Subject Made Easy - Digital Electronics

EE-204-F DIGITAL ELECTRONICS

L T P Class Work marks : 503 1 0 Theory marks : 100

Total marks : 150 Duration of Exam : 3 hr

NOTE: For setting up the question paper, Question No 1 will be set up from all the four sections which will be compulsory and of short answer type. Two questions will be set from each of the four sections. The students have to attempt first common question, which is compulsory, and one question from each of the four sections. Thus students will have to attempt 5 questions out of 9 questions.

SECTION-A

Morris Mano Chap-1: Digital system and binary numbers: Signed binary numbers, binary codes, cyclic codes, error detecting and correcting codes, hamming codes.

Morris Mano Chap-3: Gate-level minimization: The K-map method up to five variable, don’t care conditions, POS simplification, NAND and NOR implementation, Quine Mc-Clusky method (Tabular method)

SECTION-B

Morris Mano Chap-4: Combinational Logic: Combinational circuits, analysis procedure, design procedure, binary adder-subtractor, decimal adder, binary multiplier, magnitude comparator, decoders, encoders, multiplexers ,demultiplexers

SECTION –C

Morris Mano Chap-5: Synchronous Sequential logic: Sequential circuits, storage elements: latches, flip flops, analysis of clocked sequential circuits, state reduction and assignments, design procedure. Morris Mano Chap-6: Registers and counters: Shift registers, ripple counter, synchronous counter, other counters

SECTION- D

Morris Mano Chap-7: Memory and programmable logic: RAM, ROM, PLA, PAL.Morris Mano Chap-8: Design at the register transfer level: ASMs, design example, design with multiplexers. Morris Mano Chap-9: Asynchronous sequential logic: Analysis procedure, circuit with latches, design procedure, reduction of state and flow table, race Free State assignment, hazards

Text Book:

M. Morris Mano and M. D. Ciletti, “Digital Design”, 4th Edition, Pearson Education Pedroni - Digital Electronics & Design, ElsevierR.P. Jain , “Modern digital electronics” , 3rd edition , 12th reprint TMH Publication, 2007.Digital Design and computer organization: Nasib Singh Gill & J. B. Dixit

REFERENCE BOOKS :

Grout - Digital Design using FPGA'S & CPLD's, ElsevierF. Vahid: Digital Design: Wiley Student Edition, 2006 J. F. Wakerly, Digital Design Principles and Practices, Fourth Edition, Prentice-Hall, 2005. R. L. Tokheim, Digital electronics, Principles and applications, 6th Edition, Tata McGraw Hill Edition, 2003

DIGITAL ELECTRONICS: Course Objectives and Lecture Plan

Aim: At the end of the course the student will be able to analyze, design, andevaluate digital circuits, of medium complexity, that are based on SSIs, MSIs, andprogrammable logic devices.

Modules Learning Units Hoursper

topic

TotalHours

1. Morris Mano Chap-1:

Digital system and binary numbers: Signed binary numbers, binary codes, cyclic codes, error detecting and correcting codes, hamming codes.

44


Gate-level minimization: The K-map method up to five variable, don’t care conditions, POS simplification, NAND and NOR implementation, Quine Mc-Clusky method (Tabular method)

44


Combinational Logic: Combinational circuits, analysis procedure, design procedure, binary adder-subtractor, decimal adder, binary multiplier, magnitude comparator, decoders, encoders, multiplexers ,demultiplexers

66

First Session Exam along with two assignments


Synchronous Sequential logic: Sequential circuits, storage elements: latches, flip flops, analysis of clocked sequential circuits, state reduction and assignments, design procedure.

8

8


Registers and counters: Shift registers, ripple counter, synchronous counter, other counters 4

4

Second Session Exam along with two assignments


Memory and programmable logic: RAM, ROM, PLA, PAL.

44


Design at the register transfer level: ASMs, design example, design with multiplexers. 4

4


Asynchronous sequential logic: Analysis procedure, circuit with latches, design procedure, reduction of state and flow table, race Free State assignment, hazards

4

4

FINAL EXAM

Lecture Plan

Digital Systems: Motivation

A digital circuit is one that is built with devices with two well-defined states. Such circuits can process information represented in binary form. Systems based on digital circuits touch all aspects our present day lives. The present day home products including electronic games and appliances, communication and office automation products, computers with a wide range of capabilities, and industrial instrumentation and control systems, electro-medical equipment, and defence and aerospace systems are heavily dependent on digital circuits. Many fields that emerged later to digital electronics have peaked and leveled off, but the application of digital concepts appears to be still growing exponentially.

This unprecedented growth is powered by the semiconductor technology, which enables the introduction of more and complex integrated circuits. The complexity of an integrated circuit is measured in terms of the number of transistors that can be integrated into a single unit. The number of transistors in a single integrated circuit has been doubling every eighteen months (Moore’ Law) for several decades and reached the figure of almost one billion transistors per chip. This allowed the circuit designers to provide more and more complex functions in a single unit.

The introduction of programmable integrated circuits in the form of microprocessors in 70s completely transformed every facet of electronics. While fixed function integrated circuits and microprocessors coexisted for considerable time, the need to make the equipment smaller and portable lead to replacement of fixed function devices with programmable devices. With the all pervasive presence of the microprocessor and the increasing usage of other programmable circuits like PLDs (Programmable Logic devices), FPGAs (Field Programmable Gate Arrays) and ASICs (Application Specific Integrated Circuits), the very nature of digital systems is continuously changing. The central role of digital circuits in all our professional and personal lives makes it imperative that every electrical and electronics engineer acquire good knowledge of relevant basic concepts and ability to work with digital circuits. At present many of the undergraduate programmes offer two to four courses in the area of digital systems, with at least two of them being core.

The course under consideration constitutes the first course in the area of digital systems. The rate of obsolescence of knowledge, design methods, and design tools is uncomfortably high. Even the first level course in digital electronics is not exempt from this obsolescence. Any course in electronics should enable the students to design circuits to meet some stated requirements as encountered in real life situations. However, the design

approaches should be based on a sound understanding of the underlying principles. The basic feature of all design problems is that all of them admit multiple solutions. The selection of the final solution depends on a variety of criteria that could include the size and cost of the substrate on which the components are assembled, the cost of components, manufacturability, reliability, speed etc

The course contents are designed to enable the students to design digital circuits of medium level of complexity taking the functional and hardware aspects in an integrated manner within the context of commercial and manufacturing constraints. However no compromises are made with regard to theoretical aspects of the subject.

COMMON TERMS USED IN DIGITAL CIRCUITS

1. Define combinational logic

When logic gates are connected together to produce a specified output for certain

specified combinations of input variables, with no storage involved, the resulting circuit

is called combinational logic.

2. Explain the design procedure for combinational circuits

• The problem definition

• Determine the number of available input variables & required O/P variables.

• Assigning letter symbols to I/O variables

• Obtain simplified Boolean expression for each O/P.

• Obtain the logic diagram.

3. Define half adder and full adder

The logic circuit that performs the addition of two bits is a half adder. The circuit that

performs the addition of three bits is a full adder.

4. Define Decoder?

A decoder is a multiple - input multiple output logic circuits that converts coded inputs

into coded outputs where the input and output codes are different.

5. What is binary decoder?

A decoder is a combinational circuit that converts binary information from n input lines

to a maximum of 2n out puts lines.

6. Define Encoder?

An encoder has 2n input lines and n output lines. In encoder the output lines generate the binary code corresponding to the input value.

7. What is priority Encoder?

A priority encoder is an encoder circuit that includes the priority function. In priority

encoder, if 2 or more inputs are equal to 1 at the same time, the input having the highest

priority will take precedence.

8. Define multiplexer?

Multiplexer is a digital switch. If allows digital information from several sources to be

routed onto a single output line.

9. What do you mean by comparator?

A comparator is a special combinational circuit designed primarily to compare the

relative magnitude of two binary numbers.

10. List basic types of programmable logic devices.

• Read only memory

• Programmable logic Array

• Programmable Array Logic

11. Explain ROM

A read only memory (ROM) is a device that includes both the decoder and the OR gates

within a single IC package. It consists of n input lines and m output lines. Each bit

combination of the input variables is called an address. Each bit combination that comes

out of the output lines is called a word. The number of distinct addresses possible with n

input variables is 2n.

12. Define address and word:

In a ROM, each bit combination of the input variable is called on address. Each bit

combination that comes out of the output lines is called a word.

13. State the types of ROM

• Masked ROM.

• Programmable Read only Memory

• Erasable Programmable Read only memory.

• Electrically Erasable Programmable Read only Memory.

14. What is programmable logic array? How it differs from ROM?

In some cases the number of don’t care conditions is excessive, it is more economical to

use a second type of LSI component called a PLA. A PLA is similar to a ROM in

concept; however it does not provide full decoding of the variables and does not

generates all the minterms as in the ROM.

15. Which gate is equal to AND-invert Gate?

NAND gate.

16. Which gate is equal to OR-invert gate?

NOR gate.

17. Bubbled OR gate is equal to--------------

NAND gate

18. Bubbled AND gate is equal to--------------

NOR gate

19. Explain PROM.

PROM (Programmable Read Only Memory)

It allows user to store data or program. PROMs use the fuses with material

like nichrome and polycrystalline. The user can blow these fuses by

passing around 20 to 50 mA of current for the period 5 to 20µs.The

blowing of fuses is called programming of ROM. The PROMs are one

time programmable. Once programmed, the information is stored

permanent.

20. Explain EPROM.

EPROM(Erasable Programmable Read Only Memory)

EPROM use MOS circuitry. They store 1’s and 0’s as a packet of charge

in a buried layer of the IC chip. We can erase the stored data in the

EPROMs by exposing the chip to ultraviolet light via its quartz window

for 15 to 20 minutes. It is not possible to erase selective information. The

chip can be reprogrammed.

21. Explain EEPROM.

EEPROM(Electrically Erasable Programmable Read Only Memory)

EEPROM also use MOS circuitry. Data is stored as charge or no charge

on an insulated layer or an insulated floating gate in the device. EEPROM

allows selective erasing at the register level rather than erasing all the

information since the information can be changed by using electrical

signals.

22. What is RAM?

Random Access Memory. Read and write operations can be carried out.

23. What is programmable logic array? How it differs from ROM?

In some cases the number of don’t care conditions is excessive, it is more economical to

use a second type of LSI component called a PLA. A PLA is similar to a ROM in

concept; however it does not provide full decoding of the variables and does not

generates all the minterms as in the ROM.

24. What is mask - programmable?

With a mask programmable PLA, the user must submit a PLA program table to the

manufacturer.

25. What is field programmable logic array?

The second type of PLA is called a field programmable logic array. The user by means of

certain recommended procedures can program the EPLA.

26. List the major differences between PLA and PAL

PLA:

Both AND and OR arrays are programmable and Complex

Costlier than PAL

PAL

AND arrays are programmable OR arrays are

fixed Cheaper and Simpler

27. Define PLD.

Programmable Logic Devices consist of a large array of AND gates and OR gates that

can be programmed to achieve specific logic functions.

28. Give the classification of PLDs.

PLDs are classified as PROM (Programmable Read Only Memory), Programmable Logic Array

(PLA), Programmable Array Logic (PAL), and Generic Array Logic (GAL)

29. Define PROM.

PROM is Programmable Read Only Memory. It consists of a set of fixed AND gates

connected to a decoder and a programmable OR array.

30. Define PLA

PLA is Programmable Logic Array (PLA). The PLA is a PLD that consists of a

programmable AND array and a programmable OR array.

31. Define PAL

PAL is Programmable Array Logic. PAL consists of a programmable AND array and a

fixed OR array with output logic.

32. Why was PAL developed?

It is a PLD that was developed to overcome certain disadvantages of PLA, such as longer

delays due to additional fusible links that result from using two programmable arrays and

more circuit complexity.

33. Why the input variables to a PAL are buffered?

The input variables to a PAL are buffered to prevent loading by the large number of AND

gate inputs to which available or its complement can be connected.

34. What does PAL 10L8 specify?

PAL - Programmable Logic Array

10 - Ten inputs

L - Active LOW Output

8 - Eight Outputs

35. Give the comparison between PROM and PLA.

PROM PLA

1. And array is fixed and OR Both AND and OR

Array is programmable. Arrays are Programmable.

2. Cheaper and simple to use. Costliest and complex than PROMS.

36. What is the classification of sequential circuits?

The sequential circuits are classified on the basis of timing of their signals into two types.

They are, 1) Synchronous sequential circuit.2) Asynchronous sequential circuit.

37. Define Flip flop.

The basic unit for storage is flip flop. A flip-flop maintains its output state either at 1 or 0

until directed by an input signal to change its state.

38. What are the different types of flip-flop?

There are various types of flip flops. Some of them are mentioned below they

are, RS flip-flop, SR flip-flop, D flip-flop, JK flip-flop

39. What is the operation of D

flip-flop?

In D flip-flop during the occurrence of clock pulse if D=1, the output Q is set and if D=0,

the output is reset.

40. What is the operation of JK flip-flop?

When K input is low and J input is high the Q output of flip-flop is set. When K input

is high and J input is low the Q output of flip-flop is reset. When both the inputs K and

J are low the output does not change

When both the inputs K and J are high it is possible to set or reset the flip-flop (ie) the

output toggle on the next positive clock edge.

41. What is the operation of T flip-flop?

T flip-flop is also known as Toggle flip-flop.

a. When T=0 there is no change in the output.

b. When T=1 the output switch to the complement state (ie) the output toggles.

42. Define race around condition.

In JK flip-flop output is fed back to the input. Therefore change in the output results change

in the input. Due to this in the positive half of the clock pulse if both J and K are high then

output toggles continuously. This condition is called ‘race around condition’.

43.. What is a master-slave flip-flop?

A master-slave flip-flop consists of two flip-flops where one circuit serves as a master

and the other as a slave.

43. Define rise time.

The time required to change the voltage level from 10% to 90% is known as rise time(tr).

44. Define fall time.

The time required to change the voltage level from 90% to 10% is known as fall time(tf).

45. Define setup time.

The setup time is the minimum time required to maintain a constant voltage levels at the

excitation inputs of the flip-flop device prior to the triggering edge of the clock pulse in order

for the levels to be reliably clocked into the flip flop. It is denoted as tsetup.

46. Define hold time.

The hold time is the minimum time for which the voltage levels at the excitation inputs

must remain constant after the triggering edge of the clock pulse in order for the levels to

be reliably clocked into the flip flop. It is denoted as thold .

47. Define propagation delay.

A propagation delay is the time required to change the output after the application of the

input.

48. Define registers.

A register is a group of flip-flops flip-flop can store one bit information. So an n-bit

register has a group of n flip-flops and is capable of storing any binary

information/number containing n-bits.

48. Define shift registers.

The binary information in a register can be moved from stage to stage within the register

or into or out of the register upon application of clock pulses. This type of bit movement

or shifting is essential for certain arithmetic and logic operations used in microprocessors.

This gives rise to group of registers called shift registers.

49. What are the different types of shift type?

There are five types. They are,

Serial In Serial Out Shift Register

Serial In Parallel Out Shift Register

Parallel In Serial Out Shift Register

Parallel In Parallel Out Shift Register

Bidirectional Shift Register

50. Explain the flip-flop excitation tables for RS FF.

RS flip-flop

In RS flip-flop there are four possible transitions from the present state to the next

state. They are,

0 0 transition: This can happen either when R=S=0 or when R=1

and S=0.

0 1 transition: This can happen only when S=1 and R=0. 1 0

transition: This can happen only when S=0 and R=1.

1 1 transition: This can happen either when S=1 and R=0 or S=0

and R=0.

51. Define sequential circuit?

In sequential circuits the output variables dependent not only on the present input

Variables but they also depend up on the past history of these input variables.

52. Give the comparison between combinational circuits and sequential circuits.

Combinational circuits Sequential circuits

Memory unit is not required Memory unity is required

Parallel adder is a combinational Serial adder is a sequential circuit

circuit

53. What do you mean by present state?

The information stored in the memory elements at any given time defines the present

state of the sequential circuit.

54. What do you mean by next state?

The present state and the external inputs determine the outputs and the next state of the

sequential circuit.

55. State the types of sequential circuits?

a. Synchronous sequential circuits

b. Asynchronous sequential circuits

56. Define synchronous sequential circuit

In synchronous sequential circuits, signals can affect the memory elements only at

discrete instant of time.

57. Define Asynchronous sequential circuit?

In asynchronous sequential circuits change in input signals can affect memory element at

any instant of time.

58. Give the comparison between synchronous & Asynchronous sequential

circuits?

Synchronous sequential circuits Asynchronous sequential circuits.

Memory elements are clocked flip- Memory elements are either unlocked

flops flip - flops or time delay elements.

Easier to design More difficult to design

110. The following wave forms are applied to the inputs of SR latch. Determine the Q

waveform Assume initially Q = 1

Here the latch input has to be pulsed momentarily to cause a change in the latch output state,

and the output will remain in that new state even after the input pulse is over.

59. What is race around condition?

In the JK latch, the output is feedback to the input, and therefore changes in the output

results change in the input. Due to this in the positive half of the clock pulse if J and K

are both high then output toggles continuously. This condition is known as race around

condition.

60. Give the comparison between synchronous & Asynchronous counters.

Asynchronous counters Synchronous counters

In this type of counter flip-flops are In this type there is no connection between

connected in such a way that output of 1st output of first flip-flop and clock input of

flip-flop drives the clock for the next flip- the next flip - flop

flop.

All the flip-flops are Not clocked All the flip-flops are clocked

simultaneously simultaneously

SIMPLE LOGIC CIRCUITS

LOLOGIC circuits are normally composed of ‘gates’. A combination of gates make up a circuit and some digital circuits can be extremely complex. It is the logic gates that produce pulses of electrical current (1s and 0s). At school level, digital logic circuits are relatively simple. Below are simple drawings that help explain the two most popular logic gates - the AND gate and the OR gate.

The simplified AND gate shown above has two inputs, switch A and switch B. The bulb Q will only light if both switches are closed. This will allow current to flow through the bulb, illuminating the filament.

The simplified OR gate shown above has two inputs, switch A and switch B. The bulb Q will light if either switch A or B are closed. This will allow current to flow through the bulb, illuminating the filament.

When the bulb lights this represents a ‘1’ as current is running through the filament. If current is not running through the filament the bulb will not light and this represents a ‘0’ (zero).

THE ROLE OF TRANSISTORS

Transistors are vital for digital circuits to work. These components are used as very fast switches in digital logic circuits. Transistors are normally so small that hundreds of thousands fit on one processing chip on a computer motherboard. The types of transistors used in school projects are normally large enough to fit on the end of a small finger. However, the way they switched on and off is the same. When a transistor is switched on it produces a ‘1’ and when it is switched off it produces a ‘0’.

Transistors in the circuit of a computer microprocessor can switch on and off thousands of times per second. Without the invention of the transistor, computer processing power would be very limited and slow.

Two basic examples of simple transistor driven logic (AND / OR) circuits are shown below.

This is an AND gate circuit and it can be made quite easily. The example shown is built from a modular electronics kit. Both switches ‘A’ and ‘B’ must be pressed together for the bulb to light.If you construct this circuit, you may need to alter the value of the resistors. This will depend on the type of transistors used and whether to bulb or an LED is used.

This is an OR gate circuit. Either switch ‘A’ or ‘B’ must be pressed for the bulb to light. The switches do not have to be pressed together.

QUESTIONS:1. Explain how an AND gate works. Use a circuit diagram to help explain your answer.2. Explain how an OR gate works. Use a circuit diagram to help explain your answer.3. Build a simple logic circuit using a breadboard and available components. You may wish to build one of the circuits shown above.

A range of logic gates exist and they are represented as symbols, each with its own truth table (sometimes called a logic table). Gates have inputs and produce outputs and these are in the form of 1s and 0s. Remember, a 1 represents an input or output of electrical current. Each truth table clearly shows the ‘state’ of inputs and outputs at any one

time.Study the symbols and tables below. You will soon find that they can be combined to design interesting logic circuits.

The AND gate will only output current (produce a 1 at Q) if both logic states at inputs A and B change to 1.

The NAND gate has the opposite outputs to the AND gate. How does the NAND gate symbol differ to the AND gate?

The OR gate will output current at Q if either of the logic states of inputs A and B change to1.

The NOR gate has the opposite outputs to the OR gate. How does the NOR gate symbol differ to the OR gate?

The INVERTER gate reverses input. For example, if the input is 1 then the output is 0.This is a very useful gate especially when designing logic circuits.

QUESTIONS:1. Draw each of the logic gates shown above and explain how each gate works.2. Learn and remember each of the logic tables.

CASE STUDY-1

Below is the logic circuit for a simple house alarm. The alarm protects the front and back doors and six windows. Once the alarm is set if any of the doors or windows are opened the alarm will sound. OR gates have been used. The TIMER allows the house owner to enter the house by either the front or back door and turn of the alarm system before the alarm sounds.The inputs for each of the gates representing the doors and windows can be connected to a vast range of sensors (eg. movement and magnetic sensors).

On the circuit below the input states of each of the sensors are ‘0’ (false, low, off). This means that they have not detected an intruder. As a result the alarm does not sound.

The situation changes when local thug, Ed the Handyman forces window 3 open. Notice how the logic state of the input of GATE B changes from false to true. The output state of gate ‘B’ changes to true, followed by the INPUT of gate ‘E’ and its output. The input and output of gate ‘G’ also change to true. This train of events leads to the alarm sounding. Because OR gates have been used, it only takes one input to change to true at the windows or doors to activate the alarm.

CASE STUDY-2

In manufacturing industry safe use of machines is very important. All machines should be set up in such a way that it is impossible for the machine operator to have an accident.

Machine ‘A’ is unsafe because it can be turned on and used when the guard is out of position, especially if it is operated by a machinist such as Ed the Handyman (website cartoon character). This means that the operator’s hands could be seriously injured by the dangerous blade as it cuts the material.

Alternatively, machine ‘B’ has been fitted with a logic circuit. It is designed to ensure that the guard must be in the correct position and the ‘ON’ switch is pressed simultaneously, before the machine will work. This means that the operator must keep his/her spare hand on the switch or electrical power will be cut, stopping the machine working.

The animation below shows what happens when the micro-switch has been switched 'ON' as the guard is in the correct position. This means that the logic states of both inputs are 1 (true, on, high, up), consequently the output

logic state is 1 (true, on, high, up) and the machine works.Remember, for the AND gate to output 1 both inputs must be 1.

QUESTION:Draw a series of logic circuits that clearly show the logic states of inputs and outputs as the guard is put in the correct position and the 'ON' switch pressed.

Documents

Subject Made Easy - Digital Electronics