Characteristics of Digital ICs

1.Operating speed:

Speed of a logic gate depends upon the time that elapses between the application of a signal to an input terminal and the resulting change in logical state at the output terminal.It takes into consideration the transition time(rise and fall duration of  a pulse) and propagation delays. Both of these time depend upon  the loading and increase with the increase in load.The more  inputs are attached to the output of a logic gate,the more load is to be handled by that output.High operating speed is usually the main requirement of Digital ICs.

2.Fan-In:

The fan-in of a logic gate is defined as the number of inputs(coming from similar circuits)that it can handle property.

3.Fan-out:

In general, a logic circuit is required to drive several logic inputs.The fan-out(also sometimes called the loading factor) is defines as the maximum number of standard logic inputs that an output can drive reliably.For example,a logic gate that is specified to have a fan-out of 8 standard logic inputs.If this number exceeds,the output logic-level voltages cannot be guaranteed.

4.Power Dissipation:

This is the amount of power dissipated in an IC.It is determined by the current,Icc,that it draws from the Vcc supply and equal Vcc Icc where Icc is average value of Icc (0) and Icc(1).This power is specified in mW. Lower power dissipation is desirable feature for any IC.

5.Noise Immunity:

stray electric and magnetic fields can induce voltage on the connecting wires between logic circuits.These unwanted, spurious signals are known as noise and can sometimes lead to false triggering of logic levels in the circuit.The noise immunity of a logic circuits refers to the circuit's ability to tolerate noise voltage on its inputs. A quantitative measure of noise immunity is called the noise margin.Higher the noise margin,better the logic circuit.

Data Bus and control BUS In microprocessor

 Data Bus

The data bus is a group of eight lines used for data flow. Thses lines are bidirectional-data flow in both directions between the MPU and memory and pheripheral devices. The MPU uses the data bus to perform the second function:transferring data

The eight data lines enable the MPU to manipulated 8-bit data ranging from 00 to FF (256 numbers).

The  largest number that can appear on the data bus is 11111111.The 8085 microprocessor is known as an 8-bit microprocessor.Microprocessor such as the Intel 8086,Zilog Z8000,and Motorola 68000 have 16 data lines;thus they are known as 16-bit microprocessors and the Intel 80386/486 have 32 data lines;thus they are classified as 32-bit microprocessors.

Control Bus

The control bus is comprised of various single lines that carry synchronized signals.The Bus is bidirectional but the individual line is undirectional. The MPU uses such lines to perform the third function;providing timing signals .

The term bus,in relation to the control signals,is somewhat confusing.These are not groups of lines like address or data buses,but individual lines that provide a pulse to indicate an MPU operation.The MPU generates specific control signals for every operation(such as Memory Read or I/O Write) it performs.These signals are used to identify a devices type with which the MPU intends to communicate.

To communicate with a memory-for example,to read an instruction from a memory location-the MPU places the 16-bit address on the address bus.The address on the bus is decoded by an external logic circuit,and the memory location is identified.The MPU sends a pulse called Memory Read as the control signal.The pulse activates the memory chip,and the contents of the memory location (8-bit data ) are placed on the data bus and brought inside the microprocessor.

   

MICROPROCESSOR AS A CPU

we can also view the microprocessor as a primary components of a computer.Traditionally,the computer is represented in block diagram as shown in fig 1.The block diagram shows that the computer has four components: Memory,Input,Output,and the central processing unit(CPU),which consists of the Arithmetic logic Unit (ALU) and control unit.The CPU contains various registers to store data,the ALU to perform arithmetic and logical operations,instruction decoders,counters,and control lines.The CPU reads instructions from the memory and performs the tasks specified.It communicates with input/output devices either to accept or to send data,These devices are also known as peripherals.The CPU is the primary and central players in communicating with devices such as memory,input and output.However,the timing of the communication process is controlled by the group of circuits called the control unit.In the late 1920s, the CPU was designed with discrete components on various boards.with  the advent of the integrated circuit technology,it became possible to build the CPU on a single chip;this came to be known as a microprocessor,and the traditional block diagram shown  in fig 2.

THYRISTORS

Thyristors is the general name given to a family of semiconductor devices having four layers with a control mechanism,although this term is most commonly applied to the SCR(silicon controlled rectifier). This term is derived from thyratron  and transistor because the devices combines the rectification action of thyratron and control action of transistor.

 Thyristor stands for THRYatron transISTOR.

The device was first developed at the Bell Laboratories in the USA and was commercially introduced by the General Electrical Company in the USA in 1957.The device appeared in the market under different names such as SCR, thyristor, thyrode etc.Since its inception the thyristor has come to stay as a basic building block in many industrial and power system applications. Its ability to be controlled,compactness ,fast response,high reliability,better efficiency,large power handling capacity,high voltage and current ratings,good trigger sensitivity,static operation,large power gain,study construction,long life,very little maintenance and low cost of fabrication due to advancement in the field of fabrication-have given the thyristor a colorful reception in every field. Today thyristors are finding applications in the control of dc/ac motors; for the improvement of power factor;and as switching devices.They have become an integral part of HVDC transmission.They are so advantageous that our major steel plants had decided to modernise their plants by replacing MG sets and mercury-arc rectifier by thyristor converts .Thyristor   have helped in further cost reduction and in the development of drive system by changing the emphasis from dc motors to ac motors.With cycle converter and inverters, the  speed of an ac motor can also be controlled with ease and reliability.Apart from these main applications it finds use as a switching devices,particularly in the improvement of power factor of transmission lines and mains. Thyristor  can be used as a power factor of transmission lines and mains.

Digital Techniques

Merits and Limitation of Digital Techniques:

Merits
1.Digital systems are easier to design as the circuits employed are switching circuits,where exact values of voltage or current are not important,only the range(HIGH or LOW),in which they fall,is important.
2.Storage of information is easier as it is accomplished by special switching circuits that can latch into information and hold it for as long required.
3.Greater accuracy and precision as digital systems can handle as many digits of precision is usually limited to three or four digits because the values of voltage and current directly depend on the values of circuit components.
4.Programmable operation as the digital system can be easily designed for operation controllable by a set of stored instruction called a program.
Analog systems can also be programmed,but the verity and complexity of the available operations is severely limited.
5.Digital circuits are less affected by noise as spurious fluctuations in voltage (noise) are not as critical in digital systems because the exact value of voltage is not important,as long as the noise is not large enough to prevent distinguishing a HIGH from a LOW.

Limitations
There is really only one major drawback of using digital techniques and that is due to the fact that the real world is mainly analog.
Most physically quantities are analog in nature and these quantities are often the inputs and outputs that are monitored,operated on and controlled by a system.Some examples are pressure,position,velocity,temperature,liquid level and so on. We are in the habit of expressing these quantities digitally,such as when we say that the velocity is 5.2m/s ;but we are really making a digital approximation to an inherently analog quantity.


  

COUNTERS

A Counter is probably one of the most useful and versatile subsystems is a digital system.A Counter can be describe as a tallying devices that tallies or counts some number of events.A counter driven by a clock can be employed for counting the number of clock cycles.Since the clock pulses occur at fixed and known intervals,the counter can be used as a digital instrument to measure time and therefore period of frequency.Besides,wide applications in digital instrumentation,counters are also used to generate timing sequence for controlling the sequential logic operations.
A binary counter is made with a group of flip-flops arranged in a predetermined manner.Count pulses are supplied to clock input terminal of flip-flop either by a clock of fixed frequency or at random by some other source.These count pulses are counted by the counter in binary form.Since a flip-flop has two possible states,an array of n flip-flops has 2^n states and counts from 0 to 2^n-1 numbers in binary form.
Basically the counters are of two types,namely,asynchronous(or serial) and synchronous (or parallel) counters.
An example of an asynchronous counter is the ripple counter .When the output of one flip-flop drives another flip-flop drives another flip-flop,the counter is called a ripple counter.The ripple counter is simple and straight forward in construction and operation and usually needs a minimum of hardware.The drawback of such a counter is speed limitation.Each flip-flop is triggered by the previous flip,and thus the counter has a cumulative setting time.
 An increase in operating speed can be had by using a parallel or synchronous counter in which every flip-flop is triggered by the clock in synchronism.The increase in speed is usually obtained at the cost of increased hardware.
Serial and parallel counters are used in combination to compromise between speed of operation and hardware used.Serial,parallel or combination counters can be designed such that each clock pulse advances the contents of the counter by one;it is then said to e operating in count-up mode.The opposite is also possible;the counter is then said to e operating in count-down mode.

Merit,Demerits and Applications of LVDTs

Merits:

1. LVDT has infinite resolution as it gives stepless  output and it has got no mechanical element to change output in discrete  steps. Nowadays transducers are available with the resolution up to 1 micron.

2. LVDT  has almost linear characteristics within its prescribed range.Linearity upto +25 or -25% can be achieved in commercially available LVDTs.

3. LVDT has high sensitivity.It usually varies from 10mV / mm to 40 V/mm.

4.Its output is very high which,even in some cases,eliminates the need for amplification devices.

5.These devices consume very less power (less than 1 watt).

6. LVDT can be used on high frequencies up to 20 kHz.

7.Output impedance of LVDT remains constant which in case of potentiometer varies with displacement.

8.Absence of sliding contact makes LVDT a more reliable device.

9.LVDT  has very low hysteresis so a good repeat ability can  be achieved with it.

10.LVDT is very rugged devices in construction so it can tolerate shocks and variations without any adverse effect.

11.It is a very stable and easy to align and maintain due to simplicity of construction,small size and light weight.

Demerit:

1.These devices are sensitive to stray magnetic fields but its problem is overcome by using magnetic shields.

2.Relatively large displacements are required for appreciable differential output.

3.Sometimes,the transducer performance is affected by vibrations.

Applications:

LVDTs are suitable for use in applications where the displacements are too large for strain gauges to handle,for example,LDVTs can be employed for measurement of displacements that range from a fraction of a mm to a few cm. If LVDT is to be employed for measurement of mechanical displacement greater than 25mm, an appropriate mechanical ratioing(gearing) must be used.

Since the LVDTs can also be connected to other transducers,whose outputs are mechanical displacements,these are often employed together with other transducers for measurement of force,weight,pressure etc.