ΔV
°C = 2.573 • ΔV(mV) – 273°K
NOTE: The bias current for each diode temperature measurement is only on for 3 clock cycles (during the
acquisition mode) and, therefore, does not add any noticeable increase in power, especially if the temperature
measurement only occurs occasionally.
Battery Measurement
An added feature of the XPT2046 is the ability to monitor the battery voltage on the other side of the voltage
regulator(DC/DC converter), as shown in Figure 10. The battery voltage can vary from 0V to 6V, while
maintaining the voltage to the XPT2046 at 2.7V, 3.3V, etc. The input voltage (VBAT)is divided down by 4 so that
a 5.5V battery voltage is represented as 1.375V to the ADC. This simplifies the multiplexer and control logic. In
order to minimize the power consumption, the divider is only on during the sampling period when A2 = 0, A1 = 1,
and A0 = 0 (see Table 1 for the relationship between the control bits and configuration of the XPT2046).
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XPT2046
Touch Screen Controller
Figure 10. Battery Measurement Functional Block Diagram
Pressure Measurement
Measuring touch pressure can also be done with the XPT2046. 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 resolution 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 XPT2046 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 11. Using Equation (3)
calculates the touch resistance:
R 触摸＝RX 面板·
XPosition ⎛ Z 2 ⎞
− 1⎟ ………………（3）
⎜
4096 ⎝ Z1 ⎠
The second method requires knowing both the X-plate and Y-plate resistance, measurement of X-Position and
Y-Position, and Z1. Using Equation (4) also calculates the touch resistance:
⎛
⎜
⎞
⎟
⎝
⎠
Y -Position
X-Position ⎛ 4096
⎞
⎟
Rtouch＝ RX-Plate•4096
−1⎟ －RY − Plate ⎜⎜1−
⎜
⎝ Z1
⎠
⎜
4096 ⎟⎟ … … （ 4）
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XPT2046
MEASURE
X-POSITION
XP
YP
MEASURE
Z1-POSITION
XN
YN
YP
XP
TOUCH
X-POSITION
Touch Screen Controller
YP
XP
TOUCH
TOUCH
Z1-POSITION
Z2-POSITION
XN
XN
YN
YN
MEASURE
Z2-POSITION
Figure 11. Pressure Measurement Block Diagrams
Digital Interface
See Figure 12 for the typical operation of the XPT2046 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 accomplished 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 sample-and-hold goes into the hold mode and the touch panel drivers turn off (in single-ended mode). The
next 12 clock cycles accomplish the actual analogto-digital conversion. If the conversion 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 conversionr
esult. Three more clock cycles are needed to complete the last byte (DOUT will be low), which are ignored by the
converter.
Control Byte
The control byte (on DIN), as shown in Table 3, provides the start conversion, addressing, ADC resolution,
configuration,and power-down of the XPT2046. Figure 12, Table 3 and Table 4 give detailed information regarding
the order and description of these control bits within the control byte.
Initiate START—The first bit, the S bit, must always be high and initiates the start of the control byte. The
XPT2046 ignores inputs on the DIN pin until the start bit is detected.
Addressing—The next three bits (A2, A1, and A0) select the active input channel(s) of the input multiplexer (see
Table 1, Table 2, and Figure 5), touch screen drivers, and the reference inputs.
MODE—The mode bit sets the resolution of the ADC. With this bit low, the next conversion has 12 bits ofr
esolution,whereas with this bit high, the next conversion has eight bits of resolution.
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XPT2046
Touch Screen Controller
SER/DFR—The SER/DFR bit controls the reference mode, either single-ended (high) or differential (low). The
differential mode is also referred to as the ratiometric conversion mode and is preferred for X-Position,Y-Position,
and Pressure-Touch measurements for optimum performance. The reference is derived from the voltage at the
switch drivers, which is almost the same as the voltage to the touch screen. In this case, a reference voltage is not
needed as the reference voltage to the ADC is the voltage across the touch screen. In the single-ended mode, the
converter reference voltage is always the difference between the VREF and GND pins (see Table 1 and Table 2,
and Figure 5 through Figure 8, for further information).
BIT7(MSB)
S
BIT 6
BIT 5
BIT 4
A2
A1
A0
BIT 3
MODE
BIT2
——
SER/DFR
BIT 1
BIT 0(LSB)
PD1
PD0
Table 6. Order of the Control Bits in the Control Byte
BIT
NAME
DESCRIPTION
7
S
Start bit. Control byte starts with first high bit on DIN.A new control byte can start
every 15th clock cycle in 12-bit conversion mode or every 11th clock cycle in 8-bit
conversion mode (see Figure 16).
6-4
A2-A0
Channel Select bits. Along with the SER/DFR bit,these bits control the setting of the
multiplexer input,touch driver switches, and reference inputs (seeTable 1 and Figure
16).
3
MODE
12-Bit/8-Bit Conversion Select bit. This bit controls the number of bits for the next
conversion: 12-bits(low) or 8-bits (high).
2
SER/——
DFR
Single-Ended/Differential Reference Select bit. Along with bits A2-A0, this bit
controls the setting of the multiplexer input, touch driver switches, and reference
inputs (see Table 1 and Table 2).
1-0
PD1-PD0
Power-Down Mode Select bits. Refer to Table 5 fordetails.
Table 7. Descriptions of the Control Bits within the Control Byte
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XPT2046
Touch Screen Controller
CS
tACQ
DCLK
DIN
1
8
S A2 A1 A0 MODE SER/
DFR
Idle
1
1
8
8
PD1 PD0
Acquire
Conversion
Idle
BUSY
11 10 9
DOUT
Drivers 1 and 2
(SER/DFR High)
Off
7
On
6
5
4
3
2
1
0
Zero Filled...
Off
Off
Drivers 1and 2
(SER/DFR Low
8
On
Off
CS
tACQ
DCLK
DIN
1
8
S A2 A1 A0 MODE SER/
DFR
Idle
1
1
8
8
PD1 PD0
Acquire
Conversion
Idle
BUSY
11 10 9
DOUT
Drivers 1 and 2
(SER/DFR High)
Drivers 1and 2
(SER/DFR Low
Off
8
On
Off
7
6
5
4
3
2
1
0
Zero Filled...
Off
On
Off
Figure 12. Conversion Timing, 24 Clocks-per-Conversion, 8-Bit Bus Interface.
No DCLK delay required with dedicated serial port
If X-Position, Y-Position, and Pressure-Touch are measured in the single-ended mode, an external reference
voltage is needed. The XPT2046 must also be powered from the external reference. Caution should be observed
when using the single-ended mode such that the input voltage to the ADC does not exceed the internal reference
voltage, especially if the supply voltage is greater than 2.7V.
NOTE: The differential mode can only be used for X-Position, Y-Position, and Pressure-Touch measurements.
All other measurements require the single-ended mode.
PD0 and PD1—Table 5 describes the power-down and the internal reference voltage configurations. The internal
reference voltage can be turned on or off independently of the ADC. This can allow extra time for the internal
reference voltage to settle to the final value prior to making a conversion. Make sure to also allow this extra
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XPT2046
Touch Screen Controller
wake-up time if the internal reference is powered down. The ADC requires no wake-up time and can be
instantaneously used. Also note that the status of the internal reference power-down is latched into the part
(internally) with BUSY going high. In order to turn the reference off, an additional write to the XPT2046 is
required after the channel has been converted.
PD1
PD0
————
PEN IRQ
DESCRIPTION
0
0
Enabled
Power-Down Between Conversions. When each conversion is finished, the converter
enters a low-power mode. At the start of the next conversion, the device instantly
powers up to full power. There is no need for additional delays to ensure full operation,
and the very first conversion is valid. The Y− switch is on when in power-down.
0
1
Disabled
Reference is off and ADC is on.
1
0
Enabled
Reference is on and ADC is off.
1
1
Disabled
Device is always powered. Reference is on and ADC is on.
Table 8. Power-Down and Internal Reference Selection
PENIRQ Output
The pen-interrupt output function is shown in Figure 13.While in power-down mode with PD0 = 0, the Y-driver is
on and connects the Y-plane of the touch screen to GND. The PENIRQ output is connected to the X+ input
through two transmission gates. When the screen is touched, the X+ input is pulled to ground through the touch
screen.In most of the XPT2046 models, the internal pullup resistor value is nominally 50k , but this may vary
between 36k and 67k given process and temperature variations. In order to assure a logic low of 0.35 _ (+VCC)
is presented to the PENIRQ circuitry, the total resistance between the X+ and Y− terminals must be less than
21k .
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XPT2046
Touch Screen Controller
Figure 13. PENIRQ Functional Block Diagram
The −90 version of the XPT2046 uses a nominal 90k pullup resistor, which allows the total resistance between
the X+ and Y− terminals to be as high as 30k Note that the higher pullup resistance will cause a slower response
time of the PENIRQ to a screen touch, so user software should take this into account.
The PENIRQ output goes low due to the current path through the touch screen to ground, which initiates an
interrupt to the processor. During the measurement cycle for X-, Y-, and Z-Position, the X+ input is disconnected
from the PENIRQ internal pull-up resistor. This is done to eliminate any leakage current from the internal pull-up
resistor through the touch screen, thus causing no errors.
Furthermore, the PENIRQ output is disabled and low during the measurement cycle for X-, Y-, and Z-Position.
The PENIRQ output is disabled and high during the measurement cycle for battery monitor, auxiliary input, and
chip temperature. If the last control byte written to the XPT2046 contains PD0 = 1, the pen-interrupt output
function is disabled and is not able to detect when the screen is touched. In order to re-enable the pen-interrupt
output function under these circumstances, a control byte needs to be written to the XPT2046 with PD0 = 0. If the
last control byte written to the XPT2046 contains PD0 = 0, the pen-interrupt output function is enabled at the end
of the conversion. The end of the conversion occurs on the falling edge of DCLK after bit 1 of the converted data
is clocked out of the XPT2046.
It is recommended that the processor mask the interrupt PENIRQ is associated with whenever the processor sends
a control byte to the XPT2046. This prevents false triggering of interrupts when the PENIRQ output is disabled in
the cases discussed in this section.
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XPT2046
Touch Screen Controller
per-Conversion
16 Clocks-per-Conversion
The control bits for conversion n + 1 can be overlapped with conversion n to allow for a conversion every 16
clock cycles, as shown in Figure 14. This figure also shows possible serial communication occurring with other
serial peripherals between each byte transfer from the processor to the converter. This is possible, provided that
each conversion completes within 1.6ms of starting. Otherwise, the signal that is captured on the input
sample-and-hold may droop enough to affect the conversion result. Note that the XPT2046 is fully powered while
other serial communications are taking place during a conversion.
Figure 14. Conversion Timing, 16 Clocks-per-Conversion, 8-Bit Bus Interface.
No DCLK delay required with dedicated serial port
Digital Timing
Figure 12, Figure 15 and Table 6 provide detailed timing for the digital interface of the XPT2046.
Figure 15. Detailed Timing Diagram
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XPT2046
SYMBOL
Touch Screen Controller
DESCRIPTION
+VCC· 2.7V,
+VCC· IOVDD·1.5V,
CLOAD = 50pF
MIN
tACQ
TYP
UNITS
MAX
Acquisition Time
1.5
μs
tDS
DIN Valid Prior to DCLK Rising
100
ns
tDH
DIN Hold After DCLK High
50
ns
tDO
DCLK Falling to DOUT Valid
200
ns
tDV
—— Falling to DOUT Enabled
CS
200
ns
tTR
—— Rising to DOUT Disabled
CS
200
ns
tCSS
——CS Falling to First DCLK
CS
100
ns
Rising
tCSH
——Rising to DCLK Ignored
CS
10
ns
tCH
DCLK High
200
ns
tCL
DCLK Low
200
ns
tBD
DCLK Falling to BUSY
Rising/Falling
200
ns
tBDV
——Falling to BUSY Enabled
CS
200
ns
tBTR
——Rising to BUSY Disabled
CS
200
ns
Table 9. Timing Specifications,
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XPT2046
Touch Screen Controller
15 Clocks-per-Conversion
Figure 16 provides the fastest way to clock the XPT2046.This method does not work with the serial interface of
most microcontrollers and digital signal processors, as they are generally not capable of providing 15 clock cycles
per serial transfer. However, this method can be used with field-programmable gate arrays (FPGAs) or
applicationspecific integrated circuits (ASICs). Note that this effectively increases the maximum conversion rate
of the converter beyond the values given in the specification tables, which assume 16 clock cycles per conversion.
Figure 16. Maximum Conversion Rate, 15 Clocks-per-Conversion
Data Format
The XPT2046 output data is in Straight Binary format, as shown in Figure 17. This figure shows the ideal output
code for the given input voltage and does not include the ffects of offset, gain, or noise.
Figure 17. Ideal Input Voltages and Output Codes
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XPT2046
Touch Screen Controller
8-Bit Conversion
The XPT2046 provides an 8-bit conversion mode that can be used when faster throughput is needed and the
digital result is not as critical. By switching to the 8-bit mode, a conversion is complete four clock cycles earlier.
Not only does this shorten each conversion by four bits (25% faster throughput), but each conversion can actually
occur at afaster clock rate. This is because the internal settling time of the XPT2046 is not as critical—settling to
better than 8 bits is all that is needed. The clock rate can be as much as 50% faster. The faster clock rate and fewer
clock cycles combine to provide a 2x increase in conversion rate.
Power Dissipation
There are two major power modes for the XPT2046: full-power (PD0 = 1) and auto power-down
(PD0 = 0). When operating at full speed and 16 clocks-per-conversion (see Figure 14), theXPT2046
spends most of the time acquiring or converting. There is little time for auto power-down, assuming
that this mode is active. Therefore, the difference between full-power mode and auto power-down is
negligible. If the conversion rate is decreased by slowing the frequency of the DCLK input, the two
modes remain approximately equal. However, if the DCLK frequency is kept at the maximum rate
during a conversion but conversions are done less often, the difference between the two modes is
dramatic.
Figure 18 shows the difference between reducing the DCLK frequency (scaling DCLK to match the
conversion rate) or maintaining DCLK at the highest frequency and reducing the number of
conversions per second. In the latter case, the converter spends an increasing percentage of time in
power-down mode (assuming the auto power-down mode is active).
Figure 18. Supply Current versus Directly Scaling the Frequency of DCLK with Sample Rate or
Maintaining DCLK at the Maximum Possible Frequency
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XPT2046
Touch Screen Controller
Another important consideration for power dissipation is the reference mode of the converter. In the single-ended
reference mode, the touch panel drivers are ON only when the analog input voltage is being acquired (see Figure
12 and Table 1). The external device (e.g., a resistive touch screen), therefore, is only powered during the
acquisition period. In the differential reference mode, the external device must be powered throughout the
acquisition and conversion periods (see Figure 12). If the conversion rate is high, this could substantially increase
power dissipation.CS also puts the XPT2046 into power-down mode. When CS goes high, the XPT2046
immediately goes into power-down mode and does not complete the current conversion. The internal reference,
however, does not turn off with CS going high. To turn the reference off, an additional write is required before CS
goes high (PD1 = 0).When the XPT2046 first powers up, the device draws about 20μA of current until a control
byte is written to it with PD0 = 0 to put it into power-down mode. This can be avoided if the XPT2046 is powered
up with CS = 0 and DCLK = IOVDD.
Demo
IOVCC
P1
1
2
3
4
5
6
R1 R2 R3 R4
J1
Socket
J3
Socket
IOVCC
14
IOVCC
P5 Header 4
ax+
ay+
axay-
4
3
2
1
P4
R13
R15
R16
R17
0
0
0
0
Header 4
ay4
ax3
ay+
2
ax+
1
104 104 106
J5
D1
W1
Jumper
Socket
LED3 R22
X+
Y+
XY-
15pen
12AUX
PENIRQ
AUX
C4 C5 C6 C7
104 104 104 104
J6
Socket
R14
51
R23
1K C10 104 C11 106
D2
LED3
W2
Socket
Jumper
W4
Jumper
J9
+VCC U4
5
+VCC
VBAT 11
VBAT
VREF 13
VREF
IOVCC14
Socket
X+
Y+
XY-
6
7
8
9
IOVDD
X+
Y+
XY-
A1
A2
A3
A4
A5
A6
A7
A8
GND
U3A
Socket
1
C8
104
C9
104
1
2
3
IOVCC
10
R18 R19 R20 R21
47K 47K 47K 47K
M74HC07M1R
2
U3B
3
U3D
4
9
M74HC07M1R
U3C
R24
100K
5
PENIRQ
AUX
GND
4
3
2
16
1
15
12
11
CS
DOUT
IOVCC
M74HC07M1R
J7
R25 R26 R27
1K 1K 1K
AUX
C14 104 C15 104
Socket
10
M74HC07M1R
U3F
BUSY
pen
8
M74HC07M1R
U3E
6
DCLK
DIN
J2
51
51
51
2
4
6
8
11
13
15
17
Header 3
DCLK
CS
DIN
DOUT
BUSY
R5
R7
R9
1
19
SN74LVC244ADB
P6
L1
47K 47K47K 47K
OE1
OE2
Y1
Y2
Y3
Y4
Y5
Y6
Y7
Y8
J4
C12 104 C13 106
Jumper
W3
18
16
14
12
9
7
5
3
Header 4
1K
100uH
J8
IOVCC U2
20
VCC
51
51
51
51
51
10
GND
TSC2046IRGVR
P3
ay4
ax3
ay+
2
ax+
1
IOVCC
+VCC
4 DCLK R6
3 CS R8
2 DIN R10
16DOUT R11
1 BUSY R12
DCLK
CS
DIN
DOUT
BUSY
IOVDD
6
7
8
9
X+
Y+
XY-
C1 C2 C3
Header 6
+VCC U1
5
+VCC
VBAT 11
VBAT
VREF13
VREF
13
12
M74HC07M1R
10
TSC2046IRGVR
Figure 19. Demo
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1
14
2
15
3
16
4
17
5
18
6
19
7
20
8
21
9
22
10
23
11
24
12
25
13
27
26
D Connector 25
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