Contents of the Lecture

Contents of the Lecture • 1. Introduction • 2. Methods for I/O Operations • 3. Buses • 4. Liquid Crystal Displays • 5. Other Types of Displays • 6. Graphics Adapters • 7. Optical Discs Input/Output Systems and Peripheral Devices (04-1)

4. Liquid Crystal Displays • Liquid Crystals • TN Technology • Addressing Methods • Backlighting • Characteristics • VA Technology • IPS Technology Input/Output Systems and Peripheral Devices (04-1)

Liquid Crystals (1) • Liquid crystals: discovered in 1888 • Changing the state of a material known as cholesteryl benzoate from solid into liquid • Substances that exhibit anisotropyof properties  variable depending on the direction of measurement • Equilibrium state – mesomorphic • State between solid crystalline and liquid Input/Output Systems and Peripheral Devices (04-1)

Liquid Crystals (2) • Light passing through liquid crystals follows the alignment of the molecules • Applying an electric or magnetic field changes the molecular alignment of liquid crystals • Three types of liquid crystals: • Thermotropic • Lyotropic • Metallotropic Input/Output Systems and Peripheral Devices (04-1)

Liquid Crystals (3) • Thermotropicliquid crystals • Transition into several phases with temperature changes • Lyotropicliquid crystals • Present phase transitions determined primarily by the concentration of molecules in a solvent • Metallotropicliquid crystals • Composed of organic and inorganic molecules • Phase transitions also depend on the organic / inorganic composition ratio Input/Output Systems and Peripheral Devices (04-1)

Liquid Crystals (4) • Phases of thermotropic liquid crystals • High temperature: liquid (isotropic) phase • Low temperature: solid (crystalline) phase • Nematic phase • Smectic phase • Cholesteric phase • Types of ordering for the phases: • Positional order of molecules • Orientation order of molecules Input/Output Systems and Peripheral Devices (04-1)

Liquid Crystals (5) • Nematic phase (N) • Nema – thread; nemato – threadlike (Greek) • Threadlike molecules • No positional order • Approximately parallel orientation order  director • Can be easily aligned by an electric field Input/Output Systems and Peripheral Devices (04-1)

Liquid Crystals (6) • Smectic phase (Sm) • Molecules maintain the orientation order • They align in layers • Positional order along one direction • SmA (left) • SmC (right) • Other Sm phases exist Input/Output Systems and Peripheral Devices (04-1)

Liquid Crystals (7) • Cholesteric phase (N*) • Typical for cholesterol esters cholesteric • Structure similar to a stack of 2D nematic layers • The director in each layer is twisted with respect to adjacent layers • Twisted nematic(TN) Input/Output Systems and Peripheral Devices (04-1)

TN Technology • TN Technology • Principle of Operation • Color Displays • Structural Details • STN Technology • DSTN Technology • FSTN Technology Input/Output Systems and Peripheral Devices (04-1)

Principle of Operation (1) • Liquid crystal displays arepassive • Use a light source (backlight) or a mirror (to reflect ambient light) • The operation is based on the properties of polarized light • The light waves are oriented in parallel with a specific direction • Can be obtained with a polarizing filter Input/Output Systems and Peripheral Devices (04-1)

Principle of Operation (2) • The polarized light passes through a TN liquid crystal layer • The light follows the alignment of molecules • The polarizing direction is changed by the twisting of molecules Input/Output Systems and Peripheral Devices (04-1)

Principle of Operation (3) • Single pixel: TN liquid crystals introduced between two transparent electrodes • The electrodes are provided withalignment layers to control molecule alignment  grooves • The grooves on the two electrodes are perpendicular to each other • This results in a90 twist of the longitudinal axes of molecules on the two electrodes Input/Output Systems and Peripheral Devices (04-1)

Principle of Operation (4) • Two polarizing filters • Two glass plates • Two transparent electrodes • TN liquid crystal layer • The light is polarized by the first filter • The polarizing direction is twisted with 90 • The light will also pass through the second filter Input/Output Systems and Peripheral Devices (04-1)

Principle of Operation (5) • When anelectrical voltage is applied, the molecules realign • The direction of longitudinal axes tends to align in parallel to the field • The light is not twisted→ is blocked by the second filter • By controlling the voltage, different levels of gray can be obtained Input/Output Systems and Peripheral Devices (04-1)

Principle of Operation (6) • Displays for which the light is blocked in the areas with no voltage applied • The polarizing directions are parallel • The optical effect is more dependent on the thickness of display when no voltage is applied • The eye is more sensitive to variations of brightness in the dark state  spotted image • This variant may also increase power consumption Input/Output Systems and Peripheral Devices (04-1)

Principle of Operation (7) • The response of a TN cell to an applied voltage Input/Output Systems and Peripheral Devices (04-1)

Principle of Operation (8) • Percent transmission of light Input/Output Systems and Peripheral Devices (04-1)

Color Displays • Intermediate levels of brightness are required • Changing the voltage applied to the cells • The white backlight contains all the wavelengths • The color components are obtained through filtering of the white light • Each pixel is composed of three subpixels for the primary RGB colors Input/Output Systems and Peripheral Devices (04-1)

Structural Details Input/Output Systems and Peripheral Devices (04-1)

STN Technology (1) • STN – Super-Twisted Nematic • The difference between the voltages for which a cell is on / off must be very small • The TNtechnology is impractical for large sizes with conventional addressing • STNtechnology:the direction of the polarized light is rotated with an angle of 180 .. 270 • The diagram indicating the light transmission becomes more abrupt Input/Output Systems and Peripheral Devices (04-1)

STN Technology (2) Input/Output Systems and Peripheral Devices (04-1)

STN Technology (3) • Advantages of STN technology compared to the TNtechnology: • Higher contrast • Wider viewing angle • Simpler control for the percent transmission of light through the liquid crystal cells Input/Output Systems and Peripheral Devices (04-1)

STN Technology (4) • Disadvantages of STN technology: • Slower response time compared to the TN technology • Lower brightness level • Higher manufacturing costs • Early STNdisplays presented an undesirable coloration shifted transmission spectrum • In the on state: yellow • In the off state: bluish Input/Output Systems and Peripheral Devices (04-1)

DSTN Technology (1) • DSTN – DoubleSuper-Twisted Nematic • Solved the coloration problem oftheSTNtechnology by adding a new STN layer • For the second layer, the twisting direction of the polarized light is oppositeto that of the first layer • In the off state, the phase shift due to the first layer is compensated by the second layer black pixel Input/Output Systems and Peripheral Devices (04-1)

DSTN Technology (2) • The on state of the pixel is not affected by the second STNlayer  white pixel • Both layers consist of the same type of liquid crystalthe characteristics are constant • Disadvantages: • A more intense backlight is required • Higher cost • Higher thickness and weight Input/Output Systems and Peripheral Devices (04-1)

FSTN Technology • FSTN – FilmSuper-Twisted Nematic • Color compensation is achieved with a thin polymerfilm instead of the glass layer • Advantages compared to DSTN technology: • Lower cost • Lower thickness and weight • Lower-power backlight • Disadvantage: • Reduced contrast Input/Output Systems and Peripheral Devices (04-1)

Addressing Methods • Addressing Methods • Direct Addressing • Multiplexed Addressing • Passive Matrix Displays • Active Matrix Displays Input/Output Systems and Peripheral Devices (04-1)

Direct Addressing • Used for displays with a small number of display elements • Each element (segment or pixel) can be addressed or driven separately • A voltage should be applied to each element to change orientation of the crystals Input/Output Systems and Peripheral Devices (04-1)

Multiplexed Addressing (1) • Used for displays with a large number of pixels • Each pixel sits at the intersection of a row electrode and a column electrode Input/Output Systems and Peripheral Devices (04-1)

Multiplexed Addressing (2) • Advantage: • Reduced complexity of the circuits • For a matrix of 100x100 pixels, 200 drivers are needed (compared to 10,000 with direct addressing) • Disadvantage: • Reduced contrast • TNdisplays have been improved through various methods Input/Output Systems and Peripheral Devices (04-1)

Passive Matrix Displays (1) • Use a set of multiplexedtransparent electrodes • The liquid crystal layer is placed between the electrodes • The electrodes are composed of indium tin oxide(ITO) Input/Output Systems and Peripheral Devices (04-1)

Passive Matrix Displays (2) • A pixel – addressed when a voltage is applied across it • The pixelbecomes opaquewhen it is addressed • When the voltage is removed, the pixel deactivates slowly Input/Output Systems and Peripheral Devices (04-1)

Passive Matrix Displays (3) • The display controller scans across the matrix of pixels • Delay since the voltage is applied to a pixel until it is turned on response time • Inertia of the pixels after the voltage is removed • The time to scan the entire matrix must be shorter than the time needed for the pixels to deactivate (turn-off time) Input/Output Systems and Peripheral Devices (04-1)

Passive Matrix Displays (4) • Disadvantages: • Crosstalk – interference between pixels the occurrence of shadows for bright objects • The viewing angle is limited • The response time is relatively slow the current image is still maintained on the screen after a new image is displayed Input/Output Systems and Peripheral Devices (04-1)

Active Matrix Displays (1) • The front glass plate of the display is coated with a continuous electrode • The rear glass plate is coated with electrodes divided into pixels • Each pixel is connected in series with a thin film transistor (TFT) Input/Output Systems and Peripheral Devices (04-1)

Active Matrix Displays (2) • A pixel of the active matrix display • Active elements:field effect transistors (FET) • Based onamorphous silicon (a-Si) • Based onpolysilicon (p-Si) Input/Output Systems and Peripheral Devices (04-1)

Active Matrix Displays (3) Input/Output Systems and Peripheral Devices (04-1)

Active Matrix Displays (4) • An image is created by scanning the matrix: • A row of pixels is selectedby applying voltage to the row electrode connected to the transistor gates on that row • Voltages corresponding to the image are applied to the column electrodes connected to the transistor sources • The operations are repeated for each row • Refresh rate of the screen: 50 or 60 Hz Input/Output Systems and Peripheral Devices (04-1)

Active Matrix Displays (5) • Advantages (compared to passive matrix displays): • Faster response time • Higher contrast • Higher brightness level • Wider viewing angle • Disadvantages: • More intense backlight is required • Higher cost Input/Output Systems and Peripheral Devices (04-1)

Contents of the Lecture