Electronic Output Displays
Display devices are the output devices for presentation of information in text or image form. An output device is a thing that provides a way to show information to the outside world. For displaying the information in an appropriate manner these devices must be controlled by some other external devices. Controlling can be done by interfacing these displays with the controlling devices. Microcontrollers are useful to the extent that they communicate with external devices, such as switches, keypads, displays, memory and even other microcontrollers. Many interfacing techniques have been developed to solve the complex problems for communicating with displays. Some displays can show digits and alphanumeric characters only. Some displays can show images and all type of characters. Most commonly used displays along with microcontrollers are LEDs, LCD, GLCD, and 7-segment displays
Let us view details about each type of Displays Available
Display Using LED:
Light emitting diode (LED) is the most commonly used device for displaying the status of microcontroller pins. These display devices are commonly used for the indication of alarms, inputs and timers. There are two ways by we can connect LEDs to microcontroller unit. Those two ways are active high logic and active low logic. Active high logic means LED will be ON when port pin is 1 and LED will be OFF when pin is 0. Active high means LED will be OFF when port pin is 1 and LED will be ON when port pin is 0.
7-Segment LED Display:
7-Segment LED display can be used for displaying digits and few characters. A seven segment display consists of 7 LEDs arranged in the form of Square ‘8’ and a single LED as dot character. Different characters can be displayed by selecting the required LED segments. A 7 seven segment display is an electronic display, which displays 0-9 digital information. They are available in common cathode mode and common anode mode. There are state lines in LED, anode is given to positive terminal and cathode is given to negative terminal then LED will glow.
In common cathode, the negative terminals of all LEDs are connected to the common pins to ground and a particular LED glows when its corresponding pin is given high. The cathodes of all LEDs are connected together to a single terminal and the anodes of all LEDs are left alone.
In common anode arrangement, the common pin is given a high logic and the LED pins are given low to display a number. In common anode, all the anodes are connected together and all the cathodes are left alone. Thus when we gives first signal is high or 1 then only there is a lean in display if not there is no lean in display.
Dot Matrix LED Display:
Dot matrix LED display contains the group of LEDs as a two dimensional array. They can display different types of characters or a group of characters. Dot matrix display is manufactured in various dimensions. Arrangement of LEDs in the matrix pattern is made in either of the two ways: Row anode-column cathode or Row cathode-column anode. By using this dot matrix display we can reduce the number of pins required for controlling all the LEDs. A dot matrix is a two dimensional array of dots used to represent characters, symbols and messages. Dot matrix is used in displays. It is a display device used to display information on many devices like machines, clocks, railway departure indicators etc. An LED dot matrix consists of an array of LED’s which are connected such that the anode of each LED are connected together in the same column and the cathode of each LED are connected together in the same row or vice versa. An LED dot matrix display can also come with multiple LEDs of varying colors behind each dot in the matrix like red, green, blue etc.
Here each dot represents circular lenses in front of LEDs. This is done to minimize the number of pins required to drive them. For example an 8X8 matrix of LEDs would need 64 I/O pins, one for each LED pixel. By connecting all the anodes of LEDs together in a column and all the cathodes together in row, the required number of input and output pins reduced to 16. Each LED will be addressed by its row and column number. Diagram of 8X8 LED Matrix using 16 I/O pins
Controlling the LED Matrix:
Since all the LEDs in a matrix share their positive and negative terminals in each row and column, it is not possible controlling of each LED at the same time. The matrix controlled through each row very quickly by triggering the correct column pins to light the desired LED’s for that particular row. If the switching done with a fixed rate, humans can’t see the displaying message, because human eye can’t detect the images with in the milliseconds of time. Thus the displaying of a message on LED matrix must be controlled, with the rows being scanned sequentially at a rate greater than 40 MHz while sending out the column data at the exact same rate. This kind of controlling can be done my interfacing the LED matrix display with the microcontroller.
Interfacing the LED Matrix Display with Microcontroller:
Choosing a microcontroller for interfacing with LED matrix display which is to be controlled is depends on the number of input and output pins needed for controlling all the LEDs in the given matrix display, the amount of current that each pin can source and sink and the speed at which the microcontroller can send out control signals. With all these specifications, interfacing can be done for LED matrix display with a microcontroller.
Using 12 I/O pins controlling the Matrix display of 32 LEDs
In the above diagram each seven segment display is having 8 LEDs. Hence the total number of LEDs is 32. For controlling all the 32 LEDs 8 information lines and 4 control lines are needed i.e. for displaying message on the matrix of 32 LEDs, 12 lines are needed when they are connected in matrix notation. Using the microcontroller instructions can be converted into signals which turn ON or OFF lights in the matrix. Then the required message can be displayed. By controlling with the microcontroller, we can change which color LEDs are lit at even intervals.
There are several options for choosing microcontroller and LED matrix. The easiest way is first choosing the LED dot matrix and then selecting a microcontroller which needs the requirements of LEDs to be controlled. Once these selections are completed, a major part is lies in programming to scan the columns and feed the rows with appropriate values for the LED matrix to display different patterns for displaying required message.
Liquid Crystal Display (LCD):
Liquid crystal display (LCD) has material which joins together the properties of both liquid and crystals. They have a temperature range within which the particles are essentially as mobile as they might be in a liquid, however are gathered together in an order form similar to a crystal. The LCD is much more informative output device than a single LED. The LCD is a display that can easily show characters on its screen. They have a couple of lines to large displays. Some LCDs are specially designed for specific applications to display graphic images. 16×2 LCD (HD44780) module is commonly used. These modules are replacing 7-segments and other multi-segment LEDs. LCD can be easily interfaced with microcontroller to display a message or status of the device. It can be operated in two modes: 4-bit mode and 8-bit mode. This LCD has two registers namely command register and data register. It is having three selection lines and 8 data lines. By connecting the three selection lines and data lines with the microcontroller, the messages can be displayed on LCD.
In the above figure 3 selected lines EN, R/W, RS will be used for controlling the LCD display. EN pin will be used for enabling the LCD display for communicating with microcontroller. RS will be used for register selection. When RS is set microcontroller will send instructions as data and when RS is clear microcontroller will send the instructions as commands. For writing data RW should be 0 and for reading RW should be 1.
PIN Description
Interfacing 16×2 LCD with Microcontroller:
Many microcontroller devices are using smart LCD displays to output visual information. For an 8-bit data bus, the display requires a +5V supply plus 11 I/O lines. A 4 bit data bus requires supply line as well as 7 extra lines. When the LCD display is not enabled, data lines are tri-state which means they are in a state of high impedance and this means they do not interfere with the microcontroller operation when display is not used.
The three control lines are referred to as EN, RS and RW.
· The EN (Enable) control line is used to send the data to the LCD. A high to low transition at this pin will enable the module.
· When RS or Register Select is low, the data is to be treated as a command instruction. When RS is high, the data being sent is displayed on the screen. For Instance, to display any character on the screen, we set RS high.
· When RW or Read/Write Control line is low, the information on the data bus is being written to the LCD. When RW is high, the program is effectively reading the LCD. RW line will always be low.
The data bus consists of 4 or 8 lines; it depends on the mode of operation selected by the user. The lines of an 8 bit data bus are referred to as DB0,DB1,DB2,DB3,DB4,DB5,DB6 and DB7.
A Typical Application of 16×2 LCD Display:
In this application, we follow a CAN (Control Area Network) like concept generally used in cars, automobiles and industries. As the name implies control area network means that microcontroller is connected in a network fashion like computers so that it can exchange data among themselves. Here we are using 2 microcontrollers connected in a network fashion by a pair of wire connected to pin 10 and 11(i.e., P3.0, P3.1) of port 3 of each microcontroller pins for the transmission and reception of data among themselves with the help of RS232 serial communication using a pair of wire. Where first microcontroller is interfaced to 4×3 matrix keypad which is connected to input ports of first microcontroller and second microcontroller is interfaced to an LCD display to receive data from first microcontroller. An LCD which we are using is 16×2 which can display 16 characters in two lines.
For each microcontroller separate program is written in C and Hex files of it are burnt on to the respective microcontroller. When we apply power to the circuit then the LCD displays a message WAITING which means it is waiting for some data. For example a password as 1234, when 1 is pressed from keyboard then LCD displays 1 and when 2 is pressed it displays 2 and same for 3 but when 4 is pressed from keyboard they are all displayed and data communication takes place through the Rx and Tx pair to make transistor to conduct. If we enter wrong password then a buzzer will sound giving indication of wrong password.
Graphical LCD Displays:
16X2 LCDs have their own limitations. They can display characters of certain limitations. The graphical LCDs can be used to display customized characters and images. The graphical LCDs find use in many applications like video games, mobile phones, and lifts as display units. The most commonly used GLCD is JHD12864E. This LCD has a display format of 128×64 dots. These graphical LCDs are needed controllers to execute its internal operations. These LCDs are having page schemes. The page schemes can be understood using the following table. Here CS stands for control select.
The 128×64 LCD implies 128 columns and 64 rows. The images will be displayed in the form of pixels unlike normal LCDs and LEDs.
Electroluminescent Display Technology
Electroluminescent display technology is one of most widely used technique these days for display solutions. They are basically a type of flat panel display. LED and Phosphor displays are now popular which uses the principle of Electroluminescence. It is the property by the virtue of which a semiconductor emits photons or quantum of light energy when supplied with electricity. Electroluminescence results from the radioactive recombination of electrons and holes by the influence of an electric charge. In the LED, the doping material forms the p-n junction which separates the electrons and holes. When current passes through the LED, the recombination of electrons and holes takes place resulting in photon emission. But in Phosphor displays, the mechanism of light emission is different. By the influence of the electric charge, the electrons are accelerated leading to the emission of light.
Basic Principle of Operation
An electroluminescent display consists of a thin film of phosphorescent material sandwiched between two plates, one of which is coated with vertical wires and another with horizontal wire. As current passes through the wires, the material between the plates starts glowing. EL Display appears to be brighter than LED display and the brightness of the surface appears the same from all angle of view. The light from the EL display is not directional so that it cannot be measure in Lumens. The light from the EL display is Monochromatic and has very narrow bandwidth and is visible from a long distance. The EL light can be perceived well since the light is homogenous. The voltage applied to the EL device controls the light output. When the voltage and frequency increases, the light output will also increases proportionally.
Inside the EL Device:
The EL devices consist of a thin layer or material either organic or inorganic doped with a semiconductor material. It also contains do-pants to give color. Typical substances used in EL devices are Zinc Sulphide doped with Copper or Silver, Blue diamond doped with Boron, Gallium Arsenide etc. To give Yellow-Orange light, the do-pant used is Zinc and Manganese mixture.The EL Device has two electrodes – Glass electrode and Back electrode. The glass electrode is the front transparent electrode which is coated with Indium Oxide or Tin Oxide. The Back electrode is coated with a reflective material. In between the glass and back electrodes, the semiconductor material is present.
EL Device Application
One typical application of EL device is the panel lighting like automotive dash board panel. It is also used in Audio equipments and other electronic gadgets having displays. In some makes of Laptops, Powder Phosphor panel is used as the back-light. It is mostly used in portable computers these days. The lighting of EL device is more superior to that of LCD. It is also used in Keypad illumination, Watch dials, Calculators, Mobile phones etc. The power consumption of EL display is very low so that it is an ideal solution to save power in battery operated devices. The color of EL display may be Blue, Green, and White etc.