Segment LCD driving method
- Jun 12, 2018 -

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Segment code LCD LCD driver method First of all, do not think that with the microcontroller to drive the segment code screen is DC-driven, in fact, segment code screen is AC drive, what is AC? Rectangular wave, sine wave and so on. Everyone may often use the driver chip to play, such as the HT1621, but some segment screen IO ports are less, or if the IO port is sufficient, you can also save the write controller driver. The interface with the MCU is convenient, while the latter has a small drive current, low power consumption, long life, beautiful shape, clear display, large viewing angle, flexible driving mode, and wide application. However, the LCD on the control is more complicated because the relative DC value between the LCD electrodes must be 0. Otherwise, the LCD will be oxidized easily. Therefore, the LCD cannot be controlled by the level signal simply. Instead, a square wave sequence with a certain waveform is used. control.


The LCD display has two modes of static and time division. The former is simple, but requires more lines; the latter is complicated, but requires fewer lines. These two modes are determined by the electrode lead selection method. The following uses the liquid crystal display of the electronic watch as an example. The high hour of the hour is also off or on. When the high digit of the minute shows the digital number 1 to 5, the top and bottom are also off or on at the same time. The two dot points are also on or off at the same time. The driving method is divided driving with a bias ratio of 1/2, and there are 11 segment electrodes and two common electrodes. However, there is a precondition for IO analog-driven liquid crystal display, that is, IO must be three-state. Why?



LCD segment LCD screen

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The first step, the important parameters of the segment code LCD: operating voltage, duty cycle, bias ratio. These three parameters are very important and must be met.


The second step, driving method: according to the driving principle of the LCD, only the AC voltage can be added to the LCD pixel. The contrast of the LCD display is determined by subtracting the voltage value on the SEG pin from the voltage value on the COM pin. When this voltage difference Saturation voltage greater than the LCD can open the pixel, and it can turn off the pixel when the LCD threshold voltage is lower. The LCD type MCU has automatically generated the LCD drive signal from the built-in LCD driver circuit. Therefore, as long as the I/O port can simulate and output the drive signal. , you can complete the LCD driver.


Segment code LCD screen has two main pins, COM, SEG, similar to the digital tube, but the pressure difference must be alternating, such as the first moment is a positive 3V, then the second moment must be reversed 3V It is important to note that if the segment code LCD panel is powered by DC, the screen will be wasted for a long time, so be careful. Let's consider how to simulate the COM port waveform. Take 1/4D, 1/2B as an example:


At the same time, we must pay attention to, when the COM output is high, if the screen is on, SEG will output low, then when the COM output is low, SEG will output high, ensure that the pressure difference between COM and SEG is greater than 1/2B operating voltage It can be displayed


When the SEG level is reversed with the COM level, driving the segment LCD is basically successful.


Section code lcd basic knowledge


The liquid crystal display is a passive display, it can not emit light, can only use the ambient light. It shows only a small amount of energy for the pattern or character. Because of low power consumption and miniaturization, LCD has become a better display method.


The liquid crystal material used in the liquid crystal display is an organic material having both liquid and solid properties. Its rod-like structure is generally arranged in parallel within the liquid crystal cell, but it can change its alignment direction under the action of an electric field.


For a positive TN-LCD, when no voltage is applied to the electrode, the LCD is in the "OFF" state, and the light energy is transmitted through the LCD in the white state; when the voltage is applied to the electrode, the LCD is in the "ON" state, the long axis direction of the liquid crystal molecules. Arranged in the direction of the electric field, the light cannot pass through the LCD and appears black. Selectively applying a voltage across the electrodes, different patterns can be displayed.


For the STN-LCD, the twist angle of the liquid crystal is larger, so the contrast is better and the viewing angle is wider. STN-LCD is based on birefringence theory, its basic color is generally yellow-green, font blue, yellow green model. When using a purple polarizer, the base color turns gray to become a gray mold. When using a polarizing film with a compensation film, the base color will become nearly white. At this time, the STN becomes a black-and-white mode, that is, FSTN. The above mode of the polarizer rotates 90°, that is, it becomes a blue mode, and the effect will be better.




As can be seen from the figure, the liquid crystal display is a liquid crystal cell made of upper and lower two sheets of conductive glass. The cell is filled with liquid crystals, and the periphery is sealed with a sealing material-a plastic frame (usually an epoxy resin). Both sides of the cell are sealed. Polarizer attached.


The interval between the upper and lower glass plates in the liquid crystal cell, which is generally referred to as the cell thickness, is generally several micrometers (man's accuracy diameter is tens of micrometers). The inside of the upper and lower glass plates, corresponding to the display pattern portion, is coated with a transparent conductive oxide-tin oxide (ITO) conductive film, ie, a display electrode. The role of the electrode is mainly to make the external electrical signal through it to the liquid crystal.


The entire display area inside the glass pane in the liquid crystal cell is covered with an alignment layer. The role of the alignment layer is to align the liquid crystal molecules in a specific direction. This alignment layer is usually a thin layer of organic polymer and is treated by rubbing; it can also be prepared by vacuum evaporation of a silicon oxide film at an angle on the glass surface. .


The TN-type liquid crystal display is filled with a positive nematic liquid crystal. The orientation of the liquid crystal molecules is such that the long rod-type liquid crystal molecules are arranged in a fixed direction parallel to the glass surface, and the direction of the long axis of the molecules is along the orientation treatment direction. The orientation directions of the upper and lower glass surfaces are perpendicular to each other, so that the orientation of the liquid crystal molecules in the box is gradually distorted in the direction perpendicular to the surface of the glass sheet, and the glass sheet is twisted by 90° from the upper glass sheet to the lower glass sheet (see the figure below). It is the origin of the twisted nematic LCD name.




In fact, the liquid crystal molecules close to the glass surface are not completely parallel to the glass surface, but rather are at a certain angle to it. This angle is called the pretilt angle, which is generally 1°~2°.


Two polarizers are respectively attached to the outer sides of the glass sheet in the liquid crystal cell, and the polarization axes of the two polarizers are parallel to each other (the normally black type is white on a black background) or are orthogonal to each other (a normally white type is a black symbol on a white background). The orientation direction of the surface of the liquid crystal cell is parallel or perpendicular to each other. Polarizers are generally processed by polymer plastic film under certain process conditions.


Most of what we usually see is a reverse-type liquid crystal display, which has a reflective sheet behind the lower polarizer. In this way, light is incident and observed on the same side of the cell.


Display method


LCD has a variety of display methods: reflective, transmissive and transflective. A reflective plate is attached behind the bottom polarizer of the reflective LCD. It is generally used outdoors and in well-lit offices. The bottom polarizer of a transmissive LCD is a transmissive polarizer, which requires the continuous use of a backlight, and is generally used in a poor light environment. The transflective LCD is between the above two. The bottom polarizer can partially reflect light and generally has a backlight. When the light is good, the backlight can be turned off. When the light is poor, the backlight can be lit using the LCD.


LCD display is also divided into positive and negative. Positive LCDs have black letters on a white background and are best viewed in reflective and transflective LCDs; negative LCDs are shown in black on white and are generally used in transmissive LCDs. With a backlight, the fonts are clear and easy to read.


Backlight


Transmissive and semi-transmissive LCDs generally need to add a backlight source. The placement of the backlight according to the actual situation below introduces several common backlight sources:


Electroluminescence (EL): EL backlights are thin, lightweight, and emit light evenly. It can be used in different colors, but it is most commonly used in LCD white backlights. EL backlight power consumption is low, only the voltage of 80-100VAC, through the transformer to 5V, 12V or 24VDC conversion. The EL backlight has a half-life of about 2000-3000 hours.


Light Emitting Diodes (LEDs): LED backlights are mainly used for character-type modules. Longer life than EL (minimum 5000 hours), stronger light, but greater energy consumption. As a solid state device, it uses 5VDC directly. The LCD is generally arranged directly behind the LCD, and the thickness is increased by 5mm. The LEDs can emit different colors of light, the most common being yellow-green light.


Cold Cathode Fluorescent Lamp (CCFL): The CCFL can provide low power and bright white light. It emits light from a cold cathode fluorescent tube, and the light is evenly dispersed in the window area by a diffuser. The side backlight has a small size and low power consumption, but the CCFL needs a transformer to supply 270-300VAC. It is mainly used for graphic LCDs and has a lifetime of 10,000 to 15,000 hours.


TN and STN are two types of liquid crystal displays. The liquid crystal of the TN display is twisted by 90[deg.] in the liquid crystal cell and is generally used for low-channel LCD products.


The liquid crystal displayed by the STN is twisted by 180° to 360° in the liquid crystal cell. The larger the twist angle, the steeper the electro-optical curve and the closer the V on and V off values. Can be used for the production of 32 or more LCD products.


LCD perspective


The point of view is simply the angle at which the display pattern can be seen clearly. It is determined by the rubbing direction of the alignment layer and cannot be changed by rotating the polarizer. The angle of view is named after the hour hand, such as 6:00 viewing angle, 12:00 viewing angle, and so on. The 6:00 viewing angle means that the LCD in the area from the 6 o'clock to the normal direction of the hour hand is ideal; the 12:00 viewing angle is the ideal display of the 12 o'clock corn to the normal direction.


The LCD's viewing angle is determined by the position of the LCD display on the instrument. For example, a calculator is usually placed on the table or on the hand, and the LCD is made at the 6:00 viewing angle. Some instruments have an LCD screen mounted below the line of sight of the human eye and are generally made at a viewing angle of 12:00. The clock on the car is generally mounted on the right side of the driver, making the best viewing angle of 9:00.