Carry-Skip Adders

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Carry-Skip Adders

• Generalization of Manchester Adders
• With CMOS Implementation Carry Skip speed
  Comparable to CLA
• Less Area/Power
• Based on Observation that Carry Process Can
  Skip Stage for Which xi ? yi (that is pi
  xi?yi 1)
• Adder of n stages Divided into Stages of
  Selected Lengths
• With Simple Ripple Carry
• Each Group Generates Group Carry-Propagate1
  if all pi1 in Each Group
• This Signal Allows Incoming Carry to Skip
  Stages within Group and Generate Carry-out

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Fixed Block Width Carry-Skip Adders
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Carry-Skip Adder-Group Carry
GCin
cin
cin
GCout
P1
P0
P2
GCinP20
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Fixed Block Width Carry-Skip Propagation Delay
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Variable Sized Blocks
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Optimum Block Size
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Optimum Block Size
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Multilevel Carry-Skip Adders
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Multilevel Carry-Skip Adders
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Example Carry-Skip Adder
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Example Carry-Skip Adder
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Example Carry-Skip Adder
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Example Carry-Skip Adder
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Example Carry-Skip Adder
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Skip Adder Delay Model
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Example 2 Delays
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Example 2 Delays