Digital Electronics

Dr. Hasanujjaman
Assistant Professor
Department of Electronics & Communication Engineering

March 6, 2025 1
 Digital System Design
System
– orderly interconnection of parts into a
meaningful whole
– interactions and interconnections define
the system

What are the system components?

… depends entirely on your point of view
 Gajski-Kuhn
Y-Chart

Gajski, D., Kuhn, R.: New VLSI

Tools – Guest Editors’
Introduction. IEEE Computer,
Bd. 16,1983 (original)

Walker, R., Thomas, D.: A Model

for design Representation
and Synthesis. Proc. Design
Automation Conference, 1985
(refinement)
 Digital System Design
Design process …
 Start with a word problem
 Functional specification
 Define inputs, outputs, control lines
 Describe circuit function;
 Block diagram
 Pictorial
 Major functional modules and interconnections
 Virtual Lab

• https://de-iitr.vlabs.ac.in/

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 Analog and Digital

•Data can be analog or digital.

•The term analog data refers to information

that is continuous; digital data refers to
information that has discrete states.

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 Signals can be analog or digital.

– Analog signals can have an infinite number of

values in a range.

– Digital signals can have only a limited number of

values.

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March 6, 2025 8
 Digital Electronics
• Digital Electronics represents information (0, 1) with only
two discrete values.
• Ideally
“no voltage” (e.g., 0v) represents a 0 and
“full source voltage” (e.g., 5v) represents a 1
• Realistically
“low voltage” (e.g., <1v) represents a 0 and
“high voltage” (e.g., >4v) represents a 1
• We achieve these discrete values by using switches.
• We use transistor switches, which operates at high speed,
electronically, a small in size.
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• How we represent digital information in electronic
devices?

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 • By discrete voltages.

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 What is the
Basic Digital Element
in Electronics ?

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 • a Switch

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 Using Switch to Represent
Digital Information

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 Benefits of Digital over Analog

• Reproducibility
• Not effected by noise means quality
• Ease of design
• Data protection
• Programmable
• Speed
• Economy

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 Digital Revolution
• Digital systems started back in 1940s.
• Digital systems cover all areas of life:
– still pictures
– digital video
– digital audio
– telephone
– traffic lights
– Animation

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 Basic terminology
• Gate
• A device that performs a basic operation on electrical signals
• Circuits
• Gates combined to perform more complicated tasks

• How do we describe the behavior of gates and circuits?

• Boolean expressions
• Uses Boolean algebra, a mathematical notation for expressing two-
valued logic
• Logic diagrams
• A graphical representation of a circuit; each gate has its own symbol
• Truth tables
• A table showing all possible input value and the associated output
• values
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 Circuits
• Circuits can be Combinational or Sequential
• Combinational logic circuits produce a specified output
(almost) at the instant when input values are applied.
• The addition of a memory device to a combinational circuit
allows the output to be fed back into the input: Sequential
circuit
Combinational circuit

Input(s) Output(s)
circuit

memory
Sequential circuit
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 Digital Devices
• Combinational circuit
– Gates
– Multiplexer
– Demultiplexer
– Adders
– Encoder
– Decoder
• Sequential circuit
– Flip-Flops
– Registers
– Counters
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 Gates
• The most basic digital devices are called gates.
• Gates got their name from their function of
allowing or blocking (gating) the flow of digital
information.
• A gate has one or more inputs and produces an
output depending on the input(s).
• A gate is called a combinational circuit.
• Three most important gates are: AND, OR, NOT

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 NOT Gate
•A NOT gate accepts one input signal (0 or 1)
and returns the opposite signal as output

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 AND Gate
•An AND gate accepts two input signals
•If both are 1, the output is 1; otherwise the
output is 0

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 OR Gate
•An OR gate accepts two input signals
•If both are 0, the output is 0; otherwise, the
output is 1

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 XOR Gate
• An XOR gate accepts two input signals
• If both are the same, the output is 0; therwise,
the output is 1

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 NAND Gate
•The NAND gate accepts two input signals
•If both are 1, the output is 0; otherwise, the
output is 1

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 NOR Gate
•The NOR gate accepts two input
signals
•If both are 0, the output is 1; otherwise,
the output is 0

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 Universal Gates

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 • Thank You

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