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general description the mxb7843 is an industry-standard 4-wire touch- screen controller. it contains a 12-bit sampling analog- to-digital converter (adc) with a synchronous serial interface and low on-resistance switches for driving resistive touch screens. the mxb7843 uses an external reference. the mxb7843 can make absolute or ratio- metric measurements. the mxb7843 has two auxiliary adc inputs. all analog inputs are fully esd protected, eliminating the need for external transzorb devices. the mxb7843 is guaranteed to operate with a single 2.375v to 5.25v supply voltage. in shutdown mode, the typical power consumption is reduced to under 0.5?, while the typical power consumption at 125ksps throughput and a 2.7v supply is 650?. low-power operation makes the mxb7843 ideal for bat- tery-operated systems, such as personal digital assis- tants with resistive touch screens and other portable equipment. the mxb7843 is available in 16-pin qsop and tssop packages, and is guaranteed over the -40? to +85? temperature range. applications personal digital assistants portable instruments point-of-sales terminals pagers touch-screen monitors cellular phones features ? esd-protected adc inputs ?5kv iec 61000-4-2 air-gap discharge ?kv iec 61000-4-2 contact discharge ? pin compatible with mxb7846 ? +2.375v to +5.25v single supply ? 4-wire touch-screen interface ? ratiometric conversion ? spi/qspi, 3-wire serial interface ? programmable 8-/12-bit resolution ? two auxiliary analog inputs ? automatic shutdown between conversions ? low power 270? at 125ksps 115? at 50ksps 25? at 10ksps 5? at 1ksps 2? shutdown current mxb7843 2.375v to 5.25v, 4-wire touch-screen controller ________________________________________________________________ maxim integrated products 1 16 15 14 13 12 11 10 9 1 2 3 4 5 6 7 8 v dd dclk cs din busy dout penirq v dd ref top view mxb7843 qsop/tssop x+ y+ gnd x- y- in3 in4 pin configuration ordering information 19-2435; rev 1; 9/05 for pricing, delivery, and ordering information, please contact maxim/dallas direct! at 1-888-629-4642, or visit maxim? website at www.maxim-ic.com. part temp range pin-package mxb7843eee -40 c to +85 c 16 qsop MXB7843EUE -40 c to +85 c 16 tssop transzorb is a trademark of vishay intertechnology, inc. spi/qspi are trademarks of motorola, inc. typical application circuit appears at end of data sheet.
mxb7843 2.375v to 5.25v, 4-wire touch-screen controller 2 _______________________________________________________________________________________ absolute maximum ratings stresses beyond those listed under ?bsolute maximum ratings?may cause permanent damage to the device. these are stress rating s only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specificatio ns is not implied. exposure to absolute maximum rating conditions for extended periods may affect device reliability. v dd , din, cs , dclk to gnd ...................................-0.3v to +6v digital outputs to gnd...............................-0.3v to (v dd + 0.3v) v ref , x+, x-, y+, y-, in3, in4 to gnd........-0.3v to (v dd + 0.3v) maximum current into any pin .........................................?0ma maximum esd per iec-61000-4-2 (per mil std-883 hbm) x+, x-, y+, y-, in3, in4 ...........................................15kv (4kv) all other pins ..........................................................2kv (500v) continuous power dissipation (t a = +70?) 16-pin qsop (derate 8.30mw/? above +70?).........667mw 16-pin tssop (derate 5.70mw/? above +70?) .......456mw operating temperature range ...........................-40? to +85? junction temperature ......................................................+150? storage temperature range .............................-65? to +150? lead temperature (soldering, 10s) .................................+300? electrical characteristics (v dd = 2.7v to 3.6v, v ref = 2.5v, f dclk = 2mhz (50% duty cycle), f sample = 125khz, 12-bit mode, 0.1? capacitor at ref, t a = t min to t max , unless otherwise noted. typical values are at t a = +25?.) parameter sym b o l conditions min typ max units dc accuracy (note 1) resolution 12 bits no missing codes 11 12 bits relative accuracy inl (note 2) ? ? lsb differential nonlinearity dnl ? lsb offset error ? lsb gain error (note 3) ? lsb noise 70 ? rms conversion rate conversion time t conv 12 clock cycles (note 4) 6 s track/hold acquisition time t acq 3 clock cycles 1.5 ? throughput rate f sample 16 clock conversion 125 khz multiplexer settling time 500 ns aperture delay 30 ns aperture jitter 100 p s channel-to-channel isolation v in = 2.5v p-p at 50khz 100 db serial clock frequency f dclk 0.1 2.0 mhz duty cycle 40 60 % analog input (x+, x-, y+, y-, in3, in4) input voltage range 0 v ref v input capacitance 25 pf input leakage current on/off-leakage, v in = 0 to v dd ?.1 ? ? switch drivers y+, x+ 7 on-resistance (note 5) y-, x- 9 ?
mxb7843 2.375v to 5.25v, 4-wire touch-screen controller _______________________________________________________________________________________ 3 electrical characteristics (continued) (v dd = 2.7v to 3.6v, v ref = 2.5v, f dclk = 2mhz (50% duty cycle), f sample = 125khz, 12-bit mode, 0.1? capacitor at ref, t a = t min to t max , unless otherwise noted. typical values are at t a = +25?.) parameter sym b o l conditions min typ max units reference (reference applied to ref) reference input voltage range (note 6) 1 v dd v input resistance 5g ? f sample = 125khz 13 40 f sample = 12.5khz 2.5 input current f dclk = 0 3 ? digital inputs (dclk, cs , din) input high voltage v ih v dd ? 0.7 v input low voltage v il 0.8 v input hysteresis v hyst 100 mv input leakage current i in ? ? input capacitance c in 15 pf digital output (dout, busy) output voltage low v ol i sink = 250? 0.4 v output voltage high v oh i source = 250? v dd - 0.5 v penirq output low voltage v ol 50k ? pullup to v dd 0.8 v three-state leakage current i l cs = v dd 1 ?0 ? three-state output capacitance c out cs = v dd 15 pf power requirements supply voltage v dd 2.375 5.250 v f sample = 125ksps 270 650 f sample = 12.5ksps 220 supply current i dd f sample = 0 150 ? shutdown supply current i shdn dclk = cs = v dd 3a power-supply rejection ratio psrr v dd = 2.7v to 3.6v full scale 70 db
mxb7843 2.375v to 5.25v, 4-wire touch-screen controller 4 _______________________________________________________________________________________ note 1: tested at v dd = +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: adc performance is limited by the conversion noise floor, typically 300? p-p . an external reference below 2.5v can com- promise the adc performance. timing characteristics (figure 1) (v dd = 2.7v to 3.6v, v ref = 2.5v, f dclk = 2mhz (50% duty cycle), f sample = 125khz, 12-bit mode, 0.1? capacitor at ref, t a = t min to t max , unless otherwise noted. typical values are at t a = +25?.) parameter sym b o l conditions min typ max units timing characteristics (figure 1) acquisition time t acq 1.5 ? dclk clock period t cp 500 ns dclk pulse width high t ch 200 ns dclk pulse width low t cl 200 ns din-to-dclk setup time t ds 100 ns din-to-dclk hold time t dh 0ns cs fall-to-dclk rise setup time t css 100 ns cs rise-to-dclk rise ignore t csh 0ns dclk falling-to-dout valid t do c load = 50pf 200 ns cs rise-to-dout disable t tr c load = 50pf 200 ns cs fall-to-dout enable t dv c load = 50pf 200 ns dclk falling-to-busy rising t bd 200 ns cs falling-to-busy enable t bdv 200 ns cs rise-to-busy disable t btr 200 ns
mxb7843 2.375v to 5.25v, 4-wire touch-screen controller _______________________________________________________________________________________ 5 integral nonlinearity vs. digital output code mxb7843 toc01 output code inl (lsb) 3500 3000 2000 2500 1000 1500 500 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 -0.4 0400 0 differential nonlinearity vs. digital output code mxb7843 toc02 output code dnl (lsb) 3500 3000 2000 2500 1000 1500 500 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1.0 -1.0 -0.8 0 4000 change in offset error vs. supply voltage mxb7843 toc04 supply voltage (v) offset error (lsb) 5.0 4.5 3.0 3.5 4.0 -1.5 -1.0 -0.5 0 0.5 1.0 1.5 2.0 -2.0 2.5 5.5 change in offset error vs. temperature mxb7843 toc05 temperature ( c) offset error from + 25 c (lsb) 65 50 35 20 5 -10 -25 -0.5 0 0.5 1.0 -1.0 -40 80 change in gain error vs. supply voltage mxb7843 toc07 supply voltage (v) gain error (lsb) 5.0 4.5 4.0 3.5 3.0 -2 -1 0 1 2 3 -3 2.5 5.5 change in gain error vs. temperature mxb7843 toc08 temperature ( c) gain error from + 25 c (lsb) 65 50 35 20 5 -10 -25 -1.5 -1.0 -0.5 0 0.5 1.0 -2.0 -40 80 switch on-resistance vs. supply voltage (x+, y+ : + v dd to pin; x-, y- : to gnd) mxb7843 toc03 supply voltage (v) r on ( ? ) 5.0 4.5 4.0 3.5 3.0 2 4 6 8 10 12 14 0 2.5 5.5 x- y- x+ y+ switch on-resistance vs. temperature (x+, y+ : + v dd to pin; x-, y- : pin to gnd) mxb7843 toc06 temperature ( c) r on ( ? ) 35 20 5 -10 -25 2 1 3 7 6 5 4 8 9 10 11 12 0 -40 50 65 80 y+ x+ x- y- t ypical operating characteristics (v dd = 2.7v, v ref = 2.5v, f dclk = 2mhz, f sample = 125khz, c load = 50pf, 0.1? capacitor at ref, t a = +25?, unless otherwise noted.)
mxb7843 2.375v to 5.25v, 4-wire touch-screen controller 6 _______________________________________________________________________________________ t ypical operating characteristics (continued) (v dd = 2.7v, v ref = 2.5v, f dclk = 2mhz, f sample = 125khz, c load = 50pf, 0.1? capacitor at ref, t a = +25?, unless otherwise noted.) reference current vs. supply voltage mxb7843 toc12 supply voltage (v) reference current ( a) 5.0 4.5 4.0 3.5 3.0 7.8 7.9 8.0 8.1 8.2 8.3 7.7 2.5 5.5 c l = 0.1 f f sample = 125khz reference current vs. temperature mxb7843 toc13 temperature ( c) reference current ( a) 65 50 35 20 5 -10 -25 7.8 7.9 8.0 8.1 8.2 8.3 7.7 -40 80 v dd = 2.7v c l = 0.1 f f sample = 125khz reference current vs. sample rate mxb7843 toc14 sample rate (khz) reference current ( a) 100 75 50 25 1 2 3 4 5 6 7 8 9 10 0 0125 supply current vs. supply voltage mxb7843 toc18 supply voltage (v) supply current ( a) 5.0 4.5 4.0 3.5 3.0 2.5 175 200 225 250 150 2.0 5.5 f sample = 12.5khz supply current vs. temperature mxb7843 toc19 temperature ( c) supply current ( a) 65 50 -25 -10 5 20 35 255 260 265 270 275 280 285 290 250 -40 80 f sample = 125khz v dd = 2.7v supply current vs. sample rate mxb7843 toc20 sample rate (khz) supply current ( a) 100 75 50 25 125 150 175 200 225 250 100 0 125 v dd = 2.7v v ref = 2.5v shutdown current vs. supply voltage mxb7843 toc21 supply voltage (v) shutdown current (na) 4.7 4.2 3.7 3.2 100 150 200 250 300 50 2.7 5.2 dlck = cs = v dd shutdown current vs. temperature mxb7843 toc22 temperature ( c) shutdown current (na) 65 50 35 20 5 -10 -25 60 70 80 90 100 110 120 50 -40 80 dclk = cs = v dd maximum sample rate vs. supply voltage mxb7843 toc23 supply voltage (v) sample rate (khz) 5.0 4.5 4.0 3.5 3.0 2.5 10 100 1000 1 2.0 5.5
mxb7843 2.375v to 5.25v, 4-wire touch-screen controller _______________________________________________________________________________________ 7 pin description pin name function 1v dd positive supply voltage. connect to pin 10. 2x + x+ position input, adc input channel 1 3y + y+ position input, adc input channel 2 4x - x- position input 5y - y- position input 6 gnd ground 7 in3 auxiliary input to adc; adc input channel 3 8 in4 auxiliary input to adc; adc input channel 4 9 ref voltage reference input. reference voltage for analog-to-digital conversion. apply a reference voltage between 1v and v dd . bypass ref to gnd with a 0.1? capacitor. 10 v dd positive supply voltage, +2.375v to +5.25v. bypass with a 1? capacitor. connect to pin 1. 11 penirq pen interrupt output. open anode output. 10k ? to 100k ? pullup resistor required to v dd . 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%).
mxb7843 detailed description the mxb7843 uses a successive-approximation conver- sion technique to convert analog signals to a 12-bit digital output. an spi/qspi/microwire-compatible serial interface provides an easy communication to a micro- processor (?). it features a 4-wire touch-screen interface and two auxiliary adc channels ( functional diagram ). analog inputs figure 2 shows a block diagram of the analog input sec- tion that includes the input multiplexer of the mxb7843, the differential signal inputs of the adc, and the differ- ential reference inputs of the adc. the input multiplexer switches between x+, x-, y+, y-, in3, and in4. in single-ended mode, conversions are performed using ref as the reference. in differential mode, ratiometric conversions are performed with ref+ connected to x+ or y+, and ref- connected to x- or y-. configure the refer- ence and switching matrix according to tables 1 and 2. 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? source impedance is high, the acquisition time lengthens, and more time must be allowed between conversions. the acquisition time (t acq ) is the maximum time the device takes to acquire the input signal to 12-bit accuracy. calculate t acq with the following equation: where r in = 2k ? and r s is the source impedance of the input signal. source impedances below 1k ? do not significantly affect the adc? performance. accommodate higher source impedances by either slowing down dclk or by placing a 1? capacitor between the analog input and gnd. input bandwidth and anti-aliasing the adcs input tracking circuitry has a 25mhz small- signal bandwidth, so it is possible to digitize high- speed transient events. to avoid high-frequency sig- nals being aliased into the frequency band of interest, anti-alias filtering is recommended. trrpf acq s in . = + () 84 25 2.375v to 5.25v, 4-wire touch-screen controller 8 _______________________________________________________________________________________ cs dclk din dout busy t bdv t dv t css t cl t ch t ds t dh t cp t do t bd t tr t btr t csh figure 1. detailed serial interface timing microwire is a trademark of national semiconductor corp.
mxb7843 2.375v to 5.25v, 4-wire touch-screen controller _______________________________________________________________________________________ 9 6-to-1 mux v dd ref x+ x- y+ y- 12-bit adc in3 in4 serial d ata interface din cs dclk busy penirq dout functional diagram a2 a1 a0 measurement adc input connection drivers on 00 0 reserved reserved 00 1 y-position x+ y+, y- 010 in3 in3 01 1 reserved reserved 10 0 reserved reserved 10 1 x-position y+ x-, x+ 110 in4 in4 11 1 reserved reserved table 1. input configuration, single-ended reference mode (ser/ dfr high) a2 a1 a0 adc +ref connection to adc -ref connection to adc input connection to measurement performed driver on 00 1y +y -x +y position y+, y- 10 1x +x -y +x position x+, x- table 2. input configuration, differential reference mode (ser/ dfr low)
mxb7843 analog input protection internal protection diodes, which clamp the analog input to v dd and gnd, allow the analog input pins to swing from gnd - 0.3v to v dd + 0.3v without damage. analog inputs must not exceed v dd by more than 50mv or be lower than gnd by more than 50mv for accurate conversions. if an off-channel analog input voltage exceeds the supplies, limit the input current to 50ma. the analog input pins are esd protected to ?kv using the contact-discharge method and ?5kv using the air-gap method specified in iec 61000-4-2. touch-screen conversion the mxb7843 provides two conversion methods?if- ferential and single ended. the ser/ dfr bit in the con- trol word selects either mode. a logic 1 selects a single-ended conversion, while a logic 0 selects a dif- ferential conversion. differential vs. single ended changes in operating conditions can degrade the accu- racy and repeatability of touch-screen measurements. therefore, the conversion results representing x and y coordinates may be incorrect. for example, in single- ended measurement mode, variation in the touch- screen driver voltage drops results in incorrect input reading. differential mode minimizes these errors. single-ended mode figure 3 shows the switching matrix configuration for y-coordinate measurement in single-ended mode. the mxb7843 measures the position of the pointing device by connecting x+ to in+ of the adc, enabling y+ and y- dri- vers, and digitizing the voltage on x+. the adc performs a conversion with ref+ = ref and ref- = gnd. in sin- gle-ended measurement mode, the bias to the touch screen can be turned off after the acquisition to save power. the on-resistance of the x and y drivers results in a gain error in single-ended measurement mode. touch- screen resistance ranges from 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 resistive divider created by the touch screen and the on-resistance of the x and y drivers result in both an offset and a gain shift. also, the on-resis- tance of the x and y drivers does not track the resistance of the touch screen over temperature and supply. this results in further measurement errors. differential measurement mode figure 4 shows the switching matrix configuration for y-coordinate measurement. the ref+ and ref- inputs are connected directly to the y+ and y- pins, respec- tively. differential mode uses the voltage at the y+ pin as the ref+ voltage and voltage at the y- pin as ref- voltage. this conversion is ratiometric and independent of the voltage drop across the drivers and variation in the touch-screen resistance. in differential mode, the touch screen remains biased during the acquisition and conversion process. this results in additional supply current and power dissipation during conversion when compared to the absolute measurement mode. pen interrupt request ( penirq ) figure 5 shows the block diagram for the penirq func- tion. when used, penirq requires a 10k ? to 100k ? pullup to +v dd . 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 mxb7843 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 t touch . the t touch value changes for different touch-screen parasitic capacitance and resistance. the microprocessor receives this interrupt and pulls cs low to initiate a con- version. at this instant, the penirq pin should be masked, as transitions can occur due to a selected input channel or the conversion mode. the penirq pin functionality becomes valid when either the last data bit is clocked out, or cs is pulled high. external reference during conversion, an external reference at ref must deliver up to 40? dc load current. if the reference has a higher output impedance or is noisy, bypass it close to the ref pin with a 0.1? and a 4.7? capacitor. 2.375v to 5.25v, 4-wire touch-screen controller 10 ______________________________________________________________________________________
mxb7843 2.375v to 5.25v, 4-wire touch-screen controller ______________________________________________________________________________________ 11 v dd penirq ref a2?0 (shown 001 b ) ser/dfr (shown high) gnd x+ x- y+ y- converter in3 in4 +ref -ref +in -in figure 2. equivalent input circuit ref v dd gnd y+ y- x+ ref+ ref- +in -in 12-bit adc figure 3. single-ended y-coordinate measurement v dd gnd y+ y- x+ ref+ ref- +in -in 12-bit adc figure 4. ratiometric y-coordinate measurement
mxb7843 2.375v to 5.25v, 4-wire touch-screen controller 12 ______________________________________________________________________________________ open circuit penirq enable penirq touch screen v dd 100k ? y+ x+ y- on figure 5. penirq functional block diagram no response to touch ? mask penirq penirq enabled t touch screen touched here interrupt processor sa2a1a0m s/d pd1 pd0 12345678123 1 2131 41516 penirq cs din dclk figure 6. penirq timing diagram
digital interface 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 mxb7843 digital interface and places dout in a high-impedance state. pulling cs low enables the mxb7843 digital interface. start a conversion by clocking a control byte into din (table 3) with cs low. each rising edge on dclk clocks a bit from din into the mxb7843? internal shift register. after cs falls, the first arriving logic 1 bit defines the control byte? start bit. until the start bit arrives, any number of logic 0 bits can be clocked into din with no effect. the mxb7843 is compatible with spi/qspi/microwire devices. for spi, select the correct clock polarity and sampling edge in the spi control registers of the micro- controller: set cpol = 0 and cpha = 0. microwire, spi, and qspi all transmit a byte and receive a byte at the same time. the simplest software interface requires only three 8-bit transfers to perform a conversion (one 8- bit transfer to configure the adc, and two more 8-bit transfers to read the conversion result) (figure 7). simple software interface make sure the cpu? 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 simultane- ously receive byte rb2. 5) transmit a byte of all zeros ($00 hex) and simultane- ously receive byte rb3. 6) pull cs high. figure 7 shows the timing for this sequence. bytes rb2 and rb3 contain the result of the conversion, padded by four trailing zeros. the total conversion time is a func- tion of the serial-clock frequency and the amount of idle timing between 8-bit transfers. digital output the mxb7843 outputs data in straight binary format (figure 10). data is clocked out on the falling edge of the dclk, msb first. serial clock the external clock not only shifts data in and out, but it also drives the analog-to-digital conversion steps. busy pulses high for one clock period after the last bit of the control byte. successive-approximation bit deci- sions are made and appear at dout on each of the next 12 dclk falling edges. busy and dout go into a high-impedance state when cs goes high. the conversion must complete in 500? 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 conver- sion starts on dclk? falling edge, after the eighth bit of the control byte is clocked into din. the first logic 1 clocked into din after bit 6 of a conver- sion in progress is clocked onto the dout pin and is treated as a start bit (figure 8). once a start bit has been recognized, the current con- version must be completed. the fastest the mxb7843 can run with cs held continu- ously low is 15 clock conversions. figure 8 shows the serial-interface timing necessary to perform a conver- sion every 15 dclk cycles. if cs is connected low and dclk is continuous, guarantee a start bit by first clock- ing in 16 zeros. most microcontrollers (?s) require that data transfers occur in multiples of eight dclk cycles; 16 clocks per conversion is typically the fastest that a ? can drive the mxb7843. figure 9 shows the serial-interface timing nec- essary to perform a conversion every 16 dclk cycles. 8-bit conversion the mxb7843 provides an 8-bit conversion mode selected by setting the mode bit in the control byte high. in the 8-bit mode, conversions complete four clock cycles earlier than in the 12-bit output mode, resulting in 25% faster throughput. this can be used in conjunction with serial interfaces that provide 12-bit transfers, or two conversions could be accomplished with three 8-bit transfers. not only does this shorten each conversion by 4 bits, but each conversion can also mxb7843 2.375v to 5.25v, 4-wire touch-screen controller ______________________________________________________________________________________ 13
mxb7843 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. data format the mxb7843 output data is in straight binary format as shown in figure 10. this figure shows the ideal output code for the given input voltage and does not include the effects of offset, gain, or noise. applications information basic operation of the mxb7843 the 4-wire touch-screen controller works by creating a voltage gradient across the vertical or horizontal resis- tive network connected to the mxb7843, as shown in the typical application circuit . the touch screen is biased through internal mosfet switches that connect each resistive layer to v dd and ground on an alternate basis. for example, to measure the y position when a 2.375v to 5.25v, 4-wire touch-screen controller 14 ______________________________________________________________________________________ dclk 1891216 4 5 6 7 8 9 10 11 3 2 1 0 20 24 4 dout a/d state busy cs din sa2a1a0 mode on off (msb) (lsb) conversion idle idle idle tb acquire conversion idle rb1 off off off on pd1 pd0 (start) ser/ dfr drivers1 and 2 (ser/dfr high) drivers1 and 2 (ser/dfr low) t acq acquire rb2 rb3 figure 7. conversion timing, 24-clock per conversion, 8-bit bus interface 181518 15 1 dclk din dout busy s control byte 0 control byte 1 control byte 2 s s conversion result 0 conversion result 1 b11 b10 b9 b8 b7 b6 b5 b4 b3 b2 b1 b0 b11 b10 b9 b8 b7 b6 b5 b4 b3 b2 b1 b0 cs figure 8. 15-clock/conversion timing
mxb7843 2.375v to 5.25v, 4-wire touch-screen controller ______________________________________________________________________________________ 15 18 1618 16 dclk control byte 0 control byte 1 s conversion result 0 conversion result 1 b11 b10 b9 b8 b7 b6 b11 b10 b9 b8 b7 b6 b5 b4 b3 b2 b1 b0 din dout . . . . . . . . . . . . cs . . . s busy figure 9. 16-clock/conversion timing 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 description 7 start start bit 6a2 5a1 4a0 address (tables 1 and 2) 3 mode conversion resolution. 1 = 8 bits, 0 = 12 bits. 2 ser/ dfr conversion mode. 1 = single ended, 0 = differential. 1 pd1 0 pd0 power-down mode (table 4) table 3. control byte format supply current (typ) (?) pd1 pd0 penirq status during conversion after conversion 00 enabled adc is on during conversion, off between conversion 200 1 01 disabled adc is always on 200 200 10 disabled reserved 11 disabled adc is always on 200 200 table 4. power mode selection
mxb7843 pointing device presses on the touch screen, the y+ and y- drivers are turned on, connecting one side of the vertical resistive layer to v dd and the other side to ground. in this case, the horizontal resistive layer func- tions as a sense line. one side of this resistive layer gets connected to the x+ input, while the other side is left open or floating. the point where the touch screen is pressed brings the two resistive layers in contact and forms a voltage-divider at that point. the data converter senses the voltage at the point of contact through the x+ input and digitizes it. the horizontal layer resistance does not introduce any error in the conversion because no dc current is drawn. the conversion process of the analog input voltage to digital output is controlled through the serial interface between the a/d converter and the ?. the processor controls the mxb7843 configuration through a control byte (tables 3 and 4). once the processor instructs the mxb7843 to initiate a conversion, the mxb7843 biases the touch screen through the internal switches at the beginning of the acquisition period. the voltage transient at the touch screen needs to settle down to a stable volt- age before the acquisition period is over. after the acqui- sition period is over, the a/d converter goes into a conversion period with all internal switches turned off if the device is in single-ended mode. if the device is in differential mode, the internal switches remain on from the start of the acquisition period to the end of the con- version period. power-on reset when power is first applied, internal power-on circuitry resets the mxb7843. allow 10? for the first conversion after the power supplies stabilize. if cs is low, the first logic 1 on din is interpreted as a start bit. until a con- version takes place, dout shifts out zeros. power modes save power by placing the converter in one of two low- current operating modes or in full power-down between conversions. select the power-down mode through pd1 and pd0 of the control byte (tables 3 and 4). the software power-down modes take effect after the conversion is completed. the serial interface remains active while waiting for a new control byte to start a con- version and switches to full-power mode. after complet- ing its conversion, the mxb7843 enters the programmed power mode until a new control byte is received. the power-up wait before conversion period is depen- dent on the power-down state. when exiting software low-power modes, conversion can start immediately when running at decreased clock rates. upon power- on reset, the mxb7843 is in power-down mode with pd1 = 0 and pd0 = 0. when exiting software shutdown, the mxb7843 is ready to perform a conversion in 10?. pd1 = 1, pd0 = 1 in this mode, the mxb7843 is always powered. the device remains fully powered after the current conver- sion completes. pd1 = 0, pd0 = 0 in this mode, the mxb7843 powers down after the current conversion completes or on the next rising edge of cs , whichever occurs first. the next control byte received on din powers up the mxb7843. at the start of a new con- version, it instantly powers up. when each conversion is finished, the part enters power-down mode, unless other- wise indicated. the first conversion after the adc returns to full power is valid for differential conversions and sin- gle-ended measurement conversions. when operating at full speed and 16 clocks per conver- sion, the difference in power consumption between pd1 = 0, pd0 = 1, and pd1 = 0, pd0 = 0 is negligible. also, in the case where the conversion rate is decreased by slowing the frequency of the dclk input, the power consumption between these two modes is not very different. when the dclk frequency is kept at 2.375v to 5.25v, 4-wire touch-screen controller 16 ______________________________________________________________________________________ output code fs = (v ref+ - v ref- ) fs-3/2lsb full-scale transition input voltage (lsb) = [(v +in ) - (v -in )] 123 fs 0 11?11 11?10 11?01 00?11 00?10 00?01 00?00 1lsb = (v ref+ - v ref- ) 4096 figure 10. ideal input voltages and output codes
the maximum rate during a conversion, conversions are done less often. there is a significant difference in power consumption between these two modes. pd1 = 0, pd0 = 1 in this mode, the mxb7843 is powered down. this mode becomes active after the current conversion completes or on the next rising edge of cs , whichever occurs first. the next command byte received on the din returns the mxb7843 to full power. the first conver- sion after the adc returns to full power is valid. pd1 = 1, pd0 = 0 this mode is reserved. hardware power-down cs also places the mxb7843 into power-down. when cs goes high, the mxb7843 immediately powers down and aborts the current conversion. 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 elimi- nate this issue are described below. one option is to lengthen the acquisition time by stopping or slowing down dclk, allowing for the required touch- screen settling time. this method solves the settling time problem for both single-ended and differential modes. the second option is to operate the mxb7843 in the dif- ferential mode only for the touch screen, and perform additional conversions with the same address until the input signal settles. the mxb7843 can then be placed in the power-down state on the last measurement. connection to standard interface microwire interface when using the microwire- (figure 11) or spi-com- patible interface (figure 12), set the cpol = cpha = 0. two consecutive 8-bit readings are necessary to obtain the entire 12-bit result from the adc. dout data transi- tions occur on the serial clock? falling edge and are clocked into the ? on the dclk? rising edge. the first 8-bit data stream contains the first 8-bits of the current conversion, starting with the msb. the second 8-bit data stream contains the remaining 4 result bits fol- lowed by 4 trailing zeros. dout then goes high imped- ance when cs goes high. qspi/spi interface the mxb7843 can be used with the qspi/spi interface using the circuit in figure 12 with cpol = 0 and cpha = 0. this interface can be programmed to do a conver- sion on any analog input of the mxb7843. tms320lc3x interface figure 13 shows an example circuit to interface the mxb7843 to the tms320. the timing diagram for this interface circuit is shown in figure 14. use the following steps to initiate a conversion in the mxb7843 and to read the results: 1) the tms320 should be configured with clkx (trans- mit clock) as an active-high output clock and clkr (tms320 receive clock) as an active-high input clock. clkx and clkr on the tms320 are connect- ed to the mxb7843 dclk input. 2) the mxb7843? cs pin is driven low by the tms320? xf i/o port to enable data to be clocked into the mxb7843? din pin. 3) an 8-bit word (1xxxxxxx) should be written to the mxb7843 to initiate a conversion and place the device into normal operating mode. see table 3 to select the proper xxxxxxx bit values for your spe- cific application. 4) the mxb7843? busy output is monitored through the tms320? fsr input. a falling edge on the busy output indicates that the conversion is in progress and data is ready to be received from the devices. 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 mxb7843 until the next conversion is initiated. layout, grounding, and bypassing for best performance, use printed circuit (pc) boards with good layouts; wire-wrap boards are not recommend- ed. board layout should ensure that digital and analog signal lines are separated from each other. do not run analog and digital (especially clock) lines parallel to one another, or digital lines underneath the adc package. establish a single-point analog ground (star ground point) at gnd. connect all analog grounds to the star mxb7843 2.375v to 5.25v, 4-wire touch-screen controller ______________________________________________________________________________________ 17
mxb7843 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? power supply. power-supply decoupling is also crucial for optimal device performance. analog supplies can be decou- pled by placing a 10? tantalum capacitor in parallel with a 0.1? capacitor bypassed to gnd. to maximize performance, place these capacitors as close as possi- ble to the supply pin of the device. minimize capacitor lead length for best supply-noise rejection. if the supply is very noisy, a 10 ? resistor can be connected in series as a lowpass filter. while using the mxb7843, the interconnection between the converter and the touch screen should be as short as possible. since touch screens have low resistance, longer or loose connections may introduce error. noise can also be a major source of error in touch-screen applications (e.g., applications that require a backlight lcd panel). emi noise coupled through the lcd panel to the touch screen may cause flickering of the convert- ed data. utilizing a touch screen with a bottom-side metal layer connected to ground decouples the noise to ground. in addition, the filter capacitors from y+, y-, x+, and x- inputs to ground also help further reduce the noise. caution should be observed for settling time of the touch screen, especially operating in the single- ended measurement mode and at high data rates. 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 mxb7843 are mea- sured using the end-point method. differential nonlinearity differential nonlinearity (dnl) is the difference between an actual step width and the ideal value of 1lsb. a dnl error specification of less than 1lsb guarantees no missing codes and a monotonic transfer function. aperture jitter aperture jitter (t aj ) is the sample-to-sample variation in the time between the samples. aperture delay aperture delay (t ad ) is the time defined between the falling edge of the sampling clock and the instant when an actual sample is taken. chip information transistor count: 12,000 process: 0.6? bicmos 2.375v to 5.25v, 4-wire touch-screen controller 18 ______________________________________________________________________________________ microwire i/o sck mosi maskable interrupt miso cs dclk din busy dout mxb7843 figure 11. microwire interface qspi/spi i/o sck mosi maskable interrupt miso cs dclk din busy dout mxb7843 figure 12. qspi/spi interface tms320lc3x xf clkx dr dx fsr clkr cs sclk din busy dout mxb7843 figure 13. tms320 serial interface
mxb7843 2.375v to 5.25v, 4-wire touch-screen controller ______________________________________________________________________________________ 19 cs din busy high impedance high impedance dout dclk start a2 a1 a0 mode ser/def pd1 pd0 msb b10 b1 b0 figure 14. mxb7843-to-tms320 serial interface timing diagram 16 15 14 13 12 11 10 9 1 2 3 4 5 6 7 8 v dd serial/conversion clock chip select serial data in converter status serial data out pen interrupt 50k ? x+ y+ x- y- gnd in3 in4 touch screen 2.375v to 5.5v 1 f to 10 f optional 0.1 f 0.1 f mxb7843 dclk cs din penirq busy dout v dd ref t ypical application circuit
mxb7843 2.375v to 5.25v, 4-wire touch-screen controller 20 ______________________________________________________________________________________ package information (the package drawing(s) in this data sheet may not reflect the most current specifications. for the latest package outline info rmation, go to www.maxim-ic.com/packages .) qsop.eps e 1 1 21-0055 package outline, qsop .150", .025" lead pitch
mxb7843 2.375v to 5.25v, 4-wire touch-screen controller maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a maxim product. no circu it patent licenses are implied. maxim reserves the right to change the circuitry and specifications without notice at any time. maxim integrated products, 120 san gabriel drive, sunnyvale, ca 94086 408-737-7600 ____________________ 21 2005 maxim integrated products printed usa is a registered trademark of maxim integrated products, inc. package information (continued) (the package drawing(s) in this data sheet may not reflect the most current specifications. for the latest package outline info rmation, go to www.maxim-ic.com/packages .) tssop4.40mm.eps package outline, tssop 4.40mm body 21-0066 1 1 g

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