Example of modular design: ALU

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Example of modular design ALU

• ALU logic symbol

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ALU

• Exercise construct an ALU to perform the
  operations in this function table

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ALU

• Subdivide ALU function or partition ALU into bit
  slices
• A bit slice a module that can be replicated
  (arrayed) to generate a n-bit ALU

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ALU

• Further partition the bit slice into a Logic Unit
  and an Arithmetic Unit

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ALU

• Implement the logic unit module
• Truth table
• Simple multiplexer realization

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ALU

• K-map of the LU, and the minimized LU circuit

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ALU

• Implement the Arithmetic Unit
• Block diagram of the AU slice

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ALU
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ALU

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ALU

• Complete 1-bit slice

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Programmable logic devices

• Goal place as much functionality on a single
  chip as possible
• Increases speed
• Reduces power consumption, size, weight
• IC design methodologies can be classified
  according to the level of design effort expended
  in obtaining the chip
• Can range from full custom to none
• Each has its advantages and drawbacks

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IC design methodologies

• Full custom
• Design every aspect of the chip gate
  placements, routing, etc.
• Complete control over all circuit parameters
• Slowest to design and manufacture
• Semi-custom
• Use some pre-designed components, design the rest
• None
• Uses pre-fabricated SSI or LSI chips, just
  interconnect on PCB

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IC design methodologies

• Semi-custom
• Standard Cell libraries
• Use pre-designed units, just place and
  interconnect
• All processing steps need to be done (same as
  full custom)
• Fabrication delay weeks or months
• Pre-designed gate arrays
• They are ICs containing unconnected gates
• Only interconnection layer needs to be fabricated
• Fabrication delay is days or weeks
• PLDs
• Gate arrays with flexible interconnections
  included
• Fabrication delay is hours or days

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Programmable logic devices

• PLA circuits
• Built around homogeneous arrays of elementary
  components that can be configured to perform
  logical AND and OR operations
• We look at basic structures that can realize AND,
  OR functions (in reality PLAs are based on
  NAND/NOR functions in CMOS technology)
• PLAs can belong to
• Gate arrays, if interconnect layer is not
  finished
• PLDs, if fuses are provided along with a full
  interconnect layer
• FPGA are PLD