Combinational Logic Design

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Combinational Logic Design

• Sections 3-1, 3-2 Mano/Kime

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Combinational Logic

• Design Hierarchy
• Top-Down Design
• Computer-Aided Design
• Hardware Description Languages
• VHDL
• Verilog
• Logic Synthesis
• Combinational Logic Circuits
• Multiplexers
• Binary Adders
• Binary Subtractions
• Shifters
• Comparators
• Decoders and Encoders

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Combinational vs. Sequential Circuits
Combinational Circuit
n-inputs
m-outputs
(Depend only on inputs)
Combinational Circuit
Outputs
Inputs
Combinational Circuit
Storage Elements
Present state
Next state
Sequential Circuit
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Design Hierarchy
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Design Hierarchy
6
Top-Down Design
Gajski and Kuhns Y Chart
Top-Down
Bottom-Up
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Computer-Aided Design

• Schematic Capture
• Hardware Description Languages
• CUPL, ABEL
• VHDL, Verilog
• Logic Simulation
• Logic Synthesis

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VHDL

• VHDL is an acronym for VHSIC (Very High Speed
  Integrated Circuit) Hardware Description Language
• IEEE standard specification language (IEEE
  1076-1993) for describing digital hardware used
  by industry worldwide
• VHDL enables hardware modeling from the gate
  level to the system level

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VHDLs History

• Very High Speed Integrated Circuit (VHSIC)
  Program
• Launched in 1980
• Aggressive effort to advance state of the art
• Object was to achieve significant gains in VLSI
  technology
• Need for common descriptive language
• 17 Million for direct VHDL development
• 16 Million for VHDL design tools
• Woods Hole Workshop
• Held in June 1981 in Massachusetts
• Discussion of VHSIC goals
• Comprised of members of industry, government, and
  academia

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VHDLs History (Cont.)

• In July 1983, a team of Intermetrics, IBM and
  Texas Instruments were awarded a contract to
  develop VHDL
• In August 1985, the final version of the language
  under government contract was released VHDL
  Version 7.2
• In December 1987, VHDL became IEEE Standard
  1076-1987 and in 1988 an ANSI standard
• In September 1993, VHDL was restandardized to
  clarify and enhance the language
• VHDL has been accepted as a Draft International
  Standard by the IEC

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Additional Benefits of VHDL

• Allows for various design methodologies
• Provides technology independence
• Describes a wide variety of digital hardware
• Eases communication through standard language
• Allows for better design management
• Provides a flexible design language
• Has given rise to derivative standards
• WAVES, VITAL, Analog VHDL

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High-Level Flow forLogic Synthesis Tool
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Combinational Logic Circuits

• Multiplexers
• Binary Adders
• Binary Subtractions
• Shifters
• Comparators
• Code Converters
• Decoders and Encoders