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3 Different types of Displays AvailableOctober 24, 2013 by D.Mohan Kumar Leave a Comment
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 AvailableDisplay 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.
Active low LED connection with microcontroller pin
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.
LED pattern for displaying digits using 7-segment display
Interfacing of 7-segment display with 8051 microcontroller
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
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
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.
LCD instructions set for controlling the LCD display using microcontrollers
Interfacing 16×2 LCD display with 8051 microcontroller
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.
LC
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.
Page scheme for the graphical LCD JHD12864E
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 TechnologyElectroluminescent 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 OperationAn 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.
EL-LIGHT
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 ApplicationOne 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.
Photo Credit
Diagram of 8X8 LED Matrix using 16 I/O pins by sprags
2 I/O pins controlling the Matrix display of 32 LEDs by mikroe
LC by 3.bp
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