Carry-Skip Adders - PowerPoint PPT Presentation

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Carry-Skip Adders

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  1. 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-Propagate”=1 if all pi=1 in Each Group • This Signal Allows Incoming Carry to “Skip” Stages within Group and Generate Carry-out

  2. Fixed Block Width Carry-Skip Adders

  3. Carry-Skip Adder-Group Carry GCin cin cin GCout P1 P0 P2 GCin•P[2:0]

  4. Fixed Block Width Carry-Skip Propagation Delay

  5. Variable Sized Blocks

  6. Optimum Block Size

  7. Optimum Block Size

  8. Multilevel Carry-Skip Adders

  9. Multilevel Carry-Skip Adders

  10. Example Carry-Skip Adder

  11. Example Carry-Skip Adder

  12. Example Carry-Skip Adder

  13. Example Carry-Skip Adder

  14. Example Carry-Skip Adder

  15. Skip Adder Delay Model

  16. Example 2 Delays

  17. Example 2 Delays