Segment code LCD liquid crystal screen driving method
First of all, don't think that the segment screen is driven by DC when driven by a single{{0}}chip microcomputer. In fact, the segment screen is driven by AC. What is AC? Rectangular wave, sine wave, etc. You may often use driver chips, such as HT1621, etc., but some segment screens have fewer IO ports, or if there are enough IO ports, you can also save the need to write controller drivers. It is convenient to interface with the single-chip microcomputer, and the latter has small drive current, low power consumption, long life, beautiful font, clear display, large viewing angle, flexible drive mode, and wide application. However, the control of LCD is more complicated, because the DC average value of the relative voltage between the LCD electrodes must be 0, otherwise it is easy to cause LCD oxidation, so LCD can not be simply controlled by level signals, but with a certain waveform of square wave sequence. control.
LCD display has two modes: static and time division. The former is simple, but requires more ports; the latter is complicated, but requires fewer ports. These two modes are determined by the selection of electrode leads. Let's take the LCD display of an electronic watch as an example. The high digit of the hour will be off or on at the same time. When the high digit of the minute is displaying numbers 1 to 5, the top and bottom will also be off or on at the same time. The two dots will also be on or off at the same time. The driving mode is time-division driving with a bias ratio of 1/2, with 11 segment electrodes and two common electrodes in total. However, there is a prerequisite for IO analog driving segment LCD, that is, IO must be tri-state, why?
Let's talk about it carefully below:
The first step is the important parameters of the segment LCD screen: working voltage, duty cycle, and bias ratio. These three parameters are very important and must be met.
The second step, the driving method: According to the driving principle of LCD, it can be known that only AC voltage can be applied to LCD pixels. The contrast of LCD display is determined by the voltage value on the COM pin minus the voltage value on the SEG pin. When this voltage difference The pixel can be turned on when the saturation voltage of the LCD is greater than the LCD threshold voltage, and the pixel can be turned off. The LCD-type MCU has been automatically generated by the built-in LCD drive circuit to generate the LCD drive signal, so as long as the I/O port can simulate the output of the drive signal , The LCD drive can be completed.
The segment LCD screen has two main pins, COM and SEG, which are very similar to the digital tube, but the pressure difference must be alternately changed. For example, the first moment is 3V in the forward direction, and the second moment must be 3V in the reverse direction. , Pay attention, if the segment type LCD screen is connected with DC power, the screen will be useless in a short time, so be careful. Let's consider how to simulate the waveform of the COM port, taking 1/4D and 1/2B as examples:
As long as the above waveforms are simulated, the LCD screen has been more than half successful.
One article to analyze the driving method of segment code LCD liquid crystal screen
One article to analyze the driving method of segment code LCD liquid crystal screen
Enter the above code as shown, timing 2ms, and let 4 COM alternately output waveforms.
At the same time, we must pay attention that when the COM output is high, if the screen is bright, the SEG must output low, then when the COM output is low, the SEG must output high to ensure that the voltage difference between COM and SEG is greater than 1/2B working voltage Can be displayed
Let's look at the function of one of the com port output
One article to analyze the driving method of segment code LCD liquid crystal screen
Move the SEG level to the opposite of the COM level at all times, then the segment LCD is basically successfully driven.
Basic knowledge of segment LCD
The liquid crystal display is a passive display. It cannot emit light and can only use the light of the surrounding environment. It only needs a small amount of energy to display patterns or characters. It is precisely because of low power consumption and miniaturization that LCD becomes a better display method.
The liquid crystal material used in liquid crystal display is an organic substance with both liquid and solid properties. Its rod-like structure is generally arranged in parallel in the liquid crystal cell, but its arrangement direction can be changed under the action of an electric field.
For a positive TN-LCD, when no voltage is applied to the electrodes, the LCD is in the "OFF" state, and the light can pass through the LCD in a white state; when voltage is applied to the electrodes, the LCD is in the "ON" state, and the liquid crystal molecules are in the long axis direction. Arranged along the direction of the electric field, the light cannot pass through the LCD and is black. By selectively applying voltage on the electrodes, different patterns can be displayed.
For 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 the principle of birefringence for display. Its base color is generally yellow-green, and the font is blue, which becomes a yellow-green mode. When using a purple polarizer, the base color will change to gray and become a gray mold. When using a polarizer with a compensation film, the base color will become close to white. At this time, the STN becomes the black and white mode, which is FSTN. The polarizers of the above three modes turn 90 degree to turn into a blue mode, and the effect will be better. The figure below is a structural diagram of a reflective TN-type liquid crystal display.
One article to analyze the driving method of segment code LCD liquid crystal screen
As can be seen from the figure, the LCD is a liquid crystal box made of two pieces of conductive glass, the box is filled with liquid crystal, and the surroundings are sealed with a sealing material-a plastic frame (usually epoxy resin), and the two outer sides of the box A polarizer is attached.
The space between the upper and lower glass sheets in the liquid crystal cell, which is commonly referred to as the cell thickness, is generally a few microns (the diameter of human accuracy is tens of microns). The inner side of the upper and lower glass sheets, corresponding to the display pattern, are plated with a transparent oxide-tin oxide (ITO for short) conductive film, which is the display electrode. The main function of the electrode is to allow external electrical signals to be applied to the liquid crystal through it.
The entire display area inside the glass sheet in the liquid crystal cell is covered with an orientation layer. The function of the orientation layer is to align the liquid crystal molecules in a specific direction. This orientation layer is usually a thin layer of high molecular weight organic matter and is rubbed; it can also be prepared by vacuum evaporation of silicon oxide film on the glass surface at a certain angle .
Positive nematic liquid crystal is filled in TN type liquid crystal display. The orientation of the liquid crystal molecules means that the long rod-shaped 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 direction of the orientation treatment. The orientation directions of the upper and lower glass surfaces are perpendicular to each other, so that in the direction perpendicular to the surface of the glass sheet, the orientation of the liquid crystal molecules in the cell is gradually distorted, from the upper glass sheet to the lower glass sheet by 90 degree (see the figure below). This is the origin of the name of the twisted nematic liquid crystal display.
One article to analyze the driving method of segment code LCD liquid crystal screen
In fact, the liquid crystal molecules close to the glass surface are not completely parallel to the glass surface, but form a certain angle with it. This angle is called the pretilt angle, which is generally 1 degree 2 degree .
Polarizers are attached to the two outer sides of the glass sheet in the liquid crystal cell. The polarization axes of the two polarizers are parallel to each other (normally black type with white characters on a black background) or orthogonal to each other (normally white type with black characters on a white background), and The orientation direction with the surface of the liquid crystal cell is parallel or perpendicular to each other. Polarizers are generally processed by polymer plastic films under certain technological conditions.
What we usually see is a reverse-type liquid crystal display. This kind of display has a reflective sheet behind the polarizer on the bottom. In this way, the incident and observation of light are on the same side of the liquid crystal cell.
Display method
LCD has three display modes: reflective, transmissive and transflective. A reflective plate is added behind the bottom polarizer of reflective LCD, which is generally used outdoors and in well-lit offices. The bottom polarizer of a transmissive LCD is a transmissive polarizer, which requires continuous use of the backlight, and is generally used in poor light environments. The transflective LCD is in between the above two, the bottom polarizer can partially reflect light, and it is generally equipped with a backlight. When the light is good, the backlight can be turned off; when the light is poor, the backlight can be turned on and use the LCD.
One article to analyze the driving method of segment code LCD liquid crystal screen
LCD display mode is also divided into positive and negative. Positive LCD presents black characters on a white background, which is the best display among reflective and transflective LCDs; negative LCD presents white characters on a black background, and is generally used for transmissive LCDs. With backlight, the fonts are clear and easy to read.
One article to analyze the driving method of segment code LCD liquid crystal screen
Backlight
Transmissive and semi-transmissive LCDs generally need to add a backlight. The placement position is based on the actual situation. Here are several common backlights:
One article to analyze the driving method of segment code LCD liquid crystal screen
Electroluminescence (EL): EL backlight is thin in thickness, light in weight and uniform in light emission. It can be used in different colors, but is most commonly used for LCD white backlighting. The EL backlight has low power consumption and only needs a voltage of 80-100VAC, which can be obtained by converting 5V, 12V or 24VDC through a transformer. The half-life of EL backlight is about 2000-3000 hours.
Light Emitting Diode (LED): LED backlight is mainly used for character type modules. Longer life than EL (minimum 5000 hours), stronger light, but more energy consumption. As a solid-state device, it directly uses 5VDC. LCDs are generally arranged directly behind the LCD, and the thickness should be increased by 5mm. LEDs can emit light of different colors, the most common being yellow-green light.
Cold Cathode Fluorescent Lamp (CCFL): CCFL can provide low energy consumption and bright white light. It emits light from a cold cathode fluorescent tube, and the light is evenly dispersed in the window area through a diffuser. The side backlight is small in size and low in energy consumption, but CCFL needs a transformer to supply 270-300VAC power. It is mainly used for graphic LCDs with a life span of 10,000 to 15,000 hours.
TN and STN are two types of liquid crystal displays. The liquid crystal of TN display is twisted 90 degree in the liquid crystal cell, and is generally used for low-channel LCD products.
The liquid crystal displayed by STN is twisted 180 degree -360 degree in the liquid crystal cell. The larger the twist angle, the steeper the electro-optical curve and the closer the value of V on and V off. It can be used for the production of LCD products with more than 32 channels.
LCD viewing angle
The angle 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 of the hour hand, such as the angle of 6:00, the angle of 12:00, and so on. The 6:00 viewing angle means that the LCD display effect is ideal in the area from the plane direction of the 6 o'clock hand to the normal direction; the 12:00 viewing angle refers to the ideal display in the area from the cornmeal to the normal direction of the 12 o'clock hand.
The viewing angle of LCD is determined by the position of the LCD display on the instrument. For example, a calculator is generally used on the desk or held in your hand, and the LCD with a 6:00 viewing angle is the best. The LCD screen on some instruments is installed below the line of sight of the human eye, and the viewing angle is generally 12:00. The clock on the car is generally installed on the right side of the driver, and the viewing angle of 9:00 is the best.
