MXB7843EUE [MAXIM]
2.375V to 5.25V, 4-Wire Touch-Screen; 2.375V至5.25V,4线触摸屏型号: | MXB7843EUE |
厂家: | MAXIM INTEGRATED PRODUCTS |
描述: | 2.375V to 5.25V, 4-Wire Touch-Screen |
文件: | 总21页 (文件大小:500K) |
中文: | 中文翻译 | 下载: | 下载PDF数据表文档文件 |
19-2435; Rev 0; 4/02
2.375V to 5.25V, 4-Wire Touch-Screen
Controller
General Description
Features
The MXB7843 is an industry-standard 4-wire touch-
screen controller. It contains a 12-bit sampling analog-
to-digital converter (ADC) with a synchronous serial
interface and low on-resistance switches for driving
resistive touch screens. The MXB7843 uses an external
reference. The MXB7843 can make absolute or ratio-
metric measurements. The MXB7843 has two auxiliary
ADC inputs. All analog inputs are fully ESD protected,
eliminating the need for external TransZorb™ devices.
o ESD-Protected ADC Inputs
±±15k IEC ꢀ±ꢁꢁꢁ-ꢂ-ꢃ Aꢄr-ꢅGp DꢄscꢆGrꢇe
±ꢈ5k IEC ꢀ±ꢁꢁꢁ-ꢂ-ꢃ ContGct DꢄscꢆGrꢇe
o Pꢄn CompGtꢄble wꢄtꢆ MXB7ꢈꢂꢀ
o +ꢃ.371k to +1.ꢃ1k Sꢄnꢇle Supply
o ꢂ-Wꢄre Toucꢆ-Screen InterfGce
o RGtꢄometrꢄc Conversꢄon
The MXB7843 is guaranteed to operate with a single
2.375V to 5.25V supply voltage. In shutdown mode, the
typical power consumption is reduced to under 0.5µW,
while the typical power consumption at 125ksps
throughput and a 2.7V supply is 650µW.
o SPI™/QSPI™, 3-Wꢄre SerꢄGl InterfGce
o ProꢇrGmmGble ꢈ-/±ꢃ-Bꢄt Resolutꢄon
o Two AuxꢄlꢄGry AnGloꢇ Inputs
o AutomGtꢄc Sꢆutdown Between Conversꢄons
Low-power operation makes the MXB7843 ideal for bat-
tery-operated systems, such as personal digital assis-
tants with resistive touch screens and other portable
equipment. The MXB7843 is available in 16-pin QSOP
and TSSOP packages, and is guaranteed over the
-40°C to +85°C temperature range.
o Low Power
ꢃ7ꢁµA Gt ±ꢃ15sps
±±1µA Gt 1ꢁ5sps
ꢃ1µA Gt ±ꢁ5sps
1µA Gt ±5sps
Applications
ꢃµA Sꢆutdown Current
Personal Digital Assistants
Portable Instruments
Point-of-Sales Terminals
Pagers
Ordering Information
Touch-Screen Monitors
Cellular Phones
PART
TEMP RANGE
-40°C to +85°C
-40°C to +85°C
PIN-PACKAGE
16 QSOP
MXB7843EEE
MXB7843EUE
16 TSSOP
Typical Application Circuit appears at end of data sheet.
Pin Configuration
TransZorb is a trademark of General Semiconductor Industries,
Inc.
TOP VIEW
SPI/QSPI are trademarks of National Semiconductor Corp.
V
DD
X+
Y+
X-
Y-
1
2
3
4
5
6
7
8
16 DCLK
15 CS
14 DIN
MXB7843
13 BUSY
12 DOUT
11 PENIRQ
GND
IN3
10
9
V
DD
IN4
REF
QSOP/TSSOP
________________________________________________________________ Maxim Integrated Products
±
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
2.375V to 5.25V, 4-Wire Touch-Screen
Controller
ABSOLUTE MAXIMUM RATINGS
Continuous Power Dissipation (T = +70°C)
V
DD
, DIN, CS, DCLK to GND ...................................-0.3V to +6V
A
16-Pin QSOP (derate 8.30mW/°C above +70°C).........667mW
16-Pin TSSOP (derate 5.70mW/°C above +70°C) .......456mW
Operating Temperature Range ...........................-40°C to +85°C
Junction Temperature......................................................+150°C
Storage Temperature Range.............................-65°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
Digital Outputs to GND...............................-0.3V to (V
+ 0.3V)
+ 0.3V)
DD
DD
V
, X+, X-, Y+, Y-, IN3, IN4 to GND........-0.3V to (V
REF
Maximum Current into Any Pin ......................................... 50mA
Maximum ESD per IEC-61000-4-2 (per MIL STD-883 HBM)
X+, X-, Y+, Y-, IN3, IN4...........................................15kV (4kV)
All Other Pins ..........................................................2kV (500V)
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(V
= 2.7V to 3.6V, V
= 2.5V, f
= 2MHz (50% duty cycle), f
= 125kHz, 12-bit mode, 0.1µF capacitor at REF, T =
SAMPLE A
DD
REF
DCLK
T
MIN
to T
, unless otherwise noted. Typical values are at T = +25°C.)
MAX A
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
12
UNITS
DC ACCURACY (Note 1)
Resolution
Bits
Bits
LSB
LSB
LSB
LSB
No Missing Codes
Relative Accuracy
Differential Nonlinearity
Offset Error
11
12
1
INL
(Note 2)
(Note 3)
2
DNL
1
6
4
Gain Error
Noise
70
µV
RMS
CONVERSION RATE
Conversion Time
Track/Hold Acquisition Time
Throughput Rate
Multiplexer Settling Time
Aperture Delay
t
12 clock cycles (Note 4)
3 clock cycles
6
µs
µs
CONV
t
1.5
ACQ
f
16 clock conversion
125
kHz
ns
SAMPLE
500
30
ns
Aperture Jitter
100
100
ps
Channel-to-Channel Isolation
Serial Clock Frequency
Duty Cycle
V
= 2.5V
at 50kHz
dB
MHz
%
IN
P-P
f
0.1
40
2.0
60
DCLK
ANALOG INPUT (X+, X-, Y+, Y-, IN3, IN4)
Input Voltage Range
0
V
V
REF
1
Input Capacitance
25
pF
µA
Input Leakage Current
SWITCH DRIVERS
On/off-leakage, V = 0 to V
IN
0.1
DD
Y+, X+
Y-, X-
7
9
On-Resistance (Note 5)
Ω
2
_______________________________________________________________________________________
2.375V to 5.25V, 4-Wire Touch-Screen
Controller
ELECTRICAL CHARACTERISTICS (continued)
(V
= 2.7V to 3.6V, V
= 2.5V, f
= 2MHz (50% duty cycle), f
= 125kHz, 12-bit mode, 0.1µF capacitor at REF, T
A
=
DD
REF
DCLK
SAMPLE
T
MIN
to T
, unless otherwise noted. Typical values are at T = +25°C.)
MAX A
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
REFERENCE (Reference applied to REF)
Reference Input Voltage Range
Input Resistance
(Note 6)
1
V
V
DD
5
GΩ
f
f
f
= 125kHz
13
40
3
SAMPLE
SAMPLE
Input Current
µA
= 12.5kHz
2.5
= 0
DCLK
DIGITAL INPUTS (DCLK, CS, DIN)
V
✕
DD
0.7
Input High Voltage
V
IH
V
Input Low Voltage
V
0.8
1
V
IL
Input Hysteresis
V
100
15
mV
µA
pF
HYST
Input Leakage Current
Input Capacitance
I
IN
C
IN
DIGITAL OUTPUT (DOUT, BUSY)
Output Voltage Low
V
I
I
= 250µA
0.4
V
V
OL
SINK
V
0.5
-
DD
Output Voltage High
V
= 250µA
OH
SOURCE
PENIRQ Output Low Voltage
Three-State Leakage Current
Three-State Output Capacitance
POWER REQUIREMENTS
Supply Voltage
V
50kΩ pullup to V
0.8
10
V
OL
DD
I
CS = V
CS = V
1
µA
pF
L
DD
DD
C
15
OUT
V
2.375
5.250
650
V
DD
f
f
f
= 125ksps
= 12.5ksps
= 0
270
220
150
SAMPLE
SAMPLE
SAMPLE
Supply Current
I
µA
DD
Shutdown Supply Current
I
DCLK = CS = V
3
µA
dB
SHDN
DD
Power-Supply Rejection Ratio
PSRR
V
= 2.7V to 3.6V full scale
70
DD
_______________________________________________________________________________________
3
2.375V to 5.25V, 4-Wire Touch-Screen
Controller
TIMING CHARACTERISTICS (Figure 1)
(V
= 2.7V to 3.6V, V
= 2.5V, f
= 2MHz (50% duty cycle), f
= 125kHz, 12-bit mode, 0.1µF capacitor at REF, T =
SAMPLE A
DD
REF
DCLK
T
MIN
to T
, unless otherwise noted. Typical values are at T = +25°C.)
MAX A
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
TIMING CHARACTERISTICS (Figure 1)
Acquisition Time
t
1.5
500
200
200
100
0
µs
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ACQ
DCLK Clock Period
t
CP
CH
DCLK Pulse Width High
DCLK Pulse Width Low
DIN-to-DCLK Setup Time
DIN-to-DCLK Hold Time
CS Fall-to-DCLK Rise Setup Time
CS Rise-to-DCLK Rise Ignore
DCLK Falling-to-DOUT Valid
CS Rise-to-DOUT Disable
CS Fall-to-DOUT Enable
DCLK Falling-to-BUSY Rising
CS Falling-to-BUSY Enable
CS Rise-to-BUSY Disable
t
t
CL
DS
DH
t
t
t
100
0
CSS
CSH
t
t
C
C
C
= 50pF
= 50pF
= 50pF
200
200
200
200
200
200
DO
LOAD
LOAD
LOAD
t
TR
DV
BD
t
t
t
BDV
t
BTR
Note 1: Tested at V
= +2.7V.
DD
Note 2: Relative accuracy is the deviation of the analog value at any code from its theoretical value after the full-scale range has
been calibrated.
Note 3: Offset nulled.
Note 4: Conversion time is defined as the number of clock cycles multiplied by the clock period; clock has 50% duty cycle.
Note 5: Resistance measured from the source to drain of the switch.
Note 6: ADC performance is limited by the conversion noise floor, typically 300µV . An external reference below 2.5V can com-
P-P
promise the ADC performance.
4
_______________________________________________________________________________________
2.375V to 5.25V, 4-Wire Touch-Screen
Controller
Typical Operating Characteristics
(V
= 2.7V, V
= 2.5V, f
= 2MHz, f
= 125kHz, C
= 50pF, 0.1µF capacitor at REF, T = +25°C, unless otherwise
LOAD A
DD
REF
DCLK
SAMPLE
noted.)
CHANGE IN OFFSET ERROR
vs. SUPPLY VOLTAGE
INTEGRAL NONLINEARITY
vs. DIGITAL OUTPUT CODE
DIFFERENTIAL NONLINEARITY
vs. DIGITAL OUTPUT CODE
0.5
0.4
0.3
0.2
0.1
0
1.0
0.8
2.0
1.5
1.0
0.5
0
0.6
0.4
0.2
0
-0.2
-0.5
-1.0
-1.5
-2.0
-0.1
-0.2
-0.3
-0.4
-0.4
-0.6
-0.8
-1.0
0
500 1000 1500 2000 2500 3000 3500 4000
OUTPUT CODE
0
500 1000 1500 2000 2500 3000 3500 4000
OUTPUT CODE
2.5
3.0
3.5
4.0
4.5
5.0
5.5
SUPPLY VOLTAGE (V)
CHANGE IN OFFSET ERROR
vs. TEMPERATURE
CHANGE IN GAIN ERROR
vs. SUPPLY VOLTAGE
CHANGE IN GAIN ERROR
vs. TEMPERATURE
1.0
0.5
0
3
2
1.0
0.5
1
0
0
-0.5
-1.0
-1.5
-2.0
-1
-2
-3
-0.5
-1.0
-40 -25 -10
5
20 35 50 65 80
2.5
3.0
3.5
4.0
4.5
5.0
5.5
-40 -25 -10
5
20 35 50 65 80
TEMPERATURE (°C)
SUPPLY VOLTAGE (V)
TEMPERATURE (°C)
SWITCH ON-RESISTANCE vs. SUPPLY VOLTAGE
SWITCH ON-RESISTANCE vs. TEMPERATURE
(X+, Y+ : + V TO PIN; X-, Y- : TO GND)
(X+, Y+ : + V TO PIN; X-, Y- : PIN TO GND)
DD
DD
14
12
12
11
10
9
X-
X-
X+
Y+
10
8
Y-
7
8
Y-
X+
Y+
6
5
6
4
2
0
4
3
2
1
0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
-40 -25 -10
5
20 35 50 65 80
SUPPLY VOLTAGE (V)
TEMPERATURE (°C)
_______________________________________________________________________________________
5
2.375V to 5.25V, 4-Wire Touch-Screen
Controller
Typical Operating Characteristics (continued)
(V
= 2.7V, V
= 2.5V, f
= 2MHz, f
= 125kHz, C
= 50pF, 0.1µF capacitor at REF, T = +25°C, unless otherwise
LOAD A
DD
REF
DCLK
SAMPLE
noted.)
REFERENCE CURRENT
vs. SUPPLY VOLTAGE
REFERENCE CURRENT vs. TEMPERATURE
REFERENCE CURRENT vs. SAMPLE RATE
8.3
8.2
8.1
8.0
7.9
7.8
7.7
8.3
8.2
8.1
8.0
7.9
7.8
7.7
10
9
8
7
6
5
4
3
2
1
0
C = 0.1µF
L
f
= 125kHz
SAMPLE
V
= 2.7V
DD
C = 0.1µF
L
f
= 125kHz
SAMPLE
2.5
3.0
3.5
4.0
4.5
5.0
5.5
-40 -25 -10
5
20 35 50 65 80
0
25
50
75
100
125
SUPPLY VOLTAGE (V)
TEMPERATURE (°C)
SAMPLE RATE (kHz)
SUPPLY CURRENT
vs. SUPPLY VOLTAGE
SUPPLY CURRENT vs. TEMPERATURE
SUPPLY CURRENT vs. SAMPLE RATE
250
225
200
175
150
290
285
280
275
270
265
260
255
250
250
V
V
= 2.7V
= 2.5V
f
= 12.5kHz
DD
REF
f
= 125kHz
SAMPLE
SAMPLE
V
= 2.7V
DD
225
200
175
150
125
100
2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
SUPPLY VOLTAGE (V)
-40 -25 -10
5
20 35 50 65 80
0
25
50
75
100
125
TEMPERATURE (°C)
SAMPLE RATE (kHz)
MAXIMUM SAMPLE RATE
vs. SUPPLY VOLTAGE
SHUTDOWN CURRENT
vs. SUPPLY VOLTAGE
SHUTDOWN CURRENT vs. TEMPERATURE
120
110
100
90
300
250
200
150
100
50
1000
100
10
DCLK = CS = V
DD
DLCK = CS = V
DD
80
70
60
50
1
-40 -25 -10
5
20 35 50 65 80
2.7
3.2
3.7
4.2
4.7
5.2
2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
SUPPLY VOLTAGE (V)
TEMPERATURE (°C)
SUPPLY VOLTAGE (V)
6
_______________________________________________________________________________________
2.375V to 5.25V, 4-Wire Touch-Screen
Controller
Pin Description
PIN
1
NAME
FUNCTION
Positive Supply Voltage. Connect to pin 10.
X+ Position Input, ADC Input Channel 1
Y+ Position Input, ADC Input Channel 2
X- Position Input
V
DD
2
X+
3
Y+
X-
4
5
Y-
Y- Position Input
6
GND
IN3
IN4
Ground
7
Auxiliary Input to ADC; ADC Input Channel 3
Auxiliary Input to ADC; ADC Input Channel 4
8
Voltage Reference Input. Reference voltage for analog-to-digital conversion. Apply a reference
voltage between 1V and V . Bypass REF to GND with a 0.1µF capacitor.
DD
9
REF
10
11
V
Positive Supply Voltage, +2.375V to +5.25V. Bypass with a 1µF capacitor. Connect to pin 1.
DD
PENIRQ
Pen Interrupt Output. Open anode output. 10kΩ to 100kΩ pullup resistor required to V
.
DD
Serial Data Output. Data changes state on the falling edge of DCLK. High impedance when CS is
HIGH.
12
DOUT
Busy Output. BUSY pulses high for one clock period before the MSB decision. High impedance when
CS is HIGH.
13
14
15
BUSY
DIN
CS
Serial Data Input. Data clocked in on the rising edge of DCLK.
Active-Low Chip Select. Data is only clocked into DIN when CS is low. When CS is high, DOUT and
BUSY are high impedance.
Serial Clock Input. Clocks data in and out of the serial interface and sets the conversion speed (duty
cycle must be 40% to 60%).
16
DCLK
_______________________________________________________________________________________
7
2.375V to 5.25V, 4-Wire Touch-Screen
Controller
The time required for the T/H to acquire an input signal
Detailed Description
is a function of how quickly its input capacitance is
charged. If the input signal’s source impedance is high,
the acquisition time lengthens, and more time must be
allowed between conversions. The acquisition time
The MXB7843 uses a successive-approximation conver-
sion technique to convert analog signals to a 12-bit digital
output. An SPI/QSPI/MICROWIRE™-compatible serial
interface provides an easy communication to a micro-
processor (µP). It features a 4-wire touch-screen interface
and two auxiliary ADC channels (Functional Diagram).
(t
) is the maximum time the device takes to acquire
ACQ
the input signal to 12-bit accuracy. Calculate t
the following equation:
with
ACQ
Analog Inputs
Figure 2 shows a block diagram of the analog input sec-
tion that includes the input multiplexer of the MXB7843,
the differential signal inputs of the ADC, and the differ-
ential reference inputs of the ADC. The input multiplexer
switches between X+, X-, Y+, Y-, IN3, and IN4.
t
= 8.4 × R + R
× 25pF
(
)
ACQ
S
IN
where R = 2kΩ and R is the source impedance of
IN
S
the input signal.
Source impedances below 1kΩ do not significantly
affect the ADC’s performance. Accommodate higher
source impedances by either slowing down DCLK or
by placing a 1µF capacitor between the analog input
and GND.
In single-ended mode, conversions are performed using
REF as the reference. In differential mode, ratiometric
conversions are performed with REF+ connected to X+ or
Y+, and REF- connected to X- or Y-. Configure the refer-
ence and switching matrix according to Tables 1 and 2.
Input Bandwidth and Anti-Aliasing
The ADCs input tracking circuitry has a 25MHz small-
signal bandwidth, so it is possible to digitize high-
speed transient events. To avoid high-frequency sig-
nals being aliased into the frequency band of interest,
anti-alias filtering is recommended.
During the acquisition interval, the selected channel
charges the sampling capacitance. The acquisition
interval starts on the fifth falling clock edge and ends
on the eighth falling clock edge.
CS
t
CH
t
t
CP
CSS
t
CSH
t
CL
DCLK
t
t
DO
DS
t
DH
DIN
t
TR
t
DV
DOUT
t
t
BDV
BTR
BUSY
t
BD
Figure 1. Detailed Serial Interface Timing
MICROWIRE is a trademark of National Semiconductor Corp.
_______________________________________________________________________________________
8
2.375V to 5.25V, 4-Wire Touch-Screen
Controller
Functional Diagram
V
DD
X+
X-
DOUT
BUSY
Y+
Y-
PENIRQ
6-TO-1
MUX
SERIAL
DATA
INTERFACE
12-BIT ADC
DCLK
DIN
IN3
IN4
CS
REF
Table 1. Input Configuration, Single-Ended Reference Mode (SER/DFR HIGH)
A2
0
A1
0
A0
0
MEASUREMENT
Reserved
Y-Position
IN3
ADC INPUT CONNECTION
DRIVERS ON
Reserved
X+
—
Y+, Y-
—
0
0
1
0
1
0
IN3
0
1
1
Reserved
Reserved
X-Position
IN4
Reserved
Reserved
Y+
—
1
0
0
—
1
0
1
X-, X+
—
1
1
0
IN4
1
1
1
Reserved
Reserved
—
Table 2. Input Configuration, Differential Reference Mode (SER/DFR LOW)
ADC +REF
ADC -REF
ADC INPUT
MEASUREMENT
PERFORMED
A2
A1
A0
DRIVER ON
CONNECTION TO CONNECTION TO CONNECTION TO
0
1
0
0
1
1
Y+
X+
Y-
X-
X+
Y+
Y position
X position
Y+, Y-
X+, X-
_______________________________________________________________________________________
9
2.375V to 5.25V, 4-Wire Touch-Screen
Controller
Differential Measurement Mode
Analog Input Protection
Figure 4 shows the switching matrix configuration for
Y-coordinate measurement. The REF+ and REF- inputs
are connected directly to the Y+ and Y- pins, respec-
tively. Differential mode uses the voltage at the Y+ pin
as the REF+ voltage and voltage at the Y- pin as REF-
voltage. This conversion is ratiometric and independent
of the voltage drop across the drivers and variation in
the touch-screen resistance. In differential mode, the
touch screen remains biased during the acquisition and
conversion process. This results in additional supply
current and power dissipation during conversion when
compared to the absolute measurement mode.
Internal protection diodes, which clamp the analog
input to V
and GND, allow the analog input pins to
DD
swing from GND - 0.3V to V
+ 0.3V without damage.
DD
Analog inputs must not exceed V
by more than
DD
50mV or be lower than GND by more than 50mV for
accurate conversions. If an off-channel analog input
voltage exceeds the supplies, limit the input current to
50mA. The analog input pins are ESD protected to
8kV using the Contact-Discharge method and 15kV
using the Air-Gap method specified in IEC 61000-4-2.
Touch-Screen Conversion
The MXB7843 provides two conversion methods—dif-
ferential and single ended. The SER/DFR bit in the con-
trol word selects either mode. A logic 1 selects a
single-ended conversion, while a logic 0 selects a dif-
ferential conversion.
PEN Interrupt Request (PENIRQ)
Figure 5 shows the block diagram for the PENIRQ func-
tion. When used, PENIRQ requires a 10kΩ to 100kΩ
pullup to +V . If enabled, PENIRQ goes low whenever
DD
the touch screen is touched. The PENIRQ output can
be used to initiate an interrupt to the microprocessor,
which can write a control word to the MXB7843 to start
a conversion.
Differential vs. Single Ended
Changes in operating conditions can degrade the accu-
racy and repeatability of touch-screen measurements.
Therefore, the conversion results representing X and Y
coordinates may be incorrect. For example, in single-
ended measurement mode, variation in the touch-
screen driver voltage drops results in incorrect input
reading. Differential mode minimizes these errors.
Figure 6 shows the timing diagram for the PENIRQ pin
function. The diagram shows that once the screen is
touched while CS is high, the PENIRQ output goes low
after a time period indicated by t
. The t
TOUCH
TOUCH
value changes for different touch-screen parasitic
capacitance and resistance. The microprocessor
receives this interrupt and pulls CS low to initiate a con-
version. At this instant, the PENIRQ pin should be
masked, as transitions can occur due to a selected
input channel or the conversion mode. The PENIRQ pin
functionality becomes valid when either the last data bit
is clocked out, or CS is pulled high.
Single-Ended Mode
Figure 3 shows the switching matrix configuration for
Y-coordinate measurement in single-ended mode. The
MXB7843 measures the position of the pointing device by
connecting X+ to IN+ of the ADC, enabling Y+ and Y- dri-
vers, and digitizing the voltage on X+. The ADC performs
a conversion with REF+ = REF and REF- = GND. In sin-
gle-ended measurement mode, the bias to the touch
screen can be turned off after the acquisition to save
power. The on-resistance of the X and Y drivers results in
a gain error in single-ended measurement mode. Touch-
screen resistance ranges from 200Ω to 900Ω (depending
on the manufacturer), whereas the on-resistance of the X
and Y drivers is 8Ω (typ). Limit the touch-screen current to
less than 50mA by using a touch screen with a resistance
higher than 100Ω. The resistive divider created by the
touch screen and the on-resistance of the X and Y drivers
result in both an offset and a gain shift. Also, the on-resis-
tance of the X and Y drivers does not track the resistance
of the touch screen over temperature and supply. This
results in further measurement errors.
External Reference
During conversion, an external reference at REF must
deliver up to 40µA DC load current. If the reference has
a higher output impedance or is noisy, bypass it close
to the REF pin with a 0.1µF and a 4.7µF capacitor.
10 ______________________________________________________________________________________
2.375V to 5.25V, 4-Wire Touch-Screen
Controller
PENIRQ
V
REF
DD
A2–A0
(SHOWN 001 )
B
SER/DFR
(SHOWN HIGH)
X+
X-
Y+
Y-
+REF
CONVERTER
+IN
-IN
-REF
IN3
IN4
GND
Figure 2. Equivalent Input Circuit
V
V
DD
DD
Y+
X+
Y-
Y+
X+
Y-
REF
+IN
-IN
REF+
12-BIT ADC
+IN
REF+
12-BIT ADC
REF-
REF-
-IN
GND
GND
Figure 3. Single-Ended Y-Coordinate Measurement
Figure 4. Ratiometric Y-Coordinate Measurement
______________________________________________________________________________________ 11
2.375V to 5.25V, 4-Wire Touch-Screen
Controller
V
DD
100kΩ
OPEN CIRCUIT
Y+
PENIRQ
TOUCH SCREEN
X+
Y-
ON
PENIRQ
ENABLE
Figure 5. PENIRQ Functional Block Diagram
SCREEN TOUCHED HERE
PENIRQ
CS
DCLK
DIN
1
2
3
4
5
6
7
8
1
2
3
12
13
14
15
16
S
A2
A1
A0
M
S/D PD1 PD0
INTERRUPT PROCESSOR
NO RESPONSE TO TOUCHMASK PENIRQ
PENIRQ ENABLED
t
TOUCH
Figure 6. PENIRQ Timing Diagram
12 ______________________________________________________________________________________
2.375V to 5.25V, 4-Wire Touch-Screen
Controller
Digital Interface
Initialization After Power-Up and Starting a
Conversion
Digital Output
The MXB7843 outputs data in straight binary format
(Figure 10). Data is clocked out on the falling edge of
the DCLK, MSB first.
The digital interface consists of three inputs, DIN, DCLK,
CS, and one output, DOUT. A logic-high on CS disables
the MXB7843 digital interface and places DOUT in a
high-impedance state. Pulling CS low enables the
MXB7843 digital interface.
Serial Clock
The external clock not only shifts data in and out, but it
also drives the analog-to-digital conversion steps.
BUSY pulses high for one clock period after the last bit
of the control byte. Successive-approximation bit deci-
sions are made and appear at DOUT on each of the
next 12 DCLK falling edges. BUSY and DOUT go into a
high-impedance state when CS goes high.
Start a conversion by clocking a control byte into DIN
(Table 3) with CS low. Each rising edge on DCLK
clocks a bit from DIN into the MXB7843’s internal shift
register. After CS falls, the first arriving logic 1 bit
defines the control byte’s START bit. Until the START bit
arrives, any number of logic 0 bits can be clocked into
DIN with no effect.
The conversion must complete in 500µs or less; if not,
droop on the sample-and-hold capacitors can degrade
conversion results.
The MXB7843 is compatible with SPI/QSPI/MICROWIRE
devices. For SPI, select the correct clock polarity and
sampling edge in the SPI control registers of the micro-
controller: set CPOL = 0 and CPHA = 0. MICROWIRE,
SPI, and QSPI all transmit a byte and receive a byte at
the same time. The simplest software interface requires
only three 8-bit transfers to perform a conversion (one 8-
bit transfer to configure the ADC, and two more 8-bit
transfers to read the conversion result) (Figure 7).
Data Framing
The falling edge of CS does not start a conversion. The
first logic high clocked into DIN is interpreted as a start
bit and defines the first bit of the control byte. A conver-
sion starts on DCLK’s falling edge, after the eighth bit
of the control byte is clocked into DIN.
The first logic 1 clocked into DIN after bit 6 of a conver-
sion in progress is clocked onto the DOUT pin and is
treated as a START bit (Figure 8).
Simple Software Interface
Make sure the CPU’s serial interface runs in master
mode so the CPU generates the serial clock. Choose a
clock frequency from 500kHz to 2MHz:
Once a start bit has been recognized, the current con-
version must be completed.
The fastest the MXB7843 can run with CS held continu-
ously low is 15 clock conversions. Figure 8 shows the
serial-interface timing necessary to perform a conver-
sion every 15 DCLK cycles. If CS is connected low and
DCLK is continuous, guarantee a start bit by first clock-
ing in 16 zeros.
1) Set up the control byte and call it TB. TB should be
in the format: 1XXXXXXX binary, where X denotes
the particular channel, selected conversion mode,
and power mode (Tables 3, 4).
2) Use a general-purpose I/O line on the CPU to pull
Most microcontrollers (µCs) require that data transfers
occur in multiples of eight DCLK cycles; 16 clocks per
conversion is typically the fastest that a µC can drive the
MXB7843. Figure 9 shows the serial-interface timing nec-
essary to perform a conversion every 16 DCLK cycles.
CS low.
3) Transmit TB and simultaneously receive a byte; call
it RB1.
4) Transmit a byte of all zeros ($00 hex) and simultane-
ously receive byte RB2.
8-Bit Conversion
The MXB7843 provides an 8-bit conversion mode
selected by setting the MODE bit in the control byte
high. In the 8-bit mode, conversions complete four
clock cycles earlier than in the 12-bit output mode,
resulting in 25% faster throughput. This can be used in
conjunction with serial interfaces that provide 12-bit
transfers, or two conversions could be accomplished
with three 8-bit transfers. Not only does this shorten each
conversion by 4 bits, but each conversion can also
5) Transmit a byte of all zeros ($00 hex) and simultane-
ously receive byte RB3.
6) Pull CS high.
Figure 7 shows the timing for this sequence. Bytes RB2
and RB3 contain the result of the conversion, padded
by four trailing zeros. The total conversion time is a func-
tion of the serial-clock frequency and the amount of idle
timing between 8-bit transfers.
______________________________________________________________________________________ 13
2.375V to 5.25V, 4-Wire Touch-Screen
Controller
CS
TB
t
RB2
RB3
ACQ
DCLK
DIN
1
4
8
9
12
16
20
24
SER/
DFR
S
A2
IDLE
A1
A0 MODE
PD1 PD0
ACQUIRE
CONVERSION
IDLE
(START)
BUSY
DOUT
RB1
11
10
9
8
7
6
5
4
3
2
1
0
(MSB)
(LSB)
A/D STATE
IDLE
CONVERSION
IDLE
ACQUIRE
DRIVERS1 AND 2
(SER/DFR HIGH)
OFF
OFF
ON
OFF
ON
DRIVERS1 AND 2
(SER/DFR LOW)
OFF
Figure 7. Conversion Timing, 24-Clock per Conversion, 8-Bit Bus Interface
CS
1
8
15
1
8
15
1
DCLK
DIN
S
CONTROL BYTE 0
S
CONTROL BYTE 1
S
CONTROL BYTE 2
B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0
CONVERSION RESULT 1
B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0
CONVERSION RESULT 0
DOUT
BUSY
Figure 8. 15-Clock/Conversion Timing
occur at a faster clock rate since settling to better than 8
bits is all that is required. The clock rate can be as much
as 25% faster. The faster clock rate and fewer clock
cycles combine to increase the conversion rate.
Applications Information
Basic Operation of the MXB7843
The 4-wire touch-screen controller works by creating a
voltage gradient across the vertical or horizontal resis-
tive network connected to the MXB7843, as shown in
the Typical Application Circuit. The touch screen is
biased through internal MOSFET switches that connect
Data Format
The MXB7843 output data is in straight binary format as
shown in Figure 10. This figure shows the ideal output
code for the given input voltage and does not include
the effects of offset, gain, or noise.
each resistive layer to V
and ground on an alternate
DD
basis. For example, to measure the Y position when a
14 ______________________________________________________________________________________
2.375V to 5.25V, 4-Wire Touch-Screen
Controller
. . .
CS
1
8
16
1
8
16
. . .
. . .
DCLK
DIN
S
CONTROL BYTE 1
S
CONTROL BYTE 0
B6 . . .
. . .
B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0
CONVERSION RESULT 0
B11 B10 B9 B8 B7
CONVERSION RESULT 1
DOUT
BUSY
Figure 9. 16-Clock/Conversion Timing
Table 3. Control Byte Format
BIT 7
START
BIT 6
A2
BIT 5
A1
BIT 4
A0
BIT 3
MODE
BIT 2
SER/DFR
BIT 1
PD1
BIT 0
PD0
BIT
7
NAME
START
A2
DESCRIPTION
Start bit
6
5
A1
Address (Tables 1 and 2)
4
A0
3
MODE
SER/DFR
PD1
Conversion resolution. 0 = 8-Bits, 1 = 12-Bits.
2
Conversion mode. 1 = single ended, 0 = differential.
1
Power-down mode (Table 4)
0
PD0
Table 4. Power Mode Selection
SUPPLY CURRENT (typ) (µA)
PD1
PD0
PENIRQ
STATUS
DURING
CONVERSION
200
AFTER
CONVERSION
1
0
0
1
1
0
1
0
1
Enabled
Disabled
Disabled
Disabled
ADC is ON during conversion, OFF between conversion
ADC is always ON
Reserved
200
—
200
—
ADC is always ON
200
200
______________________________________________________________________________________ 15
2.375V to 5.25V, 4-Wire Touch-Screen
Controller
pointing device presses on the touch screen, the Y+
and Y- drivers are turned on, connecting one side of
the vertical resistive layer to V
and the other side to
OUTPUT CODE
DD
ground. In this case, the horizontal resistive layer func-
tions as a sense line. One side of this resistive layer
gets connected to the X+ input, while the other side is
left open or floating. The point where the touch screen
is pressed brings the two resistive layers in contact and
forms a voltage-divider at that point. The data converter
senses the voltage at the point of contact through the
X+ input and digitizes it. The horizontal layer resistance
does not introduce any error in the conversion because
no DC current is drawn.
FULL-SCALE
TRANSITION
11…111
11…110
11…101
FS = (V
(V
- V
- V
)
REF-
REF+
)
REF+
REF-
1LSB =
4096
The conversion process of the analog input voltage to
digital output is controlled through the serial interface
between the A/D converter and the µP. The processor
controls the MXB7843 configuration through a control
byte (Tables 3 and 4). Once the processor instructs the
MXB7843 to initiate a conversion, the MXB7843 biases
the touch screen through the internal switches at the
beginning of the acquisition period. The voltage transient
at the touch screen needs to settle down to a stable volt-
age before the acquisition period is over. After the acqui-
sition period is over, the A/D converter goes into a
conversion period with all internal switches turned off if
the device is in single-ended mode. If the device is in
differential mode, the internal switches remain on from
the start of the acquisition period to the end of the con-
version period.
00…011
00…010
00…001
00…000
0
1
2
3
FS
FS-3/2LSB
INPUT VOLTAGE (LSB) = [(V ) - (V )]
+IN
-IN
Figure 10. Ideal Input Voltages and Output Codes
PD1 = 0 and PD0 = 0. When exiting software shutdown,
the MXB7843 is ready to perform a conversion in 10µs.
Power-On Reset
When power is first applied, internal power-on circuitry
resets the MXB7843. Allow 10µs for the first conversion
after the power supplies stabilize. If CS is low, the first
logic 1 on DIN is interpreted as a start bit. Until a con-
version takes place, DOUT shifts out zeros.
PD1 = 1, PD0 = 1
In this mode, the MXB7843 is always powered. The
device remains fully powered after the current conver-
sion completes.
PD1 = 0, PD0 = 0
In this mode, the MXB7843 powers down after the current
conversion completes or on the next rising edge of CS,
whichever occurs first. The next control byte received on
DIN powers up the MXB7843. At the start of a new con-
version, it instantly powers up. When each conversion is
finished, the part enters power-down mode, unless other-
wise indicated. The first conversion after the ADC returns
to full power is valid for differential conversions and sin-
gle-ended measurement conversions.
Power Modes
Save power by placing the converter in one of two low-
current operating modes or in full power-down between
conversions. Select the power-down mode through
PD1 and PD0 of the control byte (Tables 3 and 4).
The software power-down modes take effect after the
conversion is completed. The serial interface remains
active while waiting for a new control byte to start a con-
version and switches to full-power mode. After complet-
ing its conversion, the MXB7843 enters the programmed
power mode until a new control byte is received.
When operating at full speed and 16 clocks per conver-
sion, the difference in power consumption between
PD1 = 0, PD0 = 1, and PD1 = 0, PD0 = 0 is negligible.
Also, in the case where the conversion rate is
decreased by slowing the frequency of the DCLK input,
the power consumption between these two modes is
not very different. When the DCLK frequency is kept at
The power-up wait before conversion period is depen-
dent on the power-down state. When exiting software
low-power modes, conversion can start immediately
when running at decreased clock rates. Upon power-
on reset, the MXB7843 is in power-down mode with
16 ______________________________________________________________________________________
2.375V to 5.25V, 4-Wire Touch-Screen
Controller
the maximum rate during a conversion, conversions are
done less often. There is a significant difference in
power consumption between these two modes.
8-bit data stream contains the first 8-bits of the current
conversion, starting with the MSB. The second 8-bit
data stream contains the remaining 4 result bits fol-
lowed by 4 trailing zeros. DOUT then goes high imped-
ance when CS goes high.
PD1 = 0, PD0 = 1
In this mode, the MXB7843 is powered down. This
mode becomes active after the current conversion
completes or on the next rising edge of CS, whichever
occurs first. The next command byte received on the
DIN returns the MXB7843 to full power. The first conver-
sion after the ADC returns to full power is valid.
QSPI/SPI Interface
The MXB7843 can be used with the QSPI/SPI interface
using the circuit in Figure 12 with CPOL = 0 and CPHA
= 0. This interface can be programmed to do a conver-
sion on any analog input of the MXB7843.
PD1 = 1, PD0 = 0
TMS320LC3x Interface
Figure 13 shows an example circuit to interface the
MXB7843 to the TMS320. The timing diagram for this
interface circuit is shown in Figure 14.
This mode is reserved.
Hardware Power-Down
CS also places the MXB7843 into power-down. When
CS goes HIGH, the MXB7843 immediately powers
down and aborts the current conversion.
Use the following steps to initiate a conversion in the
MXB7843 and to read the results:
1) The TMS320 should be configured with CLKX (trans-
mit clock) as an active-high output clock and CLKR
(TMS320 receive clock) as an active-high input
clock. CLKX and CLKR on the TMS320 are connect-
ed to the MXB7843 DCLK input.
Touch-Screen Settling
There are two key touch-screen characteristics that can
degrade accuracy. First, the parasitic capacitance
between the top and bottom layers of the touch screen
can result in electrical ringing. Second, vibration of the
top layer of the touch screen can cause mechanical
contact bouncing.
2) The MXB7843’s CS pin is driven low by the
TMS320’s XF I/O port to enable data to be clocked
into the MXB7843’s DIN pin.
External filter capacitors may be required across the
touch screen to filter noise induced by the LCD panel
or backlight circuitry, etc. These capacitors lengthen
the settling time required when the panel is touched
and can result in a gain error, as the input signal may
not settle to its final steady-state value before the ADC
samples the inputs. Two methods to minimize or elimi-
nate this issue are described below.
3) An 8-bit word (1XXXXXXX) should be written to the
MXB7843 to initiate a conversion and place the
device into normal operating mode. See Table 3 to
select the proper XXXXXXX bit values for your spe-
cific application.
4) The MXB7843’s BUSY output is monitored through
the TMS320’s FSR input. A falling edge on the
BUSY output indicates that the conversion is in
progress and data is ready to be received from the
devices.
One option is to lengthen the acquisition time by stopping
or slowing down DCLK, allowing for the required touch-
screen settling time. This method solves the settling time
problem for both single-ended and differential modes.
5) The TMS320 reads in 1 data bit on each of the next
16 rising edges of DCLK. These bits represent the
12-bit conversion result followed by 4 trailing bits.
The second option is to operate the MXB7843 in the dif-
ferential mode only for the touch screen, and perform
additional conversions with the same address until the
input signal settles. The MXB7843 can then be placed
in the power-down state on the last measurement.
6) Pull CS high to disable the MXB7843 until the next
conversion is initiated.
Layout, Grounding, and Bypassing
For best performance, use printed circuit (PC) boards
with good layouts; wire-wrap boards are not recommend-
ed. Board layout should ensure that digital and analog
signal lines are separated from each other. Do not run
analog and digital (especially clock) lines parallel to one
another, or digital lines underneath the ADC package.
Connection to Standard Interface
MICROWIRE Interface
When using the MICROWIRE- (Figure 11) or SPI-com-
patible interface (Figure 12), set the CPOL = CPHA = 0.
Two consecutive 8-bit readings are necessary to obtain
the entire 12-bit result from the ADC. DOUT data transi-
tions occur on the serial clock’s falling edge and are
clocked into the µP on the DCLK’s rising edge. The first
Establish a single-point analog ground (star ground
point) at GND. Connect all analog grounds to the star
______________________________________________________________________________________ 17
2.375V to 5.25V, 4-Wire Touch-Screen
Controller
I/O
SCK
CS
I/O
SCK
CS
DCLK
DOUT
DCLK
DOUT
MISO
MISO
MICROWIRE
QSPI/SPI
MXB7843
MXB7843
MOSI
DIN
MOSI
DIN
MASKABLE
INTERRUPT
BUSY
MASKABLE
INTERRUPT
BUSY
Figure 12. QSPI/SPI Interface
Figure 11. MICROWIRE Interface
ground. Connect the digital system ground to the star
ground at this point only. For lowest noise operation,
minimize the length of the ground return to the star
ground’s power supply.
XF
CLKX
CLKR
CS
SCLK
Power-supply decoupling is also crucial for optimal
device performance. Analog supplies can be decou-
pled by placing a 10µF tantalum capacitor in parallel
with a 0.1µF capacitor bypassed to GND. To maximize
performance, place these capacitors as close as possi-
ble to the supply pin of the device. Minimize capacitor
lead length for best supply-noise rejection. If the supply
is very noisy, a 10Ω resistor can be connected in series
as a lowpass filter.
TMS320LC3x
MXB7843
DIN
DX
DR
DOUT
BUSY
FSR
Figure 13. TMS320 Serial Interface
While using the MXB7843, the interconnection between
the converter and the touch screen should be as short
as possible. Since touch screens have low resistance,
longer or loose connections may introduce error. Noise
can also be a major source of error in touch-screen
applications (e.g., applications that require a backlight
LCD panel). EMI noise coupled through the LCD panel
to the touch screen may cause flickering of the convert-
ed data. Utilizing a touch screen with a bottom-side
metal layer connected to ground decouples the noise
to ground. In addition, the filter capacitors from Y+, Y-,
X+, and X- inputs to ground also help further reduce
the noise. Caution should be observed for settling time
of the touch screen, especially operating in the single-
ended measurement mode and at high data rates.
static linearity parameters for the MXB7843 are mea-
sured using the end-point method.
Differential Nonlinearity
Differential nonlinearity (DNL) is the difference between
an actual step width and the ideal value of 1LSB. A
DNL error specification of less than 1LSB guarantees
no missing codes and a monotonic transfer function.
Aperture Jitter
Aperture jitter (t ) is the sample-to-sample variation in
AJ
the time between the samples.
Aperture Delay
Aperture delay (t ) is the time defined between the
AD
falling edge of the sampling clock and the instant when
an actual sample is taken.
Definitions
Chip Information
TRANSISTOR COUNT: 12,000
Integral Nonlinearity
Integral nonlinearity (INL) is the deviation of the values
on an actual transfer function from a straight line. This
straight line can be either a best-straight-line fit or a line
drawn between the endpoints of the transfer function,
once offset and gain errors have been nullified. The
PROCESS: 0.6µm BiCMOS
18 ______________________________________________________________________________________
2.375V to 5.25V, 4-Wire Touch-Screen
Controller
CS
DCLK
DIN
START
A2
A1
A0
MODE
SER/DEF
PD1
PD0
BUSY
DOUT
HIGH IMPEDANCE
HIGH IMPEDANCE
MSB
B10
B1
B0
Figure 14. MXB7843-to-TMS320 Serial Interface Timing Diagram
Typical Application Circuit
2.375V TO 5.5V
1µF TO 10µF
0.1µF
OPTIONAL
1
2
3
4
V
DCLK 16
CS 15
SERIAL/CONVERSION CLOCK
CHIP SELECT
DD
X+
SERIAL DATA IN
Y+
DIN 14
CONVERTER STATUS
SERIAL DATA OUT
PEN INTERRUPT
X-
MXB7843 BUSY 13
DOUT 12
TOUCH
SCREEN
5
6
7
8
Y-
GND
IN3
IN4
PENIRQ 11
V
10
9
DD
50kΩ
REF
0.1µF
______________________________________________________________________________________ 19
2.375V to 5.25V, 4-Wire Touch-Screen
Controller
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information,
go to www.maxim-ic.com/packages.)
20 ______________________________________________________________________________________
2.375V to 5.25V, 4-Wire Touch-Screen
Controller
Package Information (continued)
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information,
go to www.maxim-ic.com/packages.)
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 21
© 2002 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.
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