v v m m o o d d t t f f t t ? ? r r e e f f e e r r e e n n c c e e m m a a n n u u a a l l revision: november 11, 2011 note: this document applies to rev c of the board. 1300 ne henley court, suite 3 pullman, wa 99163 (509) 334 6306 voice | (509) 334 6300 fax doc: 502-210 page 1 of 5 copyright digilent, inc. all rights reserved. other product and company names mentioned may be tradema rks of their respective owners. overview the vmodtft board lets you connect an lcd touchscreen to any digilent fpga system board with a vhdci connector. it acts as both an input and output device. it features a 4.3? wide-format vivid color led- backlit lcd screen. the screen has a 480272 native resolution display with a color depth of 24 bits per pixel. a four-wire resistive touchscreen with anti- glare coating covers the entire active display area. features include: ? a 4.3? color tft lcd with a resistive touchscreen ? 480272 resolution ? 24 bpp color depth ? 80hz maximum refresh rate ? an led backlight with pwm control ? a four-wire resistive touchscreen with an analog devices ad7873 touch controller ? 125 ksps maximum touch sample rate ? an on-board temperature sensor that can detect ?40c to 85c ? a vhdci connector functional description the two-plate four-wire touchscreen uses four analog lines connecting the opposite sides of the x and y plates. a touch controller gives the touchscreen an fpga interface. the controller can both drive and sample the touchscreen using a digital serial interface. the lcd is driven using a digital parallel interface. the led backlight is driven by the on-board current source. you can change its intensity digitally using a pwm (pulse width modulation) signal or you can set jumper jp1 to analog for the default backlight setting. the lcd screen and the touchscreen can be used independently. touch readings are noisier when the lcd is on, but you can filter the noise and still obtain a fast sample rate. if you require maximum precision and sample rates, you should turn the lcd off during touchscreen sampling. power-up sequence the lcd has a specific power-up sequence that needs to be followed for proper operation. the vmodtft should only be attached to the system board once the signals driven by the system board are defined. the vmodtft can only be used with fpga system boards that support 3.3v supply for the vhdci connector. for more information, refer to your system board?s reference manual at www.digilentinc.com .
vmodtft reference manual www.digilentinc.com page 2 of 5 copyright digilent, inc. all rights reserved. other product and company names mentioned may be tradema rks of their respective owners. figure 1 power-up sequence description min max unit t1 tft-en high to first pixel bus signal 0.05 100 ms t2 valid pixel data to disp high 0 200 ms t3 disp high to backlight on; backlight off to disp low 160 ms t4 tft-en low pulse 100 ms table 1 power-up/reset timing video timing to display an image, the lcd needs to be continuously driven with properly-timed data. this data consists of the lines and blanking periods that form video frames. each frame consists of 272 active lines and several vertical blanking lines. each line consists of 480 active pixel periods and several horizontal blanking periods. parameter description value unit fclk pixel clock 9 mhz tva vertical active period 272 lines tvb vertical blanking period 16 lines tha horizontal active period 480 clk periods thb horizontal blanking period 45 clk periods table 2 typical lcd video timing parameters figure 2 video timing video data is sent on a parallel interface synchronous to clk. the table below lists the timing parameters of this interface. ?data? refers to the combined pixel data from the r, g, and b pins.
vmodtft reference manual www.digilentinc.com page 3 of 5 copyright digilent, inc. all rights reserved. other product and company names mentioned may be tradema rks of their respective owners. figure 3 ac characteristics description min max unit 1/tclk lcd clock frequency 7 12 mhz tdes de setup before falling clk 10 ns tdeh de hold before falling clk 10 ns tds data setup before falling clk 10 ns tdh data hold before falling clk 10 ns table 3 lcd power-up/reset timing backlight control the led-en (tft_bklt_o) pin provides analog or digital control over the backlight intensity. jumper jp1 toggles between analog and digital control. in analog mode, the backlight intensity is set by the voltage divider formed by r7 and r10. valid values are 0.7v (dimmest) to 1.4v (brightest). in digital mode, the fpga can drive this pin with a pwm signal of 100hz-50khz. touchscreen the four-wire resistive touchscreen is on top of the lcd. it consists of two conductive transparent layers, one on top of the other, separated by a layer of air. these conductive layers form plates x and y. each plate has electrodes on opposite sides and a constant resistance. figure 4 touchscreen plates x and y when the screen is touched with sufficient force, the two plates make contact and form resistive dividers. by properly biasing the panel and measuring the voltage on the electrodes, the coordinates of the touch can be determined. for example, tying y+ and y- to a power supply, the voltage on either x+ or x- changes linearly to the y coordinate of the touch. similarly, supplying power to the x plate and measuring the voltage on the y electrode determines the x coordinate. touch controller the vmodtft uses an ad7873 touch controller to provide an interface for the analog touchscreen to the fpga. the controller supplies current to the touchscreen and measures the voltage drop using an internal 12-bit analog-to-digital converter. the digitized voltage values are read back by the fpga so the touch coordinates can be determined. theoretically, a digitized voltage drop of 000h (0v) corresponds to a touch very close to the electrode tied to ground. likewise, a voltage drop of fffh (vcc) corresponds to a touch very close to the electrode tied to the power supply. however, the actual values measured for the vmodtft are shown in table 4.
vmodtft reference manual www.digilentinc.com page 4 of 5 copyright digilent, inc. all rights reserved. other product and company names mentioned may be tradema rks of their respective owners. touch corner x reading y reading top left 096h 12ch top right f6eh 12ch bottom left 096h ed8h bottom right f6eh ed8h table 4 actual 12-bit voltage readings using these experimental values, the touch coordinates scaled to the lcd active area can be easily determined. use the following equations, where v x and v y are the voltage readings for the x and y plates. equation 1 convert voltage readings to coordinates touch can be detected on the penirq interrupt signal. a low value indicates the screen is being touched. you can also measure the pressure of the touch, referred to as the z coordinate. the measurement requires a different biasing of the touchscreen and requires some calculation. the equation uses the x and y plate resistance, which for this panel can be approximated by 1024 and 256 ohms, respectively. for information on how to use the touch controller to measure touch pressure, refer to the ad7873 datasheet at http://www.analog.com/static/imported- files/data_sheets/ad7873.pdf vhdci connector the vmodtft is only compatible with system boards using 3.3v-level signaling and should only be attached to the system board once the signals driven by the system board are defined. vhdci vmodtft vhdci vmodtft io1-p tp_busy io1-n nc io2-p tp_din io2-n tp_dout io3-p tp_cs io3-n tp_penirq io4-p tft_b[5] io4-n tft_b[7] io5-p tft_b[4] io5-n tft_b[6] io6-p tft_b[2] io6-n tft_b[3] io7-p tft_b[0] io7-n tft_b[1] io8-p tft_g[6] io8-n tft_g[7] io9-p tft_g[5] io9-n tft_g[4] io10-p tp_dclk io10-n nc io11-p tft_clk io11-n nc io12-p tft_g[1] io12-n tft_g[3] io13-p tft_g[0] io13-n tft_g[2] io14-p tft_r[6] io14-n tft_r[7] io15-p tft_r[4] io15-n tft_r[5] io16-p tft_r[2] io16-n tft_r[3] io17-p tft_r[1] io17-n tft_r[0] io18-p tft_disp io18-n tft_de io19-p tft_en io19-n led_en io20-p nc io20-n nc table 5 vhdci connector pin-out camera output camera input
vmodtft reference manual www.digilentinc.com page 5 of 5 copyright digilent, inc. all rights reserved. other product and company names mentioned may be tradema rks of their respective owners. design resources for reference designs, library components and demo projects see www.digilentinc.com.
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