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LCD Displays Created March 2008 ©Paul R. Godin Introduction LCDs are dominating the display marketplace ◊ ◊ ◊ portable electronics devices such as: ◊ digital watches & timers ◊ thermometers & meters ◊ MP3 players ◊ telephones and other communication devices ◊ cameras ◊ game platforms laptop & computer monitors high definition televisions photo PRGodin ◊ Displays from cellular phones Advantages of LCDs LCDs have significant advantages over other forms of displays: ◊ ◊ ◊ ◊ ◊ ◊ very low power requirement very compact low voltage sharp image (no focus or distortion problems) scalable integrate with digital circuits photo PRGodin ◊ Display salvaged from a VCR Disadvantages of LCD displays sensitive to temperature variations ◊ ◊ ◊ ◊ ◊ ◊ ◊ very slow response times with cold temperatures risk of permanent damage from cold or heat less robust to physical stresses color displays are susceptible to bad pixels poor viewing angles slow response times Fixed characters and image density photo PRGodin ◊ Damaged display New product development strives to minimize these issues LCD Configurations ◊ Segment displays ◊ Dot Matrix Displays ◊ Graphic Displays ◊ TFT (Thin Film Transistor) Displays LCD Diagram Protective Glass Polarizer filter 0° Front glass with electrodes Rear glass with liquid crystal and electrodes Polarizer filter 90° Reflective layer Glass for backlighting (optional) Light Fundamentals ◊ Light is electromagnetic energy. ◊ It has physical properties that are similar to both the photon (particle) model and a wave model. ◊ Light consists of two waves that are perpendicular (90°) to one another. Light as a wave ◊ The frequency of the light wave is referenced as wavelength. Wavelength is distance between cycles. C f C= Speed of Light f = frequency ◊ Visible light is between 350 ηM (ultraviolet) and 750 ηM (infrared). Polarization ◊ Light waves can be filtered using polarized filters. ◊ Polarized filters remove one of the two perpendicular waves, reducing the intensity of the light. Polarization ◊ If a second filter is added perpendicular to the orientation of the first, all the light is filtered out and the area appears dark. Passive Displays ◊ LCDs are considered passive displays as they do not emit light (except through backlighting). Instead, they rely on reflecting ambient light. Reflective layer Glass for backlighting (optional) Polarized displays ◊ LCDs rely on polarization to darken the segments. Aligned polarization from two displays Full polarization from a 90° angle between two displays Liquid Crystal ◊ Liquid crystal substances are liquids that arrange themselves in an orderly, crystal-like manner. ◊ There are a variety of liquid crystal compounds with different properties. Twisted Nematic Crystal ◊ Twisted Nematic liquid crystals align themselves in a 90 degree angle. This twists the polarized light going through the compound. ◊ If exposed to voltage the crystals untwist and become vertical allowing light to pass through without twisting. ITO Traces ◊ Indium Tin Oxide is a transparent conductive compound that is used with LCDs. ◊ Shaped, conductive traces are placed above and below the liquid crystal layer. ◊ The matrix displays have long parallel conductors on the upper side and lower side, but they are perpendicular to each other. Basic LCD construction 1-Vertical polarizing filter 2-Transparent layer with ITO (indium tin oxide) traces 3-Twisted Nematic compound 4-Transparent layer with ITO traces 5-Horizontal filter 6-Reflective backing Image under CC licence Liquid Crystal ◊ The polarization angle can be changed by controlling the twists in the twisted nematic compound with an applied voltage. ◊ When no voltage is applied, the light which has been polarized once follows the twist of the compound and encounters the second polarizing filter with the same orientation. Light gets through the filters. ◊ When the compound has voltage applied the crystal structure untwists and the two polarizing filters prevent light from passing, creating a dark segment. LCD Drivers ◊ LCDs often come with a microcontroller and driver circuit which: ◊ Decodes the ASCII code input ◊ Drives the appropriate segments ◊ Retains the values being displayed in memory to refresh the display ◊ Performs logic functions that control the display ◊ The driver outputs must oscillate. Typical frequency is approximately 30 Hz. ◊ LCD Drivers typically contain CG-ROM (CG = Character Generator) Instruction / Data ◊ Instruction Mode ◊ shift characters left-to-right, or right-to-left ◊ display the cursor ◊ move the cursor or move the characters on the display ◊ set the number of display lines ◊ direction ◊ etc... ◊ Data Mode ◊ standard ASCII ◊ extended ASCII (Japanese 8-bit for our displays) Interfacing with LCDs ◊ Most LCD devices operate with CMOS-based logic devices. ◊ Static sensitive ◊ Sensitive to reverse voltage ◊ Most displays follow a defacto standard for connectivity and programming. ◊ Charts are available for programming the displays. Activity ◊ View specification sheets for display characteristics ◊ View other reference information on how to interface displays with the output of electronic circuits ◊ Lab activity review END ©Paul R. Godin prgodin @ gmail.com