Effective Digital Design

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Effective Digital Design

• Yahya E. Osais
• COE 200 03
• Term 032

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Where are we heading?

• In previous lectures, we learned the theoretical
  principles used in combinational logic design.
• Number systems.
• Negative number representations.
• Base-r arithmetic operations.
• Boolean Algebra.
• K-maps.
• In reality, a combinational circuit may have
  dozens of inputs and outputs, and could require
  thousands of terms to describe as a sum of
  products, and millions of rows to describe in a
  truth table.
• The real combinational logic design problem is
  too large to solve by direct application of
  theoretical techniques.
• NO complex circuit can be designed simply by
  interconnecting gates one at a time.

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Hierarchical Design

• Circuit is broken up into pieces called BLOCKS.
• A BLOCK is a functional module that implements a
  basic operation in the original circuit and can
  be described by a short Boolean expression.
• Blocks are interconnected to form the circuit.

Be Careful! The functions of the blocks and the
interfaces between them are carefully defined, so
that the circuit formed by interconnecting the
blocks obeys the specifications of the original
circuit.
? A specification describes exactly what the
circuit is supposed to do. ? It gives the output
for every possible input (Truth Table). ? It does
not describe how the circuit achieves its
results, just what the results are.
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Advantages of Hierarchical Design

• Reduce the complexity of the circuit diagram.
• Designing a block is significantly easier than
  designing a circuit.
• Blocks can be reused.

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Exercise on Hierarchical Design

• Give a hierarchy for an adder circuit that adds 2
  4-bit numbers.

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Block Diagram

• A block diagram shows the inputs, outputs,
  functional modules, internal data paths, and
  important control signals of a circuit.
• A High-Level block diagram of a circuit shows
  only the inputs, outputs, and important control
  signals.
• Good Practice
• A designer should always start with a high-level
  diagram of the circuit. Then, after understanding
  requirements, a more detailed diagram is given.

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Logic Diagram
Block Diagram
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Terminology

• Top-Down Design
• Specifications of the circuit are given. Then,
  the circuit is divided into blocks. Finally,
  blocks are appropriately interconnected.
• Bottom-Up Design
• Given a set of blocks and specifications of the
  circuit, you are to build the circuit by
  appropriately interconnecting necessary blocks.
• Computer-Aided Design (CAD) Tools
• Programs designed to facilitate automate logic
  design.
• Hardware Description Language
• A programming language used to describe the
  hardware structure and behavior of circuits.
• Logic Synthesis
• The process of converting a high-level circuit
  description into an OPTIMIZED gate-level
  description.