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ECE 301 – Digital Electronics

Single-bit Adder Circuits and Multi-bit Adder Circuits (Lecture #10). ECE 301 – Digital Electronics. The Half Adder (HA). Single-bit Adder Circuits. 0 0 1 1 + 0 + 1 + 0 + 1 0 1 1 10. Sum. Carry. Sum. Binary Addition. The Half Adder.

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ECE 301 – Digital Electronics

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  1. Single-bit Adder Circuits and Multi-bit Adder Circuits (Lecture #10) ECE 301 – Digital Electronics

  2. ECE 301 - Digital Electronics The Half Adder (HA) Single-bit Adder Circuits

  3. ECE 301 - Digital Electronics 0 0 1 1 + 0 + 1 + 0 + 1 0 1 1 10 Sum Carry Sum Binary Addition

  4. ECE 301 - Digital Electronics The Half Adder

  5. ECE 301 - Digital Electronics The Half Adder

  6. ECE 301 - Digital Electronics The Full Adder (FA) Single-bit Adder Circuits

  7. ECE 301 - Digital Electronics Binary Addition 0 0 0 0 0 0 1 1 + 0 + 1 + 0 + 1 0 1 1 10 Carry-in 1 1 1 1 0 0 1 1 + 0 + 1 + 0 + 1 1 10 10 11 Carry-out Sum

  8. ECE 301 - Digital Electronics The Full Adder Cin Cout

  9. ECE 301 - Digital Electronics S Cout Cin Cin Cin Cin The Full Adder S = X xor Y xor Cin Cout = X.Y + X.Cin + Y.Cin

  10. ECE 301 - Digital Electronics X Y S Cin Cout The Full Adder

  11. ECE 301 - Digital Electronics Half Adder Half Adder Cin Cin Cin + xy Cin The Full Adder

  12. ECE 301 - Digital Electronics Multi-bit Adder Circuits

  13. ECE 301 - Digital Electronics Implementations of Multi-bit Adders: 1. Ripple Carry Adder 2. Carry Lookahead Adder

  14. ECE 301 - Digital Electronics Ripple Carry Adder Multi-bit Adder Circuits

  15. ECE 301 - Digital Electronics Ripple Carry Adder Carry ripples from one column to the next 1 1 1 Carry-in 1 0 1 0 + 1 0 0 1 1 0 1 0 0 Carry-out

  16. ECE 301 - Digital Electronics Ripple Carry Adder Carry-out Carry-in Carry ripples from one stage to the next

  17. ECE 301 - Digital Electronics Ripple Carry Adder • n-bit Ripple Carry Adder • Composed of n 1-bit Full Adders • Carries ripple from LSB stage to MSB stage • Delay ~ (n)*(delay of single FA stage) • Area required is linear in n • 4-bit Ripple Carry Adder • Composed of 4 1-bit Full Adders

  18. ECE 301 - Digital Electronics The Ripple Carry Adder is slow! Why? How can the speed of the adder be increased?

  19. ECE 301 - Digital Electronics Increasing the speed of the Adder • Method A: Include all inputs and outputs in the design • Inputs = Xi, Yi, Cin,i; Outputs = Si, Cout,i • 1-bit 3 inputs 2 outputs • 2-bit 5 inputs 3 outputs • 4-bit 9 inputs 5 outputs • n-bit 2n+1 inputs n+1 outputs • Large number of operands, but only 2 logic levels • Increase in speed • Increase in area required Use Truth Table and K-Map to derive logic functions decrease propagation delay increase # of logic gates

  20. ECE 301 - Digital Electronics Method B: Manipulate the Boolean Algebra (results in the design of the Carry Lookahead Adder) Increasing the speed of the Adder

  21. ECE 301 - Digital Electronics Carry Lookahead Adder Multi-bit Adder Circuits

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