19-2436; Rev 1; 5/04 2.375V to 5.25V, 4-Wire Touch-Screen Controller with Internal Reference and Temperature Sensor Applications 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 ♦ SPI™/QSPI™, 3-Wire Serial Interface ♦ Programmable 8-/12-Bit Resolution ♦ Auxiliary Analog Input ♦ Automatic Shutdown Between Conversions ♦ Low Power (External Reference) 270µA at 125ksps 115µA at 50ksps 25µA at 10ksps 5µA at 1ksps 2µA Shutdown Current Ordering Information Personal Digital Assistants Portable Instruments Point-of-Sales Terminals Pagers PART TEMP RANGE PIN-PACKAGE MXB7846EEE -40°C to +85°C 16 QSOP MXB7846EUE -40°C to +85°C 16 TSSOP 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. Pin Configuration TOP VIEW VDD 1 16 DCLK X+ 2 15 CS 14 DIN Y+ 3 X- 4 MXB7846 13 BUSY 12 DOUT Y- 5 GND 6 11 PENIRQ BAT 7 10 VDD AUX 8 9 REF QSOP/TSSOP ________________________________________________________________ Maxim Integrated Products For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at 1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com. 1 MXB7846 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 TransZorb™ 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 0.5µW, while the typical power consumption at 125ksps throughput and a +2.7V supply is 650µW. 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 -40°C to +85°C temperature range. MXB7846 2.375V to 5.25V, 4-Wire Touch-Screen Controller with Internal Reference and Temperature Sensor 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 = +70°C) 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 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, 0.1µF capacitor at REF, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL CONDITIONS MIN TYP 11 12 MAX UNITS DC ACCURACY (Note 1) Resolution 12 No Missing Codes Relative Accuracy INL Differential Nonlinearity DNL (Note 2) ±1 ±2 ±1 Offset Error (Note 3) Noise Including internal reference LSB LSB ±6 Gain Error Bits Bits ±4 70 LSB LSB µVRMS CONVERSION RATE Conversion Time tCONV Track/Hold Acquisition Time Throughput Rate tACQ fSAMPLE 12 clock cycles (Note 4) 3 clock cycles 6 µs 125 kHz 1.5 µs 16 clock conversion Multiplexer Settling Time 500 ns Aperture Delay 30 ns Aperture Jitter Channel-to-Channel Isolation Serial Clock Frequency VIN = 2.5VP-P at 50kHz fDCLK Duty Cycle 100 ps 100 dB 0.1 2.0 MHz 40 60 % 0 VREF V ANALOG INPUT (X+, X-, Y+, Y-, AUX) Input Voltage Range Input Capacitance 25 Input Leakage Current On/off leakage, VIN = 0 to VDD ±0.1 pF ±1 µA SWITCH DRIVERS On-Resistance (Note 5) Y+, X+ 7 Y-, X- 9 Ω INTERNAL REFERENCE Reference Output Voltage REF Output Tempco 2 VREF TCVREF VDD = 2.7V to 5.25V, TA = +25°C 2.45 2.50 2.55 V 50 ppm°/C REF Short-Circuit Current 18 mA REF Output Impedance 250 Ω _______________________________________________________________________________________ 2.375V to 5.25V, 4-Wire Touch-Screen Controller with Internal Reference and Temperature Sensor (VDD = 2.7V to 3.6V, VREF = 2.5V, fDCLK = 2MHz (50% duty cycle), fSAMPLE = 125kHz, 12-bit mode, 0.1µF capacitor at REF, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS EXTERNAL REFERENCE (Internal reference disabled, reference applied to REF) Reference Input Voltage Range (Note 7) 1 Input Resistance VDD 1 Input Current fSAMPLE = 125kHz 13 fSAMPLE = 12.5kHz 2.5 fDCLK = 0 V GΩ 40 µA ±3 BATTERY MONITOR (BAT) Input Voltage Range 0 Input Resistance During acquisition Accuracy 6 10 V kΩ VREF = 2.5V ±2 Internal reference ±3 % TEMPERATURE MEASUREMENT Resolution Accuracy Differential method (Note 8) 1.6 °C Single-conversion method 0.3 °C Differential method (Note 8) ±2 °C Single-conversion method ±3 °C DIGITAL INPUTS (DCLK, CS, DIN) Input High Voltage VIH Input Low Voltage VIL Input Hysteresis VDD ✕ 0.7 VHYST Input Leakage Current IIN Input Capacitance DIGITAL OUTPUT (DOUT, BUSY) CIN 100 15 VOL ISINK = 250µA Output Voltage High VOH ISOURCE = 250µA PENIRQ Output Low Voltage VOL 50kΩ pullup to VDD Three-State Output Capacitance V mV ±1 Output Voltage Low Three-State Leakage Current V 0.8 µA pF 0.4 VDD - 0.5 V V 0.8 IL CS = VDD 1 COUT CS = VDD 15 ±10 V µA pF POWER REQUIREMENTS Supply Voltage VDD External reference 2.375 5.250 Internal reference 2.70 5.25 External reference Supply Current IDD Internal reference fSAMPLE = 125ksps 270 fSAMPLE = 12.5ksps 220 fSAMPLE = 0 150 fSAMPLE = 125ksps 780 fSAMPLE = 12.5ksps 720 fSAMPLE = 0 650 Shutdown Supply Current ISHDN DCLK = CS = VDD Power-Supply Rejection Ratio PSRR VDD = 2.7V to 3.6V full scale 650 µA 950 µA 3 70 V µA dB _______________________________________________________________________________________ 3 MXB7846 ELECTRICAL CHARACTERISTICS (continued) MXB7846 2.375V to 5.25V, 4-Wire Touch-Screen Controller with Internal Reference and Temperature Sensor TIMING CHARACTERISTICS (Figure 1) (VDD = 2.7V to 3.6V, VREF = 2.5V, fDCLK = 2MHz (50% duty cycle), fSAMPLE = 125kHz, 12-bit mode, 0.1µF capacitor at REF, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS TIMING CHARACTERISTICS (Figure 1) Acquisition Time tACQ 1.5 µs DCLK Clock Period tCP 500 ns DCLK Pulse Width High tCH 200 ns DCLK Pulse Width Low tCL 200 ns DIN-to-DCLK Setup Time tDS 100 ns DIN-to-DCLK Hold Time tDH 0 ns CS Fall-to-DCLK Rise Setup Time tCSS 100 ns CS Rise-to-DCLK Rise Ignore tCSH 0 ns DCLK Falling-to-DOUT Valid tDO CLOAD = 50pF 200 ns CS Rise-to-DOUT Disable tTR CLOAD = 50pF 200 ns CS Fall-to-DOUT Enable tDV CLOAD = 50pF 200 ns DCLK Falling-to-BUSY Rising tBD 200 ns CS Falling-to-BUSY Enable tBDV 200 ns CS Rise-to-BUSY Disable tBTR 200 ns 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 300µVP-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 0.3 0.8 0.6 0.4 DNL (LSB) 0.1 0 -0.1 0 -0.2 -0.4 -0.6 -0.3 -0.8 -0.4 0.5 0 -0.5 -1.5 -2.0 0 500 1000 1500 2000 2500 3000 3500 4000 1.0 -1.0 -1.0 500 1000 1500 2000 2500 3000 3500 4000 2.5 3.0 3.5 4.0 4.5 OUTPUT CODE SUPPLY VOLTAGE (V) CHANGE IN OFFSET ERROR vs. TEMPERATURE CHANGE IN GAIN ERROR vs. SUPPLY VOLTAGE CHANGE IN GAIN ERROR vs. TEMPERATURE GAIN ERROR (LSB) 0 1 0 -1 -0.5 -10 5 20 35 50 65 2.5 80 0 -0.5 -1.0 -2.0 -3 -40 -25 0.5 -1.5 -2 -1.0 MXB7846 toc08 2 0.5 1.0 GAIN ERROR FROM +25°C (LSB) 3 MXB7846 toc05 1.0 3.0 3.5 4.0 4.5 5.0 -40 -25 -10 5.5 5 20 35 50 TEMPERATURE (°C) SUPPLY VOLTAGE (V) TEMPERATURE (°C) SWITCH ON-RESISTANCE vs. SUPPLY VOLTAGE (X+, Y+ : +VDD TO PIN; X-, Y- : TO GND) SWITCH ON-RESISTANCE vs. TEMPERATURE (X+, Y+ : +VDD TO PIN; X-, Y- : PIN TO GND) INTERNAL REFERENCE vs. SUPPLY VOLTAGE X10 Y- RON (Ω) 8 X+ 6 Y+ 4 0 3.0 3.5 4.0 10 9 8 7 X+ YY+ 6 5 4 3 2 2.5 X- 4.5 SUPPLY VOLTAGE (V) 5.0 5.5 80 2.6 CL = 0.1µf 2.5 INTERNAL REFERENCE (V) 12 11 65 MXB7846 toc09 12 MXB7846 toc03 14 5.5 5.0 OUTPUT CODE MXB7846 toc07 0 OFFSET ERROR FROM +25°C (LSB) 0.2 -0.2 1.5 MXB7846 toc06 INL (LSB) 0.2 2.0 OFFSET ERROR (LSB) 0.4 MXB7846 toc02 1.0 MXB7846 toc01 0.5 RON (Ω) CHANGE IN OFFSET ERROR vs. SUPPLY VOLTAGE DIFFERENTIAL NONLINEARITY vs. DIGITAL OUTPUT CODE MXB7846 toc04 INTEGRAL NONLINEARITY vs. DIGITAL OUTPUT CODE 2.4 2.3 2.2 2.1 2 1 0 2.0 -40 -25 -10 5 20 35 50 TEMPERATURE (°C) 65 80 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 SUPPLY VOLTAGE (V) _______________________________________________________________________________________ 5 MXB7846 Typical Operating Characteristics (VDD = 2.7V, VREF = 2.5VEXTERNAL, fDCLK = 2MHz, fSAMPLE = 125kHz, CLOAD = 50pF, 0.1µF capacitor at REF, TA = +25°C, unless otherwise noted.) Typical Operating Characteristics (continued) (VDD = 2.7V, VREF = 2.5VEXTERNAL, fDCLK = 2MHz, fSAMPLE = 125kHz, CLOAD = 50pF, 0.1µF capacitor at REF, TA = +25°C, unless otherwise noted.) INTERNAL REFERENCE VOLTAGE vs. TEMPERATURE 2.2 VDD = 2.7V CL = 0.1µF 2.0 1.0 0.5 CL = 1µF (1060µs) 12-BIT SETTLING 0 5 20 35 50 65 80 200 400 TEMPERATURE (°C) REFERENCE CURRENT (µA) 8.0 7.9 5 10 7.7 5.0 5.5 15 20 25 30 MXB7846 toc13 VDD = 2.7V CL = 0.1µF fSAMPLE = 125kHz EXTERNAL REFERENCE EXTERNAL REFERENCE 9 8 7 6 5 4 3 2 1 0 -40 -25 -10 5 20 35 50 65 0 80 25 50 75 SAMPLE RATE (kHz) TEMP DIODE VOLTAGE vs. TEMPERATURE TEMP0 DIODE VOLTAGE vs. SUPPLY VOLTAGE TEMP1 DIODE VOLTAGE vs. SUPPLY VOLTAGE TEMP2 0.5 0.4 0.3 0.2 TEMP0 588 587 586 MXB7846 toc17 589 705 704 TEMP1 DIODE VOLTAGE (mV) 0.6 MXB7846 toc16 MXB7846 toc15 TEMP1 0.7 590 125 100 TEMPERATURE (°C) 0.8 703 TEMP1 702 701 700 699 0.1 585 0 -40 -25 -10 5 20 35 50 TEMPERATURE (°C) 65 80 40 REFERENCE CURRENT vs. SAMPLE RATE 10 SUPPLY VOLTAGE (V) 0.9 35 TURN-ON TIME (µs) 7.9 7.7 1.0 6 NO CAPACITOR (30µs) 12-BIT SETTLING 0 8.0 7.8 4.5 0.5 1000 1200 8.1 7.8 4.0 800 8.2 TEMP0 DIODE VOLTAGE (mV) REFERENCE CURRENT (µA) 8.1 3.5 600 8.3 MXB7846 toc12 CL = 0.1µF fSAMPLE = 125kHz EXTERNAL REFERENCE 3.0 1.0 REFERENCE CURRENT vs. TEMPERATURE 8.3 2.5 1.5 TURN-ON TIME (µs) REFERENCE CURRENT vs. SUPPLY VOLTAGE 8.2 2.0 0 0 REFERENCE CURRENT (µA) -40 -25 -10 MXB7846 toc11b MXB7846 toc11a 1.5 2.5 MXB7846 toc14 2.3 2.0 3.0 INTERNAL VOLTAGE REFERENCE (V) 2.4 2.5 INTERNAL VOLTAGE REFERENCE (V) MXB7846 toc10 INTERNAL REFERENCE VOLTAGE (V) 2.5 2.1 INTERNAL VOLTAGE REFERENCE vs. TURN-ON TIME INTERNAL VOLTAGE REFERENCE vs. TURN-ON TIME 2.6 TEMP DIODE VOLTAGE (V) MXB7846 2.375V to 5.25V, 4-Wire Touch-Screen Controller with Internal Reference and Temperature Sensor 698 2.7 3.2 3.7 4.2 4.7 SUPPLY VOLTAGE (V) 5.2 2.7 3.2 3.7 4.2 4.7 SUPPLY VOLTAGE (V) _______________________________________________________________________________________ 5.2 2.375V to 5.25V, 4-Wire Touch-Screen Controller with Internal Reference and Temperature Sensor SUPPLY CURRENT vs. TEMPERATURE 200 fSAMPLE = 125kHz VDD = 2.7V 280 275 270 265 200 175 150 260 175 125 255 100 250 150 2.5 3.0 3.5 4.0 4.5 5.0 -40 -25 5.5 -10 5 20 35 50 65 0 80 25 50 75 100 SUPPLY VOLTAGE (V) TEMPERATURE (°C) SAMPLE RATE (kHz) SHUTDOWN CURRENT vs. SUPPLY VOLTAGE SHUTDOWN CURRENT vs. TEMPERATURE MAXIMUM SAMPLE RATE vs. SUPPLY VOLTAGE 110 SHUTDOWN CURRENT (nA) 250 DCLK = CS = VDD = 3V 200 150 1000 100 SAMPLE RATE (kHz) DCLK = CS = VDD 90 80 70 125 MXB7846 toc23 120 MXB7846 toc21 300 MXB7846 toc22 2.0 SHUTDOWN CURRENT (nA) VDD = 2.7V VREF = 2.5V 225 SUPPLY CURRENT (µA) SUPPLY CURRENT (µA) 225 285 SUPPLY CURRENT vs. SAMPLE RATE 250 MXB7846 toc19 fSAMPLE = 12.5kHz SUPPLY CURRENT (µA) 290 MXB7846 toc18 250 MXB7846 toc20 SUPPLY CURRENT vs. SUPPLY VOLTAGE 100 10 100 60 50 1 50 2.7 3.2 3.7 4.2 SUPPLY VOLTAGE (V) 4.7 5.2 -40 -25 -10 5 20 35 TEMPERATURE (°C) 50 65 80 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 SUPPLY VOLTAGE (V) _______________________________________________________________________________________ 7 MXB7846 Typical Operating Characteristics (continued) (VDD = 2.7V, VREF = 2.5VEXTERNAL, fDCLK = 2MHz, fSAMPLE = 125kHz, CLOAD = 50pF, 0.1µF capacitor at REF, TA = +25°C, unless otherwise noted.) 2.375V to 5.25V, 4-Wire Touch-Screen Controller with Internal Reference and Temperature Sensor MXB7846 Pin Description PIN NAME 1 VDD Positive Supply Voltage. Connect to pin 10. FUNCTION 2 X+ X+ Position Input, ADC Input Channel 1 3 Y+ Y+ Position Input, ADC Input Channel 2 4 X- X- Position Input 5 Y- Y- Position Input 6 GND Ground 7 BAT Battery Monitoring Inputs; ADC Input Channel 3 8 AUX Auxiliary Input to ADC; ADC Input Channel 4 9 REF 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 0.1µF capacitor. 10 VDD Positive Supply Voltage, +2.375V (2.70V) to +5.25V. External (internal) reference. Bypass with a 1µF capacitor. Connect to pin 1. 11 PENIRQ 12 DOUT Serial Data Output. Data changes state on the falling edge of DCLK. High impedance when CS is HIGH. 13 BUSY Busy Output. BUSY pulses high for one clock period before the MSB decision. High impedance when CS is HIGH. 14 DIN Serial Data Input. Data clocked in on the rising edge of DCLK. 15 CS Active-Low Chip Select. Data is only clocked into DIN when CS is low. When CS is HIGH, DOUT and BUSY are high impedance. 16 DCLK Serial Clock Input. Clocks data in and out of the serial interface and sets the conversion speed (duty cycle must be 40% to 60%). Pen Interrupt Output. Open anode output. 10kΩ to 100kΩ pullup resistor required to VDD. CS tCH tCSS tCL tCP tCSH DCLK tDO tDS tDH DIN tTR tDV DOUT tBDV tBTR BUSY tBD Figure 1. Detailed Serial Interface Timing 8 _______________________________________________________________________________________ 2.375V to 5.25V, 4-Wire Touch-Screen Controller with Internal Reference and Temperature Sensor The MXB7846 uses a successive-approximation conversion technique to convert analog signals to a 12-bit digital output. An SPI/QSPI/MICROWIRE™-compatible serial interface provides easy communication to a microprocessor (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). 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. t ACQ = 8.4 × (RS + RIN ) × 25pF where RIN = 2kΩ and RS is the source impedance of the input signal. Source impedances below 1kΩ do not significantly affect the ADC’s performance. Accommodate higher source impedances by either slowing down DCLK or by placing a 1µF capacitor between the analog input and GND. +VDD PENIRQ TEMP1 VREF 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- 7.5kΩ VBAT 2.5kΩ BATTERY ON AUX GND Figure 2. Equivalent Input Circuit MICROWIRE is a trademark of National Semiconductor Corp. _______________________________________________________________________________________ 9 MXB7846 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: Detailed Description 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+ Y- BUSY PENIRQ 6-TO-1 MUX SERIAL DATA INTERFACE 12-BIT ADC DCLK BATTERY MONITOR BAT AUX DIN CS 2.5V REFERENCE REF Table 1. Input Configuration, Single-Ended Reference Mode (SER/DFR HIGH) A2 A1 A0 MEASUREMENT ADC INPUT CONNECTION 0 0 0 0 0 DRIVERS ON 0 Temp0 Temp0 — 1 Y position X+ Y+, Y- 1 0 BAT BAT — 0 1 1 Z1 X+ X-, Y+ 1 0 0 Z2 Y- X-, Y+ 1 0 1 X- position Y+ X-, X+ 1 1 0 AUX AUX — 1 1 1 Temp1 Temp1 — Table 2. Input Configuration, Differential Reference Mode (SER/DFR LOW) 10 A2 A1 A0 ADC +REF CONNECTION TO ADC -REF CONNECTION TO ADC INPUT CONNECTION TO MEASUREMENT PERFORMED DRIVER ON 0 0 1 Y+ Y- 0 1 1 Y+ Y- X+ Y position Y+, Y- X+ Z1 position 0 1 0 X+ Y+, X- X- Y- Z2 position Y+, X- 1 0 1 X+ X- Y+ X position X+, X- ______________________________________________________________________________________ 2.375V to 5.25V, 4-Wire Touch-Screen Controller with Internal Reference and Temperature Sensor 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 200Ω to 900Ω (depending on the manufacturer), whereas the on-resistance of the X and Y drivers is 8Ω (typ). Limit the touch-screen current to less than 50mA by using a touch screen with a resistance higher than 100Ω. The resistive-divider created by the touch screen and the on-resistance of the X and Y drivers result in both an offset and a gain shift. Also, the on-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 10kΩ to 100kΩ 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. 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 MXB7846 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. MXB7846 2.375V to 5.25V, 4-Wire Touch-Screen Controller with Internal Reference and Temperature Sensor VDD VDD Y+ REF +IN X+ Y+ REF+ 12-BIT ADC -IN X+ REF- +IN -IN Y- REF- Y- GND GND Figure 3. Single-Ended Y-Coordinate Measurement Figure 4. Ratiometric Y-Coordinate Measurement +VDD 100kΩ OPEN CIRCUIT Y+ PENIRQ TOUCH SCREEN X+ YON PENIRQ ENABLE Figure 5. PENIRQ Functional Block Diagram 12 REF+ 12-BIT ADC ______________________________________________________________________________________ 2.375V to 5.25V, 4-Wire Touch-Screen Controller with Internal Reference and Temperature Sensor MXB7846 SCREEN TOUCHED HERE PENIRQ CS DCLK 1 DIN S 2 3 A2 A1 4 5 A0 6 M S/D 7 PD1 8 1 2 3 12 13 14 15 16 PD0 INTERRUPT PROCESSOR NO RESPONSE TO TOUCHMASK PENIRQ PENIRQ ENABLED tTOUCH Figure 6. PENIRQ Timing Diagram MEASURE X- POSITION V - SENSE LINE + Y+ FORCED LINE X+ RTOUCH Z X RTOUCH = (RXPLATE ) × POSITION × 2 − 1 4096 Z1 X- POSITION X- 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: YOPEN CIRCUIT MEASURE Z1 SENSE LINE X+ Y+ R 4096 X RTOUCH = XPLATE × POSITION × −1 Z1 4096 Z1 YPOSITION − RYPLATE × 4096 + RTOUCH V FORCED LINE - X- YOPEN CIRCUIT OPEN CIRCUIT X+ Internal Temperature Sensor Y+ + RTOUCH V FORCED LINE - X- 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: 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 for- ______________________________________________________________________________________ 13 MXB7846 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 0.3°C/LSB and a typical accuracy of ±3°C. 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 × (T1 − VREF T0) × 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 1.6°C/LSB. 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 0.1µF 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.) External Reference DC/DC CONVERTER +2.375V TO +5.25V BATTERY 0 TO 6.0V VDD BAT 0 TO 1.5V 7.5kΩ 12-BIT ADC 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 current. If the reference has a higher output impedance or is noisy, bypass it close to the REF pin with a 0.1µF and a 4.7µF capacitor. Temperature measurements are always performed using the internal reference. Digital Interface 2.5kΩ 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 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0 START A2 A1 A0 MODE SER/DFR PD1 PD0 BIT NAME 7 START 6 A2 5 A1 4 A0 3 MODE 2 SER/DFR 1 PD1 0 PD0 DESCRIPTION Start bit Address (Tables 1 and 2) 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 500µs 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 MXB7846 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 0 0 Enabled ADC is ON during conversion, OFF between conversion 0 1 Disabled 1 0 Disabled 1 1 Disabled STATUS DURING CONVERSION 200 AFTER CONVERSION 1 ADC is always ON, reference is always OFF 200 200 ADC is always OFF, reference is always ON 400 400 ADC is always ON, reference is always ON 600 600 CS TB RB2 RB3 tACQ DCLK 1 DIN S (START) 4 A2 A1 8 SER/ A0 MODE DFR PD1 IDLE 9 12 16 20 24 PD0 ACQUIRE CONVERSION IDLE BUSY RB1 11 DOUT 10 9 8 7 6 5 A/D STATE IDLE CONVERSION ACQUIRE DRIVERS 1 AND 2 (SER/DFR HIGH) OFF DRIVERS 1 AND 2 (SER/DFR LOW) OFF ON 4 3 2 1 0 (LSB) (MSB) IDLE 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 (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 necessary to perform a conversion every 16 DCLK cycles. 16 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. ______________________________________________________________________________________ 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 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 µP. The processor controls the MXB7846 configuration through a control byte (see Tables 3 and 4). Once the processor instructs 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 8 15 1 8 15 1 DCLK DIN S CONTROL BYTE 0 S CONTROL BYTE 1 CONTROL BYTE 2 B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0 B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0 DOUT S CONVERSION RESULT 0 CONVERSION RESULT 1 BUSY Figure 10. 15-Clock/Conversion Timing ... CS 1 8 16 1 8 16 ... DCLK DIN S CONTROL BYTE 0 DOUT S ... CONTROL BYTE 1 B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0 B11 B10 B9 B8 B7 B6 CONVERSION RESULT 0 CONVERSION RESULT 1 ... ... BUSY Figure 11. 16-Clock/Conversion Timing ______________________________________________________________________________________ 17 MXB7846 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. MXB7846 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 10µs 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. Power-On Reset 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. 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 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 = 00…011 00…010 (VREF+ - VREF-) 4096 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. 00…001 00…000 0 1 2 3 FS FS-3/2LSB INPUT VOLTAGE (LSB) = [(V+IN) - (V-IN)] PD1 = 0, PD0 = 1 This mode turns the internal reference off and leaves the ADC on to perform conversions using an external reference. 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 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. 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 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 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. I/O MXB7846 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. CS SCK DCLK MISO DOUT MICROWIRE MXB7846 MOSI MASKABLE INTERRUPT DIN BUSY Figure 13. MICROWIRE Interface I/O CS SCK DCLK MISO DOUT QSPI/SPI MXB7846 MOSI MASKABLE INTERRUPT DIN BUSY Figure 14. QSPI/SPI Interface XF CLKX CS SCLK CLKR TMS320LC3x MXB7846 DX DIN DR DOUT FSR BUSY 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 MXB7846 MXB7846 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 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. 20 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 parallel with a 0.1µF 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 10Ω 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. ______________________________________________________________________________________ 2.375V to 5.25V, 4-Wire Touch-Screen Controller with Internal Reference and Temperature Sensor 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. Aperture Jitter Aperture jitter (tAJ) is the sample-to-sample variation in the time between the samples. Chip Information TRANSISTOR COUNT: 12,000 PROCESS: 0.6µm BiCMOS Typical Application Circuit 2.375V TO 5.5V 1µF TO 10µF OPTIONAL 0.1µF SERIAL/CONVERSION CLOCK +VDD 2 X+ CS 15 CHIP SELECT 3 Y+ DIN 14 SERIAL DATA IN 4 XTOUCH SCREEN DCLK 16 1 5 Y- MXB7846 BUSY 13 CONVERTER STATUS DOUT 12 SERIAL DATA OUT TO BATTERY GND PENIRQ 11 7 BAT +VDD 10 8 AUX REF 9 6 AUXILIARY INPUT PEN INTERRUPT 50kΩ 0.1µF VOLTAGE REGULATOR ______________________________________________________________________________________ 21 MXB7846 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. 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 MXB7846 2.375V to 5.25V, 4-Wire Touch-Screen Controller with Internal Reference and Temperature Sensor PACKAGE OUTLINE, QSOP .150", .025" LEAD PITCH 21-0055 22 E 1 ______________________________________________________________________________________ 1 2.375V to 5.25V, 4-Wire Touch-Screen Controller with Internal Reference and Temperature Sensor TSSOP4.40mm.EPS 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. MXB7846 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.)