MXB7846EEE+T [MAXIM]
暂无描述;型号: | MXB7846EEE+T |
厂家: | MAXIM INTEGRATED PRODUCTS |
描述: | 暂无描述 传感器 温度传感器 控制器 |
文件: | 总23页 (文件大小:964K) |
中文: | 中文翻译 | 下载: | 下载PDF数据表文档文件 |
19-2436; Rev 1; 5/04
2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor
General Description
Features
The MXB7846 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 MXB7846 uses an internal
+2.5V reference or an external reference. The
MXB7846 can make absolute or ratiometric measure-
ments. In addition, this device has an on-chip tempera-
ture sensor, a battery-monitoring channel, and has the
ability to perform touch-pressure measurements without
external components. The MXB7846 has one auxiliary
ADC input. All analog inputs are fully ESD protected,
eliminating the need for external TransZorb™ devices.
♦ ESD-Protected ADC Inputs
±15kV IEC 61000-4-2 Air-Gap Discharge
±8kV IEC 61000-4-2 Contact Discharge
♦ Pin Compatible with MXB7843
♦ +2.375V to +5.25V Single Supply
♦ Internal +2.5V Reference
♦ Direct Battery Measurement (0 to 6V)
♦ On-Chip Temperature Measurement
♦ Touch-Pressure Measurement
♦ 4-Wire Touch-Screen Interface
♦ Ratiometric Conversion
The MXB7846 is guaranteed to operate with a supply
voltage down to +2.375V when used with an external
reference or +2.7V with an internal reference. In shut-
down 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.
♦ SPI™/QSPI™, 3-Wire Serial Interface
♦ Programmable 8-/12-Bit Resolution
♦ Auxiliary Analog Input
♦ Automatic Shutdown Between Conversions
Low-power operation makes the MXB7846 ideal for bat-
tery-operated systems, such as personal digital assis-
tants with resistive touch screens and other portable
equipment. The MXB7846 is available in 16-pin QSOP
and TSSOP packages, and is guaranteed over the
-40°C to +85°C temperature range.
♦ Low Power (External Reference)
270µA at 125ksps
115µA at 50ksps
25µA at 10ksps
5µA at 1ksps
2µA Shutdown Current
Applications
Personal Digital Assistants
Portable Instruments
Point-of-Sales Terminals
Pagers
Ordering Information
PART
TEMP RANGE
-40°C to +85°C
-40°C to +85°C
PIN-PACKAGE
16 QSOP
MXB7846EEE
MXB7846EUE
16 TSSOP
Touch-Screen Monitors
Cellular Phones
Pin Configuration
Typical Application Circuit appears at end of data sheet.
TOP VIEW
V
DD
X+
Y+
X-
Y-
1
2
3
4
5
6
7
8
16 DCLK
15 CS
TransZorb is a trademark of Vishay Intertechnology, Inc.
SPI/QSPI are trademarks of Motorola, Inc.
14 DIN
MXB7846
13 BUSY
12 DOUT
11 PENIRQ
GND
BAT
AUX
10
9
V
DD
REF
QSOP/TSSOP
________________________________________________________________ Maxim Integrated Products
1
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
with Internal Reference and Temperature Sensor
ABSOLUTE MAXIMUM RATINGS
Continuous Power Dissipation (T = +70°C)
A
V
DD
, VBAT, DIN, CS, DCLK to GND ........................-0.3V to +6V
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-, AUX 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-, VBAT, AUX......................................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
Including internal reference
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-, AUX)
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
INTERNAL REFERENCE
Reference Output Voltage
V
V
= 2.7V to 5.25V, T = +25°C
2.45
2.50
50
2.55
V
REF
DD
A
REF Output Tempco
TCV
ppm°/C
mA
REF
REF Short-Circuit Current
REF Output Impedance
18
250
Ω
2
_______________________________________________________________________________________
2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor
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 =
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
EXTERNAL REFERENCE (Internal reference disabled, reference applied to REF)
Reference Input Voltage Range
Input Resistance
(Note 7)
1
V
V
DD
1
GΩ
f
f
f
= 125kHz
= 12.5kHz
13
2.5
40
3
SAMPLE
SAMPLE
µA
Input Current
= 0
DCLK
BATTERY MONITOR (BAT)
Input Voltage Range
Input Resistance
0
6
V
During acquisition
= 2.5V
10
kΩ
V
±2
±3
REF
Accuracy
%
Internal reference
TEMPERATURE MEASUREMENT
Resolution
Differential method (Note 8)
Single-conversion method
Differential method (Note 8)
Single-conversion method
1.6
0.3
±2
°C
°C
°C
°C
Accuracy
±3
DIGITAL INPUTS (DCLK, CS, DIN)
Input High Voltage
♦
V
V
0.7
V
V
IH
DD
Input Low Voltage
V
0.8
1
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
Output Voltage High
V
= 250µA
V
- 0.5
DD
OH
SOURCE
PENIRQ Output Low Voltage
Three-State Leakage Current
Three-State Output Capacitance
POWER REQUIREMENTS
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
External reference
2.375
2.70
5.250
5.25
650
Supply Voltage
V
V
DD
DD
Internal reference
f
= 125ksps
= 12.5ksps
= 0
270
220
150
780
720
650
SAMPLE
SAMPLE
SAMPLE
SAMPLE
SAMPLE
SAMPLE
External
reference
µA
f
f
f
Supply Current
I
= 125ksps
= 12.5ksps
= 0
950
3
Internal
reference
f
µA
f
Shutdown Supply Current
I
DCLK = CS = V
µA
dB
SHDN
DD
Power-Supply Rejection Ratio
P
V
= 2.7V to 3.6V full scale
DD
70
SRR
_______________________________________________________________________________________
3
2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor
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: External load should not change during conversion for specified accuracy.
Note 7: 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.
Note 8: Difference between Temp0 and Temp1. No calibration necessary.
4
_______________________________________________________________________________________
2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor
Typical Operating Characteristics
(V
= 2.7V, V
= 2.5V
, f
= 2MHz, f
= 125kHz, C
= 50pF, 0.1µF capacitor at REF, T = +25°C, unless
DD
REF
EXTERNAL DCLK
SAMPLE
LOAD A
otherwise noted.)
INTEGRAL NONLINEARITY
vs. DIGITAL OUTPUT CODE
DIFFERENTIAL NONLINEARITY
vs. DIGITAL OUTPUT CODE
CHANGE IN OFFSET ERROR
vs. SUPPLY VOLTAGE
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
0
500 1000 1500 2000 2500 3000 3500 4000
OUTPUT CODE
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 GAIN ERROR
vs. SUPPLY VOLTAGE
CHANGE IN GAIN ERROR
vs. TEMPERATURE
CHANGE IN OFFSET ERROR
vs. TEMPERATURE
1.0
0.5
1.0
0.5
0
3
2
0
1
-0.5
-1.0
-1.5
-2.0
0
-1
-2
-3
-0.5
-1.0
-40 -25 -10
5
20 35 50 65 80
-40 -25 -10
5
20 35 50 65 80
2.5
3.0
3.5
4.0
4.5
5.0
5.5
TEMPERATURE (°C)
TEMPERATURE (°C)
SUPPLY VOLTAGE (V)
INTERNAL REFERENCE
vs. SUPPLY VOLTAGE
SWITCH ON-RESISTANCE vs. SUPPLY VOLTAGE
SWITCH ON-RESISTANCE vs. TEMPERATURE
(X+, Y+ : +V TO PIN; X-, Y- : PIN TO GND)
(X+, Y+ : +V TO PIN; X-, Y- : TO GND)
DD
DD
2.6
2.5
2.4
2.3
2.2
2.1
2.0
14
12
10
8
12
C = 0.1µf
L
11
10
9
X-
Y-
X-
X+
8
7
Y+
Y-
X+
Y+
6
6
5
4
4
3
2
2
1
0
0
2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
SUPPLY VOLTAGE (V)
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
with Internal Reference and Temperature Sensor
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
DD
REF
EXTERNAL DCLK
SAMPLE
LOAD A
otherwise noted.)
INTERNAL REFERENCE VOLTAGE
vs. TEMPERATURE
INTERNAL VOLTAGE REFERENCE
vs. TURN-ON TIME
INTERNAL VOLTAGE REFERENCE
vs. TURN-ON TIME
2.6
2.5
2.4
2.3
2.2
2.1
2.0
2.5
2.0
1.5
1.0
0.5
3.0
2.5
2.0
1.5
1.0
0.5
0
C = 1µF
V
= 2.7V
L
DD
NO CAPACITOR
(30µs) 12-BIT SETTLING
(1060µs) 12-BIT SETTLING
C = 0.1µF
L
0
0
-40 -25 -10
5
20 35 50 65 80
200 400 600 800 1000 1200
0
5
10 15 20 25 30 35 40
TEMPERATURE (°C)
TURN-ON TIME (µs)
TURN-ON TIME (µs)
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
EXTERNAL REFERENCE
C = 0.1µF
= 125kHz
EXTERNAL REFERENCE
L
f
SAMPLE
V
= 2.7V
DD
C = 0.1µF
L
f
= 125kHz
SAMPLE
EXTERNAL REFERENCE
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)
TEMP1 DIODE VOLTAGE
vs. SUPPLY VOLTAGE
TEMP0 DIODE VOLTAGE
vs. SUPPLY VOLTAGE
TEMP DIODE VOLTAGE
vs. TEMPERATURE
590
589
588
587
586
585
705
704
703
702
701
700
699
698
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
TEMP1
TEMP0
TEMP1
TEMP2
2.7
3.2
3.7
4.2
4.7
5.2
2.7
3.2
3.7
4.2
4.7
5.2
-40 -25 -10
5
20 35 50 65 80
SUPPLY VOLTAGE (V)
SUPPLY VOLTAGE (V)
TEMPERATURE (°C)
6
_______________________________________________________________________________________
2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor
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
DD
REF
EXTERNAL DCLK
SAMPLE
LOAD A
otherwise noted.)
SUPPLY CURRENT
vs. SUPPLY VOLTAGE
SUPPLY CURRENT vs. TEMPERATURE
SUPPLY CURRENT vs. SAMPLE RATE
290
285
280
275
270
265
260
255
250
250
225
200
175
150
125
100
250
225
200
175
150
V
V
= 2.7V
= 2.5V
DD
REF
f
V
= 125kHz
= 2.7V
f
= 12.5kHz
SAMPLE
DD
SAMPLE
-40 -25 -10
5
20 35 50 65 80
0
25
50
75
100
125
2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
SUPPLY VOLTAGE (V)
TEMPERATURE (°C)
SAMPLE RATE (kHz)
MAXIMUM SAMPLE RATE
vs. SUPPLY VOLTAGE
SHUTDOWN CURRENT
vs. SUPPLY VOLTAGE
SHUTDOWN CURRENT vs. TEMPERATURE
300
250
200
150
100
50
1000
100
10
120
110
100
90
DCLK = CS = V = 3V
DD
DCLK = CS = V
DD
80
70
60
1
50
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)
-40 -25 -10
5
20 35 50 65 80
SUPPLY VOLTAGE (V)
TEMPERATURE (°C)
_______________________________________________________________________________________
7
2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor
Pin Description
PIN
1
NAME
FUNCTION
V
Positive Supply Voltage. Connect to pin 10.
X+ Position Input, ADC Input Channel 1
Y+ Position Input, ADC Input Channel 2
X- Position Input
DD
2
X+
3
Y+
X-
4
5
Y-
Y- Position Input
6
GND
BAT
AUX
Ground
7
Battery Monitoring Inputs; ADC Input Channel 3
Auxiliary Input to ADC; ADC Input Channel 4
8
Voltage Reference Output/Input. Reference voltage for analog-to-digital conversion. In internal
reference mode, the reference buffer provides a 2.50V nominal output. In external reference mode,
9
REF
apply a reference voltage between 1V and V . Bypass REF to GND with a 0.1µF capacitor.
DD
Positive Supply Voltage, +2.375V (2.70V) to +5.25V. External (internal) reference. Bypass with a 1µF
capacitor. Connect to pin 1.
10
11
12
V
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.
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
CS
t
CH
t
t
CP
t
CSS
CSH
t
CL
DCLK
t
DO
t
DS
t
DH
DIN
t
TR
t
DV
DOUT
t
t
BDV
BTR
BUSY
t
BD
Figure 1. Detailed Serial Interface Timing
_______________________________________________________________________________________
8
2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor
During the acquisition interval, the selected channel
Detailed Description
charges the sampling capacitance. The acquisition
The MXB7846 uses a successive-approximation conver-
interval starts on the fifth falling clock edge and ends
on the eighth falling clock edge.
sion technique to convert analog signals to a 12-bit digi-
tal output. An SPI/QSPI/MICROWIRE™-compatible serial
interface provides easy communication to a micro-
processor (µP). It features an internal 2.5V reference, an
on-chip temperature sensor, a battery monitor, and a
4-wire touch-screen interface (Functional Diagram).
The time required for the T/H to acquire an input signal
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
Analog Inputs
Figure 2 shows a block diagram of the analog input sec-
tion that includes the input multiplexer of the MXB7846,
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-, AUX, BAT, and the
internal temperature sensor.
(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
t
= 8.4 × R + R
× 25pF
(
)
ACQ
S
IN
where R = 2kΩ and R is the source impedance of
IN
S
the input signal.
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.
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.
+V
DD
V
REF
PENIRQ
TEMP1
TEMP0
MXB7846
A2–A0
(SHOWN 001 )
SER/DFR
(SHOWN HIGH)
B
X+
X-
REF ON/OFF
Y+
Y-
REF+
12-BIT ADC
-IN
+IN
REF-
2.5V
REFERENCE
7.5kΩ
V
BAT
2.5kΩ
BATTERY
ON
AUX
GND
Figure 2. Equivalent Input Circuit
MICROWIRE is a trademark of National Semiconductor Corp.
_______________________________________________________________________________________
9
2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor
Functional Diagram
V
DD
PENIRQ
X+
X-
TEMPERATURE
SENSOR
DOUT
BUSY
Y+
Y-
PENIRQ
6-TO-1
MUX
SERIAL
DATA
INTERFACE
12-BIT ADC
DCLK
DIN
BATTERY
MONITOR
BAT
CS
AUX
2.5V
REFERENCE
REF
Table 1. Input Configuration, Single-Ended Reference Mode (SER/DFR HIGH)
A2
A1
A0
MEASUREMENT
Temp0
Y position
BAT
ADC INPUT CONNECTION
DRIVERS ON
0
0
0
Temp0
X+
—
Y+, Y-
—
0
0
1
0
1
0
BAT
X+
0
1
1
Z1
X-, Y+
X-, Y+
X-, X+
—
1
0
0
Z2
Y-
1
0
1
X- position
AUX
Y+
1
1
0
AUX
Temp1
1
1
1
Temp1
—
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
0
0
1
0
1
1
0
1
1
0
1
Y+
Y+
X+
X+
Y-
Y-
X-
X-
X+
X+
Y-
Y position
Z1 position
Z2 position
X position
Y+, Y-
Y+, X-
Y+, X-
X+, X-
Y+
10 ______________________________________________________________________________________
2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor
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-resistance of the X
and Y drivers does not track the resistance of the touch
screen over temperature and supply. This results in fur-
ther measurement errors.
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.
Differential Measurement Mode
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.
Analog Input Protection
Internal protection diodes, which clamp the analog input
to V
and GND, allow the analog input pins to swing
DD
from GND - 0.3V to V
+ 0.3V without damage. Analog
DD
inputs must not exceed V
by more than 50mV or be
DD
lower than GND by more than 50mV for accurate con-
version. 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 MXB7846 provides two conversion methods—differ-
ential and single ended. The SER/DFR bit in the control
word selects either mode. A logic 1 selects a single-
ended conversion, while a logic 0 selects a differential
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 MXB7846 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
MXB7846 measures the position of the pointing device
by connecting X+ to IN+ of the ADC, enabling Y+ and
Y- drivers, and digitizing the voltage on X+. The ADC
performs a conversion with REF+ = REF and REF- =
GND. In single-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
Touch-Pressure Measurement
The MXB7846 provides two methods for measuring the
pressure applied to the touch screen (Figure 7). By
measuring R
, it is possible to differentiate
TOUCH
between a finger or stylus in contact with the touch
screen. Although 8-bit resolution is typically sufficient,
the following calculations use 12-bit resolution demon-
strating the maximum precision of the MXB7846.
______________________________________________________________________________________ 11
2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor
V
DD
V
DD
Y+
X+
Y-
REF
Y+
X+
Y-
+IN
-IN
REF+
+IN
-IN
REF+
12-BIT ADC
12-BIT ADC
REF-
REF-
GND
GND
Figure 3. Single-Ended Y-Coordinate Measurement
Figure 4. Ratiometric Y-Coordinate Measurement
+V
DD
100kΩ
OPEN CIRCUIT
Y+
PENIRQ
TOUCH SCREEN
X+
Y-
ON
PENIRQ
ENABLE
Figure 5. PENIRQ Functional Block Diagram
12 ______________________________________________________________________________________
2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor
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
The first method performs pressure measurements
using a known X-plate resistance. After completing
three conversions (X-position, Z1, and Z2), use the fol-
MEASURE X- POSITION
X+
Y+
lowing equation to calculate R
:
TOUCH
+
-
R
Z
TOUCH
X
POSITION
4096
2
R
= R
(
×
×
−1
)
V
TOUCH
XPLATE
Z
1
X- POSITION
The second method requires knowing both the X-plate
and Y-plate resistance. Three conversions are required in
this method: the X-position, Y-position, and Z1-position.
X-
Y-
OPEN CIRCUIT
MEASURE Z1
Use the following equation to calculate R
TOUCH:
X+
Y+
R
X
4096
Z
1
XPLATE
POSITION
4096
+
−1
R
=
×
×
TOUCH
R
TOUCH
Z
1
V
-
Y
POSITION
4096
− R
×
YPLATE
X-
Y-
OPEN CIRCUIT
Internal Temperature Sensor
OPEN CIRCUIT
X+
The MXB7846 provides two temperature measurement
options: single-ended conversion and differential con-
version. Both temperature measurement techniques rely
on the semiconductor junction’s temperature character-
Y+
+
R
TOUCH
V
-
istics. The forward diode voltage (V ) vs. temperature
BE
is a well-defined characteristic. The ambient tempera-
ture can be calculated by knowing the value of V at a
BE
X-
Y-
fixed temperature and then monitoring the change in
that voltage as the temperature changes. The single
conversion method requires calibration at a known tem-
perature, but only needs a single reading to calculate
the ambient temperature. First, the PENIRQ diode for-
SENSE LINE
MEASURE Z2
Figure 7. Pressure Measurement Block Diagram
______________________________________________________________________________________ 13
2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor
ward bias voltage is measured by the ADC with an
address of A2 = 0, A1 = 0, and A0 = 0 at a known tem-
perature. Subsequent diode measurements provide an
estimate of the ambient temperature through extrapola-
tion. This assumes a temperature coefficient of
-2.1mV/°C. The single conversion method results in a
resolution of 0.3°C/LSB and a typical accuracy of 3°C.
Battery Voltage Monitor
A dedicated analog input (BAT) allows the MXB7846 to
monitor the system battery voltage. Figure 8 shows the
battery voltage monitoring circuitry. The MXB7846 mon-
itors battery voltages from 0 to 6V. An internal resistor
network divides down V
by 4 so that a 6.0V battery
BAT
voltage results in 1.5V at the ADC input. To minimize
power consumption, the divider is only enabled during
The differential conversion method uses two measure-
ment points. The first measurement (Temp0) is per-
formed with a fixed bias current into the PENIRQ diode.
The second measurement (Temp1) is performed with a
fixed multiple of the original bias current with an
address of A2 = 1, A1 = 1, and A0 = 1. The voltage dif-
ference between the first and second conversion is
proportional to the absolute temperature and is
expressed by the following formula:
the sampling of V
.
BAT
Internal Reference
Enable the internal 2.5V reference by setting PD1 in the
control byte to a logic 1 (see Tables 3 and 4). The
MXB7846 uses the internal reference for single-ended
measurement mode, battery monitoring, temperature
measurement, and for measurement on the auxiliary
input. To minimize power consumption, disable the inter-
nal reference by setting PD1 to a logic 0 when performing
ratiometric position measurements. The internal 2.5V ref-
erence typically requires 10ms to settle (with no external
load). For optimum performance, connect a 0.1µF capac-
itor from REF to GND. This internal reference can be over-
driven with an external reference. For best performance,
the internal reference should be disabled when the exter-
nal reference is applied. The internal reference of the
MXB7846 must also be disabled to maintain compatibility
with the MXB7843. To disable the internal reference of the
MXB7846 after power-up, a control byte with PD1 = 0 is
required. (See Typical Operating Characteristics for
power-up time of the reference from power down.)
VREF
4096
T(°C)= 2.60 × (T1 − T0)
× 1000
− 273
where T0 (Temp0) and T1 (Temp1) are the conversion
results.
This differential conversion method can provide much
improved absolute temperature measurement; however,
the resolution is reduced to 1.6°C/LSB.
External Reference
Although the internal reference may be overdriven with
an external reference, the internal reference should be
disabled (PD1 = 0) for best performance when using
an external reference. During conversion, an external
reference at REF must deliver up to 40µA DC load cur-
rent. 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. Temperature measurements are
always performed using the internal reference.
DC/DC
CONVERTER
+2.375V TO +5.25V
BATTERY
0 TO 6.0V
V
DD
BAT
0 TO 1.5V
12-BIT ADC
7.5kΩ
Digital Interface
Initialization After Power-Up and Starting a
Conversion
2.5kΩ
The digital interface consists of three inputs, DIN, DCLK,
CS, and one output, DOUT. A logic-high on CS disables
the MXB7846 digital interface and places DOUT in a
high-impedance state. Pulling CS low enables the
MXB7846 digital interface.
BATTERY
MEASUREMENT ON
Figure 8. Battery Measurement Functional Block Diagram
14 ______________________________________________________________________________________
2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor
Table 3. Control Byte Format
BIT 7
BIT 6
A2
BIT 5
A1
BIT 4
A0
BIT 3
BIT 2
BIT 1
PD1
BIT 0
PD0
START
MODE
SER/DFR
BIT
7
NAME
START
A2
DESCRIPTION
Start bit
Address (Tables 1 and 2)
6
5
A1
4
A0
3
MODE
SER/DFR
PD1
Conversion resolution: 1 = 8 bits, 0 = 12 bits
2
Conversion mode: 1 = single ended, 0 = differential
1
Power-down mode (Table 4)
0
PD0
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 MXB7846’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.
Figure 9 shows the timing for this sequence. Byte RB2
and RB3 contain the result of the conversion, padded
with four trailing zeros. The total conversion time is a
function of the serial-clock frequency and the amount of
idle timing between 8-bit transfers.
Digital Output
The MXB7846 outputs data in straight binary format. Data
is clocked out on the falling edge of the DCLK MSB first.
The MXB7846 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 9).
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.
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:
The conversion must complete in 500µs or less; if not,
droop on the sample-and-hold capacitors can degrade
conversion results.
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).
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.
2) Use a general-purpose I/O line on the CPU to pull
CS low.
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 10).
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.
Once a start bit has been recognized, the current con-
version must be completed.
5) Transmit a byte of all zeros ($00 hex) and simultane-
ously receive byte RB3.
6) Pull CS high.
______________________________________________________________________________________ 15
2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor
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, reference is always OFF
ADC is always OFF, reference is always ON
ADC is always ON, reference is always ON
200
400
600
200
400
600
CS
T
R
R
B3
B
B2
t
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
DRIVERS 1 AND 2
(SER/DFR HIGH)
OFF
OFF
ON
OFF
ON
DRIVERS 1 AND 2
(SER/DFR LOW)
OFF
Figure 9. Conversion Timing, 24-Clock per Conversion, 8-Bit Bus Interface
8-Bit Conversion
The fastest the MXB7846 can run with CS held continu-
The MXB7846 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
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.
ously low is 15 clock conversions. Figure 10 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.
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
MXB7846. Figure 11 shows the serial interface timing nec-
essary to perform a conversion every 16 DCLK cycles.
16 ______________________________________________________________________________________
2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor
and Y- drivers are turned on, connecting one side of
the vertical resistive layer to V and the other side to
Data Format
The MXB7846 output data is in straight binary format as
shown in Figure 12. This figure shows the ideal output
code for the given input voltage and does not include
the effects of offset, gain, or noise.
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.
Applications Information
Basic Operation of the MXB7846
The 4-wire touch-screen controller works by creating a
voltage gradient across the vertical or horizontal resis-
tive network connected to the MXB7846, as shown in
the Typical Application Circuit. The touch screen is
biased through internal MOSFET switches that connect
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 MXB7846 configuration through a control
byte (see Tables 3 and 4). Once the processor instructs
each resistive layer to V
and ground on an alternate
DD
basis. For example, to measure the Y position when a
pointing device presses on the touch screen, the Y+
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 10. 15-Clock/Conversion Timing
. . .
. . .
. . .
CS
1
8
16
1
8
16
DCLK
DIN
S
CONTROL BYTE 0
S
CONTROL BYTE 1
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 11. 16-Clock/Conversion Timing
______________________________________________________________________________________ 17
2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor
the MXB7846 to initiate a conversion, the MXB7846
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 voltage before the acquisition period is over.
After the acquisition 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 conversion period.
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 MXB7846 is in power-down mode with
PD1 = 0 and PD0 = 0. When exiting software shutdown,
the MXB7846 is ready to perform a conversion in 10µs
with an external reference. When using the internal ref-
erence, allow enough time for reference to settle to 12-
bit accuracy when exiting full power-down mode, as
shown in the Typical Operating Characteristics.
Power-On Reset
When power is first applied, internal power-on circuitry
resets the MXB7846. 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. On power-
up, allow time for the reference to stabilize.
PD1 = 1, PD0 = 1
In this mode, the MXB7846 is always powered up and
both the reference and the ADC are always on. The
device remains fully powered after the current conver-
sion completes.
PD1 = 0, PD0 = 0
In this mode, the MXB7846 powers down after the cur-
rent conversion completes or on the next rising edge of
CS, whichever occurs first. The next control byte
received on DIN powers up the MXB7846. At the start
of a new conversion, it instantly powers up. When each
conversion is finished, the part enters power-down
mode, unless otherwise indicated. The first conversion
after the ADC returns to full power is valid for differen-
tial conversions and single-ended measurement con-
versions when using an external reference.
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 MXB7846 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 maximum rate during a conversion, conversions are
done less often. There is a significant difference in
power consumption between these two modes.
OUTPUT CODE
FULL-SCALE
TRANSITION
11…111
11…110
11…101
FS = (V
- V )
REF-
REF+
PD1 = 1, PD0 = 0
In this mode, the MXB7846 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 MXB7846 to full power. The first conver-
sion after the ADC returns to full power is valid.
(V
- V
)
REF+
REF-
1LSB =
4096
00…011
00…010
00…001
PD1 = 0, PD0 = 1
This mode turns the internal reference off and leaves
the ADC on to perform conversions using an external
reference.
00…000
0
1
2
3
FS
FS-3/2LSB
INPUT VOLTAGE (LSB) = [(V ) - (V )]
+IN -IN
Figure 12. Ideal Input Voltages and Output Codes
18 ______________________________________________________________________________________
2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor
Hardware Power-Down
CS also places the MXB7846 into power-down. When
I/O
SCK
CS
CS goes HIGH, the MXB7846 immediately powers
down and aborts the current conversion. The internal
reference does not turn off when CS goes high. To dis-
able the internal reference, an additional command
byte is required before CS goes high (PD1 = 0). Set
PD1 = 0 before CS goes high.
DCLK
DOUT
MISO
MICROWIRE
MXB7846
MOSI
DIN
MASKABLE
INTERRUPT
BUSY
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.
Figure 13. MICROWIRE Interface
I/O
SCK
CS
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.
DCLK
DOUT
MISO
QSPI/SPI
MXB7846
MOSI
DIN
MASKABLE
INTERRUPT
BUSY
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.
Figure 14. QSPI/SPI Interface
The second option is to operate the MXB7846 in the dif-
ferential mode only for the touch screen, and perform
additional conversions with the same address until the
input signal settles. The MXB7846 can then be placed
in the power-down state on the last measurement.
XF
CLKX
CLKR
CS
SCLK
TMS320LC3x
MXB7846
DIN
DX
DR
Connection to Standard Interface
DOUT
BUSY
MICROWIRE Interface
When using the MICROWIRE- (Figure 13) or SPI-com-
patible interface (Figure 14), 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
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.
FSR
Figure 15. TMS320 Serial Interface
TMS320LC3x Interface
Figure 15 shows an example circuit to interface the
MXB7846 to the TMS320. The timing diagram for this
interface circuit is shown in Figure 16.
Use the following steps to initiate a conversion in the
MXB7846 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 MXB7846 DCLK input.
QSPI/SPI Interface
The MXB7846 can be used with the QSPI/SPI interface
using the circuit in Figure 14 with CPOL = 0 and CPHA
= 0. This interface can be programmed to do a conver-
sion on any analog input of the MXB7846.
______________________________________________________________________________________ 19
2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor
CS
DCLK
DIN
START
A2
A1
A0
MODE
SER/DEF
PD1
PD0
BUSY
DOUT
HIGH IMPEDANCE
HIGH IMPEDANCE
MSB
B10
B1
B0
Figure 16. MXB7846-to-TMS320 Serial Interface Timing Diagram
Power-supply decoupling is also crucial for optimal
device performance. A good way to decouple analog
supplies is to place a 10µF tantalum capacitor in paral-
lel with a 0.1µF capacitor bypassed to GND. To maxi-
mize performance, place these capacitors as close as
possible 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 connect-
ed in series as a lowpass filter.
2) The MXB7846’s CS pin is driven low by the TMS320’s
XF I/O port to enable data to be clocked into the
MXB7846’s DIN pin.
3) An 8-bit word (1XXXXXXX) should be written to the
MXB7846 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 applications.
4) The MXB7846’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 device.
While using the MXB7846, 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.
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.
6) Pull CS high to disable the MXB7846 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.
Definitions
Establish a single-point analog ground (star ground
point) at GND. Connect all analog grounds to the star
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.
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
static linearity parameters for the MXB7846 are mea-
sured using the end-point method.
20 ______________________________________________________________________________________
2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor
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 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.
Chip Information
TRANSISTOR COUNT: 12,000
Aperture Jitter
Aperture jitter (t ) is the sample-to-sample variation in
AJ
PROCESS: 0.6µm BiCMOS
the time between the samples.
Typical Application Circuit
2.375V TO 5.5V
1µF TO 10µF
0.1µF
OPTIONAL
1
2
3
4
+V
X+
Y+
X-
Y-
DCLK 16
CS 15
SERIAL/CONVERSION CLOCK
CHIP SELECT
DD
SERIAL DATA IN
DIN 14
CONVERTER STATUS
SERIAL DATA OUT
PEN INTERRUPT
MXB7846 BUSY 13
DOUT 12
TOUCH
5
6
7
8
SCREEN
TO BATTERY
GND
BAT
AUX
PENIRQ 11
+V
10
9
DD
50kΩ
AUXILIARY
INPUT
REF
0.1µF
VOLTAGE
REGULATOR
______________________________________________________________________________________ 21
2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor
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.)
PACKAGE OUTLINE, QSOP .150", .025" LEAD PITCH
1
21-0055
E
1
22 ______________________________________________________________________________________
2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor
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 ____________________ 23
© 2004 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.
相关型号:
MXB7846EEE-T
ADC, Successive Approximation, 12-Bit, 1 Func, 2 Channel, Serial Access, BICMOS, PDSO16, 0.150 INCH, 0.025 INCH PITCH, QSOP-16
MAXIM
MXB7846EUE
2.375V to 5.25V, 4-Wire Touch-Screen Controller with Internal Reference and Temperature Sensor
MAXIM
MXB7846EUE+T
ADC, Successive Approximation, 12-Bit, 1 Func, 2 Channel, Serial Access, BICMOS, PDSO16, 4.40 MM, 0.65 MM PITCH, ROHS COMPLIANT, MO-153AC, TSSOP-16
MAXIM
©2020 ICPDF网 联系我们和版权申明