Introduction to Digital Logic Design

1. Digital Logic Design
CHAPTER – I
Introduction to Digital Logic Design
Addis Ababa Science and Technology University
College of Electrical and Mechanical Engineering
Department of Software Engineering
Lecturer: Biruk T.
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2. The Evolution of Computer Hardware
◼Modern-day electronics began with the invention of the
transfer resistor (transistor) in 1947.
◼By John Bardeen, Walter Brattain, and William Shockley at
Bell Laboratories.
◼A transistor is a semiconductor device used to amplify and
switch electronic signals and electronic power.

3. ◼In digital circuits, transistors are arranged to function as logical
switch.
◼Logical switch has only 2 values: 1 (on/high) and 0 (off/low).
◼In general, a switch has three parts: source input, control input,
and output.
◼The control input is represented by a voltage that will decide
whether current can flow through the switch or not.
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The Evolution of Computer Hardware
“off”
“on”
output
source
input
output
source
input
control
input
control
input
relay vacuum
tube
discrete
transistor integrated
circuit

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◼ In 1958 the first integrated-circuit (IC) was born when Jack Kilby
at Texas Instruments successfully interconnected, by hand,
several transistors, resistors and capacitors on a single substrate.
◼ Interconnection between components are not integrated.
◼ Low connectivity between components.
The Evolution of Computer Hardware

5. The Evolution of Computer Hardware
◼ Advances in technology increase the performance of computer
hardware and decrease the production cost enormously.
Year Technology
Relative Performance/
Unit Cost
1951 Vacuum Tube 1
1965 Transistor 35
1975 Integrated Circuit (IC) 900
1995 Very Large Scale IC (VLSI) 2,400,000
2005 Ultra VLSI 6,200,000,000

6. 6
◼ The scope of this lecture is the theory and implementation of
digital design, in the hardware layer.
The Evolution of Computer Hardware

7. Numerical Representations
◼Analog representations:
◼A quantity is represented by a continuously variable,
proportional indicator.
◼Physical quantities are coupled to an indicator by purely
mechanical means or proportional voltage or current.
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◼Digital representations:
◼A quantity is represented by symbols called digits.
◼The quantity changes in discrete steps.

8. ◼Advantages:
◼Digital systems are generally easier to design.
◼Information storage is easy.
◼Accuracy and precision are easier to maintain throughout
the system.
◼Operation can be programmed.
◼Digital circuits are less affected by noise.
◼More digital circuitry can be fabricated on IC chips.
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◼Limitations:
◼The real world is analog.
◼Processing digitized signals takes time
Numerical Representations

9. Digital and Analog Systems
●
The term “Digital” has become part of our everyday
vocabulary because of the dramatic way that the digital
circuits and digital techniques have become so widely used in
almost all areas of our life:
●
●
●
Computers
Automation
Robots and so on
●
Generally digital circuits are favorable in speed, ease
of design, Programmability, flexibility and functionality.
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10. Digital and Analog Quantity
● Analog Quantity:
– The quantity that has continuous values is called
as the analog quantity.
– Example of analog quantity are:
● Time, Temperature, Pressure, Distance and Sound.
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11. Digital and Analog Quantity
● Digital Quantity:
– The quantity that has a discrete set of values is
called as the digital quantity.
– Digital system will take information at certain
defined times and display it in the form digits and
symbols on some display.
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12. ◼To take advantage of digital techniques when dealing with analog
inputs and outputs, four steps must be followed:
1. Convert the physical variable to an electric signal (analog).
2. Convert the electric signal (analog into digital form).
3. Process (operate on) the digital information.
4. Convert the digital outputs back to real-world analog form.
Digital and Analog Systems

13. Advantage of Digital over Analog
●
Digital data can be processed and transmitted
more efficiently and reliably than analog.
●
It can store huge amount of data in lesser
space and with least chance of error.
●
Noise does not affect digital data as compare to
analog signals.
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14. Binary Digit, Logic Level and
Digital Waveform.
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15. Binary Digit
●
●
●
Digital electronics involves circuits and system in which
there are two types possible states.
These states are represented by two different voltage
levels: HIGH and LOW.
In digital systems such as computers; combinations of
two states, called codes, are used to represent
●
numbers, symbols, alphabetic character and other types
of information.
The two-state number system is called binary and its
two digit are called Binary Digit: 0 and 1.
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16. Binary Digit
●
●
Each of the Binary digit, 0 and 1, is called bit.
In digital circuits, two different voltage levels are
used to represent the two bits.
●
Generally;
– 1 is represented by the higher voltage (HIGH) and
– 0 is represented by the lower voltage (LOW).
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17. Logic Levels
●
●
The voltage used to represent a 1 and 0 are logic
levels.
Ideally, one voltage level represents a HIGH and
another voltage level represents a LOW.
● In a practical digital circuit, however, a HIGH can be any
voltage between a specified minimum value and
specified maximum value. Likewise for a LOW voltage
levels.
●
There can be no overlap between the accepted range of
HIGH levels and the accepted range of LOW levels.
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18. Logic Levels
●
●
●
The voltage values
between VH(min) and VL(max)
are unacceptable for
proper operation.
The voltage values
between VH(max) and
VH(min) are HIGH.
The voltage values
between VL(max) and
VL(min) are LOW.
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21. Digital Waveform
●
●
Digital waveforms consists of voltage levels that
are changing back and forth between the HIGH
and LOW levels or states.
Usually these waveforms are represented as
timing diagrams used to represent wave
behavior relative to time.
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22. Digital Waveform
●
Pulse have two edge:
– A leading edge where pulse goes to HIGH from
LOW state and falling edge where pulse revert.
– Sudden transition from one state to other is
impractical. Depending on practical consideration
it will take small amount of time to get from one
state to another.
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23. Digital Waveform
● Important terms.
●
●
●
●
Rise Time
Fall Time
Amplitude
Pulse Width
Rise Time
Fall Time
→ time required for pulse to go from LOW level to HIGH.
→ time required for pulse to go from HIGH level to LOW.
Amplitude → the maximum value of a digital waveform.
Pulse Width → the time gap between the raising edge and falling edge.
fig.1.3: practical signal representation of a pulse.
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24. Digital Logic Design
● Digital logic design is a system in electrical and
●
●
computer engineering that uses simple number value to
produce input and output operations.
Digital logics are rooted in binary code.
This system facilitates the design of electronics circuits
that convey information, including logic gates with
functions that include AND, OR and NOT commands.
● The value system translates input signals into specific
output. These functions facilitate computing, robotics,
and other electronics applications.
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25. Electronic Components
• An electronic component is any basic discrete device or
physical entity in an electronic system used to affect
electrons or their associated fields.
• Two main types:
Active Passive
Electronic
Components
25
Review on Electronic Components

26. Passive electronic components
26

27. Active electronic components
• Can inject power into a circuit by itself or using another
power source and hence can amplify a signal or change
its power/voltage level.
27

28. Electric current
• Current (I) is the amount of charge (Q) that flows past a
point in a unit of time (t).
• One ampere is a number of electrons having a total
charge of 1 C move through a given cross section in 1 s.
Q
I
t
=
What is the current if 2 C passes a point in 5 s? 0.4 A
28

29. Ideal Voltage Source
• Voltage across it is independent of the current
through it.
• Ideal voltage source has no internal resistance
• Real voltage sources do have a finite internal
resistance
29

30. Voltage
• One volt is the potential difference (voltage) between two
points when one joule of work is used to move one
coulomb of charge from one point to the other.
W
V
Q
=
Voltage is responsible for establishing
current.
30

31. current
charge
voltage
resistance
power
ampere
coulomb
volt
ohm
watt
A
C
V
W
W
Quantity Unit Symbol
Some Important Electrical Units
31

32. End of Chapter I
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33. Individual Assignment:
1- Define the practical representation of logic levels using
voltage (1Min)
2- List advantages of Digital over analog (1Min)
3- Mention advantage of Analog over digital (30Sec)
4- Depending on practical consideration it will take small
amount of time to get from one state to another, draw the
practical signal representation of a pulse (2.30Min)
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