View MXB7846EEE_195160.PDF datasheet online --- IC-ON-LINE

Datasheet File OCR Text:

19-2436; Rev 1; 5/04
2.375V to 5.25V, 4-Wire Touch-Screen Controller with Internal Reference and Temperature Sensor
General Description
The MXB7846 is an industry-standard 4-wire touchscreen controller. It contains a 12-bit sampling analogto-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 measurements. In addition, this device has an on-chip temperature 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 TransZorbTM devices. 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 shutdown mode, the typical power consumption is reduced to under 0.5W, while the typical power consumption at 125ksps throughput and a +2.7V supply is 650W. Low-power operation makes the MXB7846 ideal for battery-operated systems, such as personal digital assistants with resistive touch screens and other portable equipment. The MXB7846 is available in 16-pin QSOP and TSSOP packages, and is guaranteed over the -40C to +85C temperature range.
Features
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 SPITM/QSPITM, 3-Wire Serial Interface Programmable 8-/12-Bit Resolution Auxiliary Analog Input Automatic Shutdown Between Conversions Low Power (External Reference) 270A at 125ksps 115A at 50ksps 25A at 10ksps 5A at 1ksps 2A Shutdown Current
MXB7846
Applications
Personal Digital Assistants Portable Instruments Point-of-Sales Terminals Pagers Touch-Screen Monitors Cellular Phones
Typical Application Circuit appears at end of data sheet. TransZorb is a trademark of Vishay Intertechnology, Inc. SPI/QSPI are trademarks of Motorola, Inc.
Ordering Information
PART MXB7846EEE MXB7846EUE TEMP RANGE -40C to +85C -40C to +85C PIN-PACKAGE 16 QSOP 16 TSSOP
Pin Configuration
TOP VIEW
VDD 1 X+ 2 Y+ 3 X- 4 Y- 5 GND 6 BAT 7 AUX 8 16 DCLK 15 CS 14 DIN
MXB7846
13 BUSY 12 DOUT 11 PENIRQ 10 VDD 9 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 MXB7846
ABSOLUTE MAXIMUM RATINGS
VDD, VBAT, DIN, CS, DCLK to GND ........................-0.3V to +6V Digital Outputs to GND...............................-0.3V to (VDD + 0.3V) VREF, X+, X-, Y+, Y-, AUX to GND..............-0.3V to (VDD + 0.3V) 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) Continuous Power Dissipation (TA = +70C) 16-Pin QSOP (derate 8.30mW/C above +70C).........667mW 16-Pin TSSOP (derate 5.70mW/C above +70C) .......456mW Operating Temperature Range ...........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10s) .................................+300C
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
(VDD = 2.7V to 3.6V, VREF = 2.5V, fDCLK = 2MHz (50% duty cycle), fSAMPLE = 125kHz, 12-bit mode, 0.1F capacitor at REF, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER DC ACCURACY (Note 1) Resolution No Missing Codes Relative Accuracy Differential Nonlinearity Offset Error Gain Error Noise CONVERSION RATE Conversion Time Track/Hold Acquisition Time Throughput Rate Multiplexer Settling Time Aperture Delay Aperture Jitter Channel-to-Channel Isolation Serial Clock Frequency Duty Cycle ANALOG INPUT (X+, X-, Y+, Y-, AUX) Input Voltage Range Input Capacitance Input Leakage Current SWITCH DRIVERS On-Resistance (Note 5) INTERNAL REFERENCE Reference Output Voltage REF Output Tempco REF Short-Circuit Current REF Output Impedance VREF TCVREF VDD = 2.7V to 5.25V, TA = +25C 2.45 2.50 50 18 250 2.55 V ppm/C mA Y+, X+ Y-, X7 9 On/off leakage, VIN = 0 to VDD 0 25 0.1 1 VREF V pF A fDCLK VIN = 2.5VP-P at 50kHz 0.1 40 tCONV tACQ fSAMPLE 12 clock cycles (Note 4) 3 clock cycles 16 clock conversion 500 30 100 100 2.0 60 1.5 125 6 s s kHz ns ns ps dB MHz % (Note 3) Including internal reference 70 INL DNL (Note 2) 11 12 1 1 6 4 2 12 Bits Bits LSB LSB LSB LSB VRMS SYMBOL CONDITIONS MIN TYP MAX UNITS
2
_______________________________________________________________________________________
2.375V to 5.25V, 4-Wire Touch-Screen Controller with Internal Reference and Temperature Sensor
ELECTRICAL CHARACTERISTICS (continued)
(VDD = 2.7V to 3.6V, VREF = 2.5V, fDCLK = 2MHz (50% duty cycle), fSAMPLE = 125kHz, 12-bit mode, 0.1F capacitor at REF, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER Reference Input Voltage Range Input Resistance fSAMPLE = 125kHz Input Current BATTERY MONITOR (BAT) Input Voltage Range Input Resistance Accuracy TEMPERATURE MEASUREMENT Resolution Accuracy DIGITAL INPUTS (DCLK, CS, DIN) Input High Voltage Input Low Voltage Input Hysteresis Input Leakage Current Input Capacitance DIGITAL OUTPUT (DOUT, BUSY) Output Voltage Low Output Voltage High PENIRQ Output Low Voltage Three-State Leakage Current Three-State Output Capacitance POWER REQUIREMENTS Supply Voltage VDD External reference Internal reference External reference Supply Current IDD Internal reference Shutdown Supply Current Power-Supply Rejection Ratio ISHDN PSRR fSAMPLE = 125ksps fSAMPLE = 12.5ksps fSAMPLE = 0 fSAMPLE = 125ksps fSAMPLE = 12.5ksps fSAMPLE = 0 DCLK = CS = VDD VDD = 2.7V to 3.6V full scale 70 2.375 2.70 270 220 150 780 720 650 3 A dB 950 A 5.250 5.25 650 A V VIH VIL VHYST IIN CIN VOL VOH VOL IL COUT ISINK = 250A ISOURCE = 250A 50k pullup to VDD CS = VDD CS = VDD 1 15 VDD - 0.5 0.8 10 15 0.4 100 1 VDD 0.7 0.8 V V mV A pF V V V A pF Differential method (Note 8) Single-conversion method Differential method (Note 8) Single-conversion method 1.6 0.3 2 3 C C C C During acquisition VREF = 2.5V Internal reference 0 10 2 3 6 V k % fSAMPLE = 12.5kHz fDCLK = 0 SYMBOL (Note 7) CONDITIONS MIN 1 1 13 2.5 3 40 TYP MAX VDD UNITS V G A
MXB7846
EXTERNAL REFERENCE (Internal reference disabled, reference applied to REF)
_______________________________________________________________________________________
3
2.375V to 5.25V, 4-Wire Touch-Screen Controller with Internal Reference and Temperature Sensor MXB7846
TIMING CHARACTERISTICS (Figure 1)
(VDD = 2.7V to 3.6V, VREF = 2.5V, fDCLK = 2MHz (50% duty cycle), fSAMPLE = 125kHz, 12-bit mode, 0.1F capacitor at REF, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER Acquisition Time DCLK Clock Period 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 SYMBOL tACQ tCP tCH tCL tDS tDH tCSS tCSH tDO tTR tDV tBD tBDV tBTR CLOAD = 50pF CLOAD = 50pF CLOAD = 50pF CONDITIONS MIN 1.5 500 200 200 100 0 100 0 200 200 200 200 200 200 TYP MAX UNITS s ns ns ns ns ns ns ns ns ns ns ns ns ns
TIMING CHARACTERISTICS (Figure 1)
Note 1: Tested at VDD = 2.7V. 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 300VP-P. An external reference below 2.5V can compromise 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
(VDD = 2.7V, VREF = 2.5VEXTERNAL, fDCLK = 2MHz, fSAMPLE = 125kHz, CLOAD = 50pF, 0.1F capacitor at REF, TA = +25C, unless otherwise noted.)
INTEGRAL NONLINEARITY vs. DIGITAL OUTPUT CODE
MXB7846 toc01
MXB7846
DIFFERENTIAL NONLINEARITY vs. DIGITAL OUTPUT CODE
MXB7846 toc02
CHANGE IN OFFSET ERROR vs. SUPPLY VOLTAGE
1.5 OFFSET ERROR (LSB) 1.0 0.5 0 -0.5 -1.0 -1.5 -2.0
MXB7846 toc04
0.5 0.4 0.3 0.2 0.1 0 -0.1 -0.2 -0.3 -0.4 0 INL (LSB)
1.0 0.8 0.6 0.4 DNL (LSB) 0.2 0 -0.2 -0.4 -0.6 -0.8 -1.0
2.0
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
MXB7846 toc05
CHANGE IN GAIN ERROR vs. SUPPLY VOLTAGE
MXB7846 toc07
CHANGE IN GAIN ERROR vs. TEMPERATURE
MXB7846 toc08
1.0 OFFSET ERROR FROM +25C (LSB)
3 2 GAIN ERROR (LSB) 1 0 -1 -2
1.0 GAIN ERROR FROM +25C (LSB) 0.5 0 -0.5 -1.0 -1.5 -2.0
0.5
0
-0.5
-1.0 -40 -25 -10 5 20 35 50 65 80 TEMPERATURE (C)
-3 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
TEMPERATURE (C)
SWITCH ON-RESISTANCE vs. SUPPLY VOLTAGE (X+, Y+ : +VDD TO PIN; X-, Y- : TO GND)
MXB7846 toc03
SWITCH ON-RESISTANCE vs. TEMPERATURE (X+, Y+ : +VDD TO PIN; X-, Y- : PIN TO GND)
MXB7846 toc06
INTERNAL REFERENCE vs. SUPPLY VOLTAGE
CL = 0.1f 2.5 INTERNAL REFERENCE (V) 2.4 2.3 2.2 2.1 2.0
MXB7846 toc09
14 12 X10 RON ()
12 11 10 9 8 7 6 5 4 3 2 1 0 -40 -25 -10 5 20 35 50 65 80 TEMPERATURE (C) XX+ YY+
2.6
Y6 4 2 0 2.5 3.0 3.5
X+
Y+
RON ()
8
4.0
4.5
5.0
5.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
SUPPLY VOLTAGE (V)
SUPPLY VOLTAGE (V)
_______________________________________________________________________________________
5
2.375V to 5.25V, 4-Wire Touch-Screen Controller with Internal Reference and Temperature Sensor MXB7846
Typical Operating Characteristics (continued)
(VDD = 2.7V, VREF = 2.5VEXTERNAL, fDCLK = 2MHz, fSAMPLE = 125kHz, CLOAD = 50pF, 0.1F capacitor at REF, TA = +25C, unless otherwise noted.)
INTERNAL REFERENCE VOLTAGE vs. TEMPERATURE
MXB7846 toc10
INTERNAL VOLTAGE REFERENCE vs. TURN-ON TIME
MXB7846 toc11a
INTERNAL VOLTAGE REFERENCE vs. TURN-ON TIME
MXB7846 toc11b
2.6 INTERNAL REFERENCE VOLTAGE (V) 2.5 2.4 2.3 2.2 2.1 2.0 -40 -25 -10 5 20 35 50 65 80 TEMPERATURE (C)
2.5 INTERNAL VOLTAGE REFERENCE (V)
3.0 INTERNAL VOLTAGE REFERENCE (V) 2.5 2.0 1.5 1.0 0.5 0
2.0
1.5
1.0
0.5 CL = 1F (1060s) 12-BIT SETTLING 0 0 200 400 600 800 1000 1200 TURN-ON TIME (s)
VDD = 2.7V CL = 0.1F
NO CAPACITOR (30s) 12-BIT SETTLING 0 5 10 15 20 25 30 35 40
TURN-ON TIME (s)
REFERENCE CURRENT vs. SUPPLY VOLTAGE
MXB7846 toc12
REFERENCE CURRENT vs. TEMPERATURE
MXB7846 toc13
REFERENCE CURRENT vs. SAMPLE RATE
9 REFERENCE CURRENT (A) 8 7 6 5 4 3 2 1 0 EXTERNAL REFERENCE
MXB7846 toc14
8.3 8.2 REFERENCE CURRENT (A) 8.1 8.0 7.9 7.8 7.7 2.5 3.0 3.5 4.0 4.5 5.0 CL = 0.1F fSAMPLE = 125kHz EXTERNAL REFERENCE
8.3 8.2 REFERENCE CURRENT (A) 8.1 8.0 7.9 7.8 7.7 VDD = 2.7V CL = 0.1F fSAMPLE = 125kHz EXTERNAL REFERENCE -40 -25 -10 5 20 35 50 65 80
10
5.5
0
25
50
75
100
125
SUPPLY VOLTAGE (V)
TEMPERATURE (C)
SAMPLE RATE (kHz)
TEMP DIODE VOLTAGE vs. TEMPERATURE
0.9 TEMP DIODE VOLTAGE (V) 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 -40 -25 -10 5 20 35 50 65 80 TEMPERATURE (C) 585 2.7 TEMP2 TEMP1
MXB7846 toc15
TEMP0 DIODE VOLTAGE vs. SUPPLY VOLTAGE
MXB7846 toc16
TEMP1 DIODE VOLTAGE vs. SUPPLY VOLTAGE
MXB7846 toc17
1.0
590
705 704 TEMP1 DIODE VOLTAGE (mV) 703 TEMP1 702 701 700 699 698
TEMP0 DIODE VOLTAGE (mV)
589 TEMP0 588
587
586
3.2
3.7
4.2
4.7
5.2
2.7
3.2
3.7
4.2
4.7
5.2
SUPPLY VOLTAGE (V)
SUPPLY VOLTAGE (V)
6
_______________________________________________________________________________________
2.375V to 5.25V, 4-Wire Touch-Screen Controller with Internal Reference and Temperature Sensor
Typical Operating Characteristics (continued)
(VDD = 2.7V, VREF = 2.5VEXTERNAL, fDCLK = 2MHz, fSAMPLE = 125kHz, CLOAD = 50pF, 0.1F capacitor at REF, TA = +25C, unless otherwise noted.)
SUPPLY CURRENT vs. SUPPLY VOLTAGE
MXB7846 toc18
MXB7846
SUPPLY CURRENT vs. TEMPERATURE
MXB7846 toc19
SUPPLY CURRENT vs. SAMPLE RATE
VDD = 2.7V VREF = 2.5V
MXB7846 toc20
250 fSAMPLE = 12.5kHz SUPPLY CURRENT (A) 225
290 285 SUPPLY CURRENT (A) 280 275 270 265 260 255 fSAMPLE = 125kHz VDD = 2.7V
250 225 SUPPLY CURRENT (A) 200 175 150 125 100
200
175
150 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 SUPPLY VOLTAGE (V)
250 -40 -25 -10 5 20 35 50 65 80 TEMPERATURE (C)
0
25
50
75
100
125
SAMPLE RATE (kHz)
SHUTDOWN CURRENT vs. SUPPLY VOLTAGE
MXB7846 toc21
SHUTDOWN CURRENT vs. TEMPERATURE
DCLK = CS = VDD = 3V 110 SHUTDOWN CURRENT (nA) 100 90 80 70 60
MXB7846 toc22
MAXIMUM SAMPLE RATE vs. SUPPLY VOLTAGE
MXB7846 toc23
300 DCLK = CS = VDD SHUTDOWN CURRENT (nA) 250
120
1000
SAMPLE RATE (kHz)
100
200
150
10
100
50 2.7 3.2 3.7 4.2 4.7 5.2 SUPPLY VOLTAGE (V)
50 -40 -25 -10 5 20 35 50 65 80 TEMPERATURE (C)
1 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 SUPPLY VOLTAGE (V)
_______________________________________________________________________________________
7
2.375V to 5.25V, 4-Wire Touch-Screen Controller with Internal Reference and Temperature Sensor MXB7846
Pin Description
PIN 1 2 3 4 5 6 7 8 9 NAME VDD X+ Y+ XYGND BAT AUX REF Positive Supply Voltage. Connect to pin 10. X+ Position Input, ADC Input Channel 1 Y+ Position Input, ADC Input Channel 2 X- Position Input Y- Position Input Ground Battery Monitoring Inputs; ADC Input Channel 3 Auxiliary Input to ADC; ADC Input Channel 4 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, apply a reference voltage between 1V and VDD. Bypass REF to GND with a 0.1F capacitor. Positive Supply Voltage, +2.375V (2.70V) to +5.25V. External (internal) reference. Bypass with a 1F capacitor. Connect to pin 1. Pen Interrupt Output. Open anode output. 10k to 100k pullup resistor required to VDD. Serial Data Output. Data changes state on the falling edge of DCLK. High impedance when CS is HIGH. Busy Output. BUSY pulses high for one clock period before the MSB decision. High impedance when CS is HIGH. 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%). FUNCTION
10 11 12 13 14 15 16
VDD PENIRQ DOUT BUSY DIN CS DCLK
CS tCSS DCLK tDO tDH tTR tDV DOUT tCH tCL tCP tCSH
tDS DIN
tBDV BUSY tBD
tBTR
Figure 1. Detailed Serial Interface Timing 8 _______________________________________________________________________________________
2.375V to 5.25V, 4-Wire Touch-Screen Controller with Internal Reference and Temperature Sensor
Detailed Description
The MXB7846 uses a successive-approximation conversion technique to convert analog signals to a 12-bit digital output. An SPI/QSPI/MICROWIRETM-compatible serial interface provides easy communication to a microprocessor (P). It features an internal 2.5V reference, an on-chip temperature sensor, a battery monitor, and a 4-wire touch-screen interface (Functional Diagram). 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. 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 (tACQ) is the maximum time the device takes to acquire the input signal to 12-bit accuracy. Calculate tACQ with the following equation: t ACQ = 8.4 x (RS + RIN ) x 25pF where RIN = 2k and RS is the source impedance of the input signal. Source impedances below 1k do not significantly affect the ADC's performance. Accommodate higher source impedances by either slowing down DCLK or by placing a 1F capacitor between the analog input and GND.
MXB7846
Analog Inputs
Figure 2 shows a block diagram of the analog input section that includes the input multiplexer of the MXB7846, the differential signal inputs of the ADC, and the differential reference inputs of the ADC. The input multiplexer switches between X+, X-, Y+, Y-, AUX, BAT, and the internal temperature sensor. 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 reference and switching matrix according to Tables 1 and 2.
PENIRQ
+VDD
VREF
TEMP1
TEMP0
MXB7846
A2-A0 (SHOWN 001B)
SER/DFR (SHOWN HIGH)
X+ X-
REF ON/OFF Y+ Y2.5V REFERENCE
+IN REF+ 12-BIT ADC -IN REF-
7.5k VBAT 2.5k
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 MXB7846
Functional Diagram
VDD
PENIRQ X+ XTEMPERATURE SENSOR
DOUT Y+ Y6-TO-1 MUX 12-BIT ADC SERIAL DATA INTERFACE DCLK BAT AUX BATTERY MONITOR DIN CS BUSY PENIRQ
REF
2.5V REFERENCE
Table 1. Input Configuration, Single-Ended Reference Mode (SER/DFR HIGH)
A2 0 0 0 0 1 1 1 1 A1 0 0 1 1 0 0 1 1 A0 0 1 0 1 0 1 0 1 MEASUREMENT Temp0 Y position BAT Z1 Z2 X- position AUX Temp1 ADC INPUT CONNECTION Temp0 X+ BAT X+ YY+ AUX Temp1 DRIVERS ON -- Y+, Y-- X-, Y+ X-, Y+ X-, X+ -- --
Table 2. Input Configuration, Differential Reference Mode (SER/DFR LOW)
A2 0 0 0 1 A1 0 1 1 0 A0 1 1 0 1 ADC +REF CONNECTION TO Y+ Y+ X+ X+ ADC -REF CONNECTION TO YYXXADC INPUT CONNECTION TO X+ X+ YY+ MEASUREMENT PERFORMED Y position Z1 position Z2 position X position DRIVER ON Y+, YY+, XY+, XX+, X-
10
______________________________________________________________________________________
2.375V to 5.25V, 4-Wire Touch-Screen Controller with Internal Reference and Temperature Sensor
Input Bandwidth and Anti-Aliasing
The ADCs input tracking circuitry has a 25MHz smallsignal bandwidth, so it is possible to digitize highspeed transient events. To avoid high-frequency signals being aliased into the frequency band of interest, anti-alias filtering is recommended. 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 further measurement errors. 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, respectively. Differential mode uses the voltage at the Y+ pin as the REF+ voltage and voltage at the Y- pin as REFvoltage. 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. PEN Interrupt Request (PENIRQ) Figure 5 shows the block diagram for the PENIRQ function. When used, PENIRQ requires a 10k to 100k pullup to +VDD. If enabled, PENIRQ goes low whenever 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. 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 tTOUCH. The tTOUCH value changes for different touch-screen parasitic capacitance and resistance. The microprocessor receives this interrupt and pulls CS low to initiate a conversion. 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.
MXB7846
Analog Input Protection
Internal protection diodes, which clamp the analog input to VDD and GND, allow the analog input pins to swing from GND - 0.3V to VDD + 0.3V without damage. Analog inputs must not exceed VDD by more than 50mV or be lower than GND by more than 50mV for accurate conversion. 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--differential and single ended. The SER/DFR bit in the control word selects either mode. A logic 1 selects a singleended conversion, while a logic 0 selects a differential conversion. Differential vs. Single Ended Changes in operating conditions can degrade the accuracy and repeatability of touch-screen measurements. Therefore, the conversion results representing X and Y coordinates may be incorrect. For example, in singleended measurement mode, variation in the touch-screen driver voltage drops results in incorrect input reading. Differential mode minimizes these errors. 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 200 to 900 (depending on the manufacturer), whereas the on-resistance of the X and Y drivers is 8 (typ). Limit the touch-screen current to less than 50mA by using a touch screen with a resistance higher than 100. The
Touch-Pressure Measurement
The MXB7846 provides two methods for measuring the pressure applied to the touch screen (Figure 7). By measuring R TOUCH , it is possible to differentiate 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 demonstrating the maximum precision of the MXB7846.
______________________________________________________________________________________
11
2.375V to 5.25V, 4-Wire Touch-Screen Controller with Internal Reference and Temperature Sensor MXB7846
VDD VDD
Y+
REF
Y+
X+
+IN -IN
REF+ 12-BIT ADC REF-
X+
+IN -IN
REF+ 12-BIT ADC REF-
Y-
Y-
GND
GND
Figure 3. Single-Ended Y-Coordinate Measurement
Figure 4. Ratiometric Y-Coordinate Measurement
+VDD
OPEN CIRCUIT Y+
100k PENIRQ
TOUCH SCREEN
X+
YON
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 MXB7846
SCREEN TOUCHED HERE
PENIRQ
CS DCLK
1
2
3
4
5
6
7
8
1
2
3
12
13
14
15
16
DIN
S
A2
A1
A0
M
S/D
PD1
PD0
INTERRUPT PROCESSOR NO RESPONSE TO TOUCHMASK PENIRQ tTOUCH PENIRQ ENABLED
Figure 6. PENIRQ Timing Diagram
MEASURE X- POSITION X+ FORCED LINE Y+ SENSE LINE
The first method performs pressure measurements using a known X-plate resistance. After completing three conversions (X-position, Z1, and Z2), use the following equation to calculate RTOUCH: Z X RTOUCH = (RXPLATE ) x POSITION x 2 - 1 4096 Z1 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. Use the following equation to calculate RTOUCH: R 4096 X RTOUCH = XPLATE x POSITION x -1 Z1 4096 Z1 YPOSITION - RYPLATE x 4096
+ V -
RTOUCH
X- POSITION XYOPEN CIRCUIT
MEASURE Z1 X+ SENSE LINE Y+
RTOUCH V
+ FORCED LINE
X-
YOPEN CIRCUIT
OPEN CIRCUIT X+
Internal Temperature Sensor
Y+
RTOUCH V
+ -
X-
Y-
SENSE LINE
MEASURE Z2
Figure 7. Pressure Measurement Block Diagram
The MXB7846 provides two temperature measurement options: single-ended conversion and differential conversion. Both temperature measurement techniques rely on the semiconductor junction's temperature characteristics. The forward diode voltage (VBE) vs. temperature is a well-defined characteristic. The ambient temperature can be calculated by knowing the value of VBE at a fixed temperature and then monitoring the change in that voltage as the temperature changes. The single conversion method requires calibration at a known temperature, but only needs a single reading to calculate the ambient temperature. First, the PENIRQ diode for13
FORCED LINE
______________________________________________________________________________________
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 temperature. Subsequent diode measurements provide an estimate of the ambient temperature through extrapolation. This assumes a temperature coefficient of -2.1mV/C. The single conversion method results in a resolution of 0.3C/LSB and a typical accuracy of 3C. The differential conversion method uses two measurement points. The first measurement (Temp0) is performed 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 difference between the first and second conversion is proportional to the absolute temperature and is expressed by the following formula: T(C) = 2.60 x (T1
-
MXB7846
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 monitors battery voltages from 0 to 6V. An internal resistor network divides down VBAT by 4 so that a 6.0V battery voltage results in 1.5V at the ADC input. To minimize power consumption, the divider is only enabled during the sampling of VBAT.
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 internal reference by setting PD1 to a logic 0 when performing ratiometric position measurements. The internal 2.5V reference typically requires 10ms to settle (with no external load). For optimum performance, connect a 0.1F capacitor from REF to GND. This internal reference can be overdriven with an external reference. For best performance, the internal reference should be disabled when the external 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 T0) x 1000 - 273 4096
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 1.6C/LSB.
External Reference
DC/DC CONVERTER BATTERY 0 TO 6.0V VDD BAT 0 TO 1.5V 7.5k 12-BIT ADC +2.375V TO +5.25V
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 40A DC load current. If the reference has a higher output impedance or is noisy, bypass it close to the REF pin with a 0.1F and a 4.7F capacitor. Temperature measurements are always performed using the internal reference.
Digital Interface
2.5k
BATTERY MEASUREMENT ON
Initialization After Power-Up and Starting a Conversion 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.
Figure 8. Battery Measurement Functional Block Diagram 14 ______________________________________________________________________________________
2.375V to 5.25V, 4-Wire Touch-Screen Controller with Internal Reference and Temperature Sensor MXB7846
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 6 5 4 3 2 1 0
NAME START A2 A1 A0 MODE SER/DFR PD1 PD0 Address (Tables 1 and 2) Start bit
DESCRIPTION
Conversion resolution: 1 = 8 bits, 0 = 12 bits Conversion mode: 1 = single ended, 0 = differential Power-down mode (Table 4)
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. 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 microcontroller: 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 8bit transfer to configure the ADC, and two more 8-bit transfers to read the conversion result; Figure 9). 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: 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 CS low. 3) Transmit TB and simultaneously receive a byte; call it RB1. 4) Transmit a byte of all zeros ($00 hex) and simultaneously receive byte RB2. 5) Transmit a byte of all zeros ($00 hex) and simultaneously receive byte RB3. 6) Pull CS high.
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.
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 decisions 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. The conversion must complete in 500s or less; if not, droop on the sample-and-hold capacitors can degrade conversion results.
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 conversion 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 conversion in progress is clocked onto the DOUT pin and is treated as a START bit (Figure 10). Once a start bit has been recognized, the current conversion must be completed.
______________________________________________________________________________________
15
2.375V to 5.25V, 4-Wire Touch-Screen Controller with Internal Reference and Temperature Sensor MXB7846
Table 4. Power-Mode Selection
SUPPLY CURRENT (typ) (A) PD1 0 0 1 1 PD0 0 1 0 1 PENIRQ Enabled Disabled Disabled Disabled STATUS 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 DURING CONVERSION 200 200 400 600 AFTER CONVERSION 1 200 400 600
CS TB tACQ DCLK
1 4 8 9 12 16 20 24
RB2
RB3
DIN
S
A2
A1
SER/ A0 MODE DFR PD1
PD0
(START) BUSY
IDLE
ACQUIRE
CONVERSION
IDLE
RB1 DOUT A/D STATE DRIVERS 1 AND 2 (SER/DFR HIGH) IDLE ACQUIRE
11 (MSB) 10 9 8 7 6 5 4 3 2 1 0
(LSB) CONVERSION IDLE
OFF
ON
OFF
DRIVERS 1 AND 2 (SER/DFR LOW)
OFF
ON
OFF
Figure 9. Conversion Timing, 24-Clock per Conversion, 8-Bit Bus Interface
The fastest the MXB7846 can run with CS held continuously low is 15 clock conversions. Figure 10 shows the serial-interface timing necessary to perform a conversion every 15 DCLK cycles. If CS is connected low and DCLK is continuous, guarantee a start bit by first clocking in 16 zeros. Most microcontrollers (Cs) require that data transfers occur in multiples of eight DCLK cycles; 16 clocks per conversion is typically the fastest that a C can drive the MXB7846. Figure 11 shows the serial interface timing necessary to perform a conversion every 16 DCLK cycles.
8-Bit Conversion
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.
16
______________________________________________________________________________________
2.375V to 5.25V, 4-Wire Touch-Screen Controller with Internal Reference and Temperature Sensor
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. and Y- drivers are turned on, connecting one side of the vertical resistive layer to VDD and the other side to ground. In this case, the horizontal resistive layer functions 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. The conversion process of the analog input voltage to digital output is controlled through the serial interface between the A/D converter and the P. The processor controls the MXB7846 configuration through a control byte (see Tables 3 and 4). Once the processor instructs
MXB7846
Applications Information
Basic Operation of the MXB7846
The 4-wire touch-screen controller works by creating a voltage gradient across the vertical or horizontal resistive network connected to the MXB7846, as shown in the Typical Application Circuit. The touch screen is biased through internal MOSFET switches that connect each resistive layer to VDD and ground on an alternate basis. For example, to measure the Y position when a pointing device presses on the touch screen, the Y+
CS 1 DCLK 8 15 1 8 15 1
DIN
S
CONTROL BYTE 0
S
CONTROL BYTE 1
S
CONTROL BYTE 2
DOUT
B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0 CONVERSION RESULT 0
B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0 CONVERSION RESULT 1
BUSY
Figure 10. 15-Clock/Conversion Timing
CS 1 DCLK 8 16 1 8 16
... ... ...
B11 B10 B9 B8 B7 B6 CONVERSION RESULT 1
DIN
S
CONTROL BYTE 0
S
CONTROL BYTE 1
DOUT
B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0 CONVERSION RESULT 0
... ...
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 dependent on the power-down state. When exiting software low-power modes, conversion can start immediately when running at decreased clock rates. Upon poweron 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 10s with an external reference. When using the internal reference, allow enough time for reference to settle to 12bit accuracy when exiting full power-down mode, as shown in the Typical Operating Characteristics. 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 conversion completes. PD1 = 0, PD0 = 0 In this mode, the MXB7846 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 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 differential conversions and single-ended measurement conversions when using an external reference. When operating at full speed and 16 clocks per conversion, 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. 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 conversion after the ADC returns to full power is valid. PD1 = 0, PD0 = 1 This mode turns the internal reference off and leaves the ADC on to perform conversions using an external reference.
MXB7846
Power-On Reset
When power is first applied, internal power-on circuitry resets the MXB7846. Allow 10s 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 conversion takes place, DOUT shifts out zeros. On powerup, allow time for the reference to stabilize.
Power Modes
Save power by placing the converter in one of two lowcurrent 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 conversion and switches to full-power mode. After completing its conversion, the MXB7846 enters the programmed power mode until a new control byte is received.
OUTPUT CODE FULL-SCALE TRANSITION 11...111 11...110 11...101 FS = (VREF+ - VREF-)
1LSB =
(VREF+ - VREF-) 4096
00...011 00...010 00...001 00...000 0 1 2 3 FS-3/2LSB INPUT VOLTAGE (LSB) = [(V+IN) - (V-IN)] FS
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 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 disable the internal reference, an additional command byte is required before CS goes high (PD1 = 0). Set PD1 = 0 before CS goes high.
MXB7846
I/O SCK MISO MICROWIRE
CS DCLK DOUT MXB7846 DIN BUSY
MOSI MASKABLE INTERRUPT
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. 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 eliminate this issue are described below. One option is to lengthen the acquisition time by stopping or slowing down DCLK, allowing for the required touchscreen settling time. This method solves the settling time problem for both single-ended and differential modes. The second option is to operate the MXB7846 in the differential 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.
Figure 13. MICROWIRE Interface
I/O SCK MISO QSPI/SPI
CS DCLK DOUT MXB7846
MOSI MASKABLE INTERRUPT
DIN BUSY
Figure 14. QSPI/SPI Interface
XF CLKX CLKR TMS320LC3x DX DR FSR
CS SCLK
MXB7846 DIN DOUT BUSY
Connection to Standard Interface
MICROWIRE Interface When using the MICROWIRE- (Figure 13) or SPI-compatible 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 transitions occur on the serial clock's falling edge and are clocked into the P 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 followed by 4 trailing zeros. DOUT then goes high impedance when CS goes high. 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 conversion on any analog input of the MXB7846.
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 (transmit 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 connected to the MXB7846 DCLK input.
19
______________________________________________________________________________________
2.375V to 5.25V, 4-Wire Touch-Screen Controller with Internal Reference and Temperature Sensor MXB7846 MXB7846
CS
DCLK
DIN
START
A2
A1
A0
MODE
SER/DEF
PD1
PD0
BUSY HIGH IMPEDANCE
DOUT
MSB
B10
B1
B0
HIGH IMPEDANCE
Figure 16. MXB7846-to-TMS320 Serial Interface Timing Diagram
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 specific 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. 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 recommended. 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. 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.
Power-supply decoupling is also crucial for optimal device performance. A good way to decouple analog supplies is to place a 10F tantalum capacitor in parallel with a 0.1F capacitor bypassed to GND. To maximize 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 10 resistor can be connected in series as a lowpass filter. 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 converted 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 singleended measurement mode and at high data rates.
Definitions
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 measured 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 (tAD) is the time defined between the falling edge of the sampling clock and the instant when an actual sample is taken.
MXB7846
Chip Information
TRANSISTOR COUNT: 12,000 PROCESS: 0.6m BiCMOS
Aperture Jitter
Aperture jitter (tAJ) is the sample-to-sample variation in the time between the samples.
Typical Application Circuit
2.375V TO 5.5V 1F TO 10F OPTIONAL 0.1F
1 2
+VDD X+
DCLK 16 CS 15 DIN 14
SERIAL/CONVERSION CLOCK CHIP SELECT SERIAL DATA IN CONVERTER STATUS SERIAL DATA OUT PEN INTERRUPT 50k 0.1F
3 Y+ 4 XTOUCH SCREEN 5 YTO BATTERY 6 AUXILIARY INPUT GND
MXB7846
BUSY 13 DOUT 12 PENIRQ 11 +VDD 10 REF 9
7 BAT 8 AUX
VOLTAGE REGULATOR
______________________________________________________________________________________
21
2.375V to 5.25V, 4-Wire Touch-Screen Controller with Internal Reference and Temperature Sensor MXB7846
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.)
QSOP.EPS
PACKAGEOUTLINE,QSOP.150",.025"LEADPITCH
21-0055
E
1
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.)
MXB7846
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 (c) 2004 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
TSSOP4.40mm.EPS

▲Up To Search▲

Price & Availability of MXB7846EEE

	To Download MXB7846EEE Datasheet File
If you can't view the Datasheet, Please click here to try to view without PDF Reader .