Conditional Codes
The condition code flags are used to store the results of certain condition when certain operation are performed during execution of the program.
The condition code flags are stored in the status registers. The status register is also referred is also referred to as flag register. ALU operations and certain register operations may set or reset one or more bits in the status register. Status bits lead to a new set of microprocessor instructions. These instructions permit the execution of a program to change flow on the basis of the condition of bits in the status register. So the condition bits in the status register can be used to take logical decisions within the program. Some of the common condition code flags are:
- Carry/Borrow : The carry bit is set when the summation of two 8-bit numbers is greater than 1111 1111(FFH). A borrow is generated when a large number is subtracted from a smaller number.
- Zero : The zero bit is set when the contents of register are zero after any operation. This happens not only when you decrement the register, but also when any arithmetic or logic operation causes the contents of register to be zero.
- Negative or sign : In 2's compliment arithmetic, the most significant bit is a sign bit. If this bit is logic 1, the number is negative number, otherwise a positive number. The negative bit or sign bit is set when any arithmetic or logical operation gives a negative result.
- Auxiliary carry : The auxiliary carry bit of status register is set when an addition in the first four bits causes a carry into the 5th bit. This is often referred as half carry or intermediate carry. This is used in the BCD arithmetic.
- Overflow flag : In 2's complement arithmetic, most significant bit is used to represent sign and remaining bits are used to represent magnitude of a number. This flag is set if the result of a signed operation is too large to fit in the number of bits available to represent it.
- Parity : When the result of an operation leave the indicated register with an even number of 1's, parity bit is set.
Light Emitting Diode(LED)
Working Principle
Applications Of LED
The LED is a PN junction diode which emits light when an electric current passes through it in the forward direction.
A PN junction can convert the absorbed light energy into electric current. Now if we reverse the same process that is the pn junction emits light when electric energy is applied to it.
This phenomenon is generally called electroluminescene. Electroluminescene is a property of the material to convert electrical energy into light energy.
Construction
The semiconductor layer of p-type is placed above n-type because the charge carrier recombination occurs in p-type. Besides, it is the surface of the device, thus the light emitted can be easily seen on the surface. If p-type is placed below the light will be emitted from the surface of p-type but we will not be able to see it. This is the reason that p-type is placed above. The below figure shows the cross sectional view of diffused LED.
The p-type layer is formed from diffusion of semiconductor material. On the other side in n-type region, the layer is grown on n-type substrate. The mental film is used on the p-type layerr to provide anode connection to the diode.
The significance of gold-film layer, gold film layer on n-type also provide reflection from the bottom surface of the diode. If any significance part of radiated light tends to hit bottom surface then that will be reflected from the bottom surface to the device top surface. This increases LED's efficiency.
Working Principle
The charge carriers recombine in a forward biased p-n junction as the electrons cross from the n region and recombine with the holes existing in the p-region. Free electrons are in the conduction band of the energy levels, while holes are in the valence energy band. Thus the energy level of the holes is less than the energy levels of the electrons. Some portion must be dissipated to recombine the electrons and the holes. The energy is emitted in the form of heat and light.
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| Working Of LED |
The working of LED depends on the quantum theory. The quantum theory states that when the energy of electrons decreases from the higher level to lower level, it emits energy in the form of photons. The energy of the photos is equal to the gap between the higher and lower level as shown above.
The LED is connected in the forward biased, which allows the current to flows in the forward direction. The flow of current is because of the movement of the electrons in the opposite direction. The recombination shows that the electrons move from the conduction band to valence band and they emits electromagnetic energy in the form of photons. The energy of photons is equal to the gap between the valence and the conduction band. Color of light was determined by the band gap of semiconductor material.
Application
- They are used in remote control systems such as TV or LCD remote.
- Used in electronic calculators for showing the digital data.
- Used in traffic signals for controlling the traffic crowds in cities.
- Used in digital computers for displaying the computer data.
- Used in digital watches and automotive heat lamps.
Purpose Of Embedded Systems
Data Connection, storage and representation
- Purpose of embedded system design is data collection. It performs acquisition of data from the external world.
- Data collection is usually done for storage, analysis, manipulation and transmission.
- Data can be analog and digital.
- Analog is directly connected from the analog signal. There is no need to convert data.
- The embedded systems which have data capturing techniques collects data directly in the form of analog signals whereas embedded systems with digital data collection mechanism convert the analog signal to the digital signal using analog to digital converters.
- Digital camera is an example of embedded systems
Data Communication
- Embedded data communication systems are in home networking systems to complex.
- Data transmission is in the form of wire medium or wireless medium. Data can either be transmitted by analog means or by digital means.
- Bluetooth, ZigBee and Wifi is an example of wireless communication and USB, TCP/IP is an example of wired communication.
- Network hubs, routers, switches are example of dedicated data transmission embedded systems.
Data Processing
- Embedded systems with signals processing functionalities are employed in application demanding signals processing like speech coding, audio video codec, transmission application etc..
- A digital hearing aid is a typical example of data processing
- Digital hearing aid improves the hearing capacity of hearing impairment person.
Monitoring
- Medical domain embedded products come under monitoring function. Patient heartbeat is monitored by electro cardiogram machine but it cannot impose control over the heartbeat.
- Digital CRO, digital multi-meters and logic analyzers are the example of monitoring embedded devices.
Control
- In embedded system, sensor and actuators are used for controlling the system. Sensor are connected to input port. Actuators are connected to output port.
- Sensor captures the changes in the environment variable. Middle system process information. Actuators are changed according to the input variable. It display the output.
- Example of control is air conditioners systems. It control the room temperature to a specific limit.
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