Displays
(6 182)The display is one of the basic elements of the user interface. Nowadays, it is often combined with a touch interface, which allows for a variety of functionalities.
Simple LED segment displays show single graphic characters – usually numbers or letters. Slightly more elaborate ones, such as matrix displays, can show single graphic signs composed of luminous dots. Alphanumeric display modules show character strings, typically in 1 to 4 lines. The most powerful version are graphic displays that, depending on the resolution and the number of available colours, can show information in the form of graphic images with a quality that depends on the resolution and technology used to manufacture the display model. The use of a graphical display also gives you the ability to use scalable-height fonts.
Types of displays
Displays can be divided according to the manufacturing technology, the way of presenting information, the built-in controller and the type of housing.
The most popular displays based on LED technology are segment and matrix displays. Both the number of LED segments and pixels may differ between different display models. A wide range of sizes and colours are also available. The most commonly used ones are 7-segment LED displays and matrices with a resolution of 5x7 LED points, but there are also displays with, for example, 16 segments, with which you can display almost any character from the ASCII palette. Usually, the LED display does not have a built-in controller, and you have to make it yourself based on digital circuits or using specialized drivers for LED displays.
LCD display
Nowadays, most of the displays used in consumer devices use the LCD technology. These include: digital, alphanumeric, graphic, and special displays, i.e. ones that can display application-specific symbols. Often, symbols are combined with variable content. For example, there are displays for CD players that can show numbers, an analogue clock, symbols of a record, musical notes, headphones, etc.
LCD displays do not shine with their own light – they require background illumination. The backlighting takes many forms – from passive backlighting, achieved by means of reflective material, to active illumination with high-voltage electroluminescent foil or LED diodes. The image can be displayed as positive (light background, darkened characters) or negative (dark background, lightened characters). The image obtained on the negative LCD screen resembles that on the OLED screen and sometimes it is difficult to distinguish between display models made with the application of these two technologies.
Currently, the best quality of the colour image is provided by LCD TFT displays. They are composed of many millions of RGB pixels which, when properly controlled, can produce up to 16.7M colours. Such displays, like other LCDs, require backlighting.
LCD display control
LCD displays may be controlled in a variety of ways. The least expensive displays have no built-in controller and require control by external electronic circuits. These are usually used in mass-produced consumer devices. More expensive display modules usually have built-in controllers with a parallel interface, where the number of bits depends on the resolution and number of colours of the display. For example, text-based, monochrome display modules most often have a 4- or 8-bit interface and some additional control signals (R/W, E, RS). Smaller graphic displays may have an 8-bit or 16-bit interface, which is controlled by additional control signals. Miniature graphic and text modules are more and more often equipped with an alternative, high-speed serial interface, e.g. SPI, I2C or other, specialized one. LCD-TFT displays most often have a standardized, digital, 24-bit RGB interface.
OLED displays
OLED displays are built with the use of the OLED technology, so they do not require backlighting. Thanks to this, they are much thinner, with vivid colours and deep blacks. They are very popular in wearable electronics. This type of a display provides excellent image quality that is almost independent of the ambient light. Monochrome OLED displays are becoming more and more popular. These include, among other types, modules that are functional equivalents of LCD display modules, but at the same time are thinner and provide a much better readability of the presented messages also in the sunlight.
Vacuum Fluorescent Displays (VFD)
In VFD displays, the luminophore that covers the cathode glows, and its structure resembles a triode. Signs are attached by means of a grid placed above the luminophore, and electrons are excited by a very thin, incandescent filament. Older models of VFD displays required complex, elaborate power supply (AC power supply to the filament, high voltage supply to the segments) and high voltage drivers. Newer models are manufactured as a module powered by single voltage (the necessary inverters are built on its board) and controlled via a digital interface. Thus, from the designer's point of view, the control is compatible with the standard used in other popular LCD families. The VFD display provides very good image readability – in the full sunlight it operates a wider temperature range than LCD displays, while at the same time there is no "smudging" at low temperatures. However, due to the filament, the VFD display requires a considerable supply current, so it is worth remembering to provide an adequately efficient power source.
E-paper
E-paper (e-ink) displays show information by changing the polarity of particles. They require power only to change the state, making them very energy efficient. Due to their significant inertia, these displays are primarily suitable for showing static information, such as pages of e-books and e-newspapers in e-readers or product labels. The first displays of this type were monochromatic, but now colour displays are also available and in use.
Smart Displays
Smart displays are mostly TFT displays combined into one module with an advanced control board. The board contains a specialized controller with an embedded microcontroller, additional memory, expansion connectors, etc. They have built-in patterns of displayed elements or make it possible to upload user’s custom-designed elements. Very often, they are integrated with a touch panel (resistive or capacitive) and have a basic sound signalling function (buzzer). This gives the display much more functionality, with easier operation and less load on the main system. In some of these display models, additional module boards can be attached to extend functionality. The memory of some displays can be expanded with an SD or microSD card – you can store scripts, graphics definitions and fonts used on the screen. Smart displays can also have their own enclosure and an interface to an external control device, not necessarily a local one, such as RS485 to a PC or PLC. Functionally, they resemble HMI panels.
Selecting a display for your application
When choosing a display for your application, you need to pay attention to many factors. Among them, the most important one is the type of information to be displayed, its number and the type of interface available. These will directly affect the size of the display and its type (digital, alphanumeric, graphic, special, etc.).
It is also impossible to choose a proper display without taking into consideration the lighting conditions and the temperature range in which the device will be used. For example, LCD displays do poorly at low temperatures, where VFDs, LEDs and OLEDs will do well. OLED displays, which present information in almost the same visual way as VFDs, are becoming more popular than expensive VFDs and replace them in a variety of vehicles.
The next factor to be take into account is the available power supply, that is, whether the display will work in a portable or stationary device. The number of voltages required for power supply should also be considered – the more voltages there are, the more difficult it is to use such a display in a portable device.
The type of the interface by which the display will be controlled is also an important factor. For example, if the device design has a powerful microcontroller and will be mass-produced, it makes sense to use a low-cost LCD display without a built-in controller and control it with the aforementioned processor, which will significantly reduce the price of the target solution.
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