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In a 4-bit carry look ahead adder, the propagation delay of Ex-OR gate is 20ns ,AND and OR gates is 10 ns.The sum and carry output of full adder takes 20ns and 10ns respectively.The total propagation delay of the above adder in ns is __________
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20ns (External Ex-OR GATE) +  10ns (Internal AND GATE) + 10ns (Next level OR GATE + 20ns (External Ex_OR GATE)

Total : 20ns + 10ns + 10ns + 20ns = 60ns

by Veteran (60.8k points)
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Just need more clearity about calculations.
Is it like this? :

  • the left side of the box contains XOR (20 ns) and AND (10 ns). 20 ns > 10 ns, so we consider 20 ns
  • inside the box there is a AND-OR stage. Each AND and OR takes 10ns, hence total 20 ns
  • right side of the box contains XOR (20 ns) stage.

So total 20+20+20 = 60 ns


but here when i calculated for 2 bit

xor delay: 1.6 sec

and/or delay :1.0 sec

it was giving 6.2

@bikram sir ?

The problem says "above adder" and there is no "above adder" in the quesion. The problem should gives delays of
gates: AND, OR and XOR
and also of CARRY and SUM.
Only one should be given, but not both.
Given fact that we are dealing with carry look ahead adder and given gate delays, we are supposed to find sum and carry delays. It makes no sense to also give carry and sum delays itself in the question.

Also the given carry and sum delays seems to be un-achievable with given gate delays.
Also it says "The sum and carry output of full adder takes 20ns and 10ns respectively.", but here we are dealing with look ahead adder. Standalone full adder carry - sum delays does makes sense here.

Can someone explain how this fact "The sum and carry output of full adder takes 20ns and 10ns respectively" can be used?

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