Chapter4_2 - Chapter 4 (Lect 2) Adders Carry propagation...

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Chapter 4 (Lect 2) Adders Carry propagation Subtraction Overflow
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Timing and Carry propagation: Standard 4-bit ripple adder A 0 B 0 A 1 B 1 A 2 B 2 A 3 B 3 S 0 C 0 S 1 S 2 S 3 C 1 C 2 C 3 C 4 When are the inputs applied When are the correct outputs available What is the limiting factor Total carry propagation delay =
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The problem the carry bits take time to propagate through the adder. Could choose faster logic to reduce delay Could change circuitry so that the carry information propagates at the same time as the other information The circuit is optimized in terms of complexity, if we want the speed increase we must increase the complexity, assuming the fastest gates are not fast enough The textbook shows one possible solution The approach is to separate out the dependencies of the carry bits on one another, of course this will require additional circuitry for each carry bit
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End result (Carry Lookahead Generator) What’s important, path for each carry bit is an additional AND OR gate?
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Chapter4_2 - Chapter 4 (Lect 2) Adders Carry propagation...

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