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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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18 votes
18 votes






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

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18 votes
18 votes
carry look ahead generator has inputs P(i) =A(i) EXOR B(i) , Gi = A(i) AND B(i)    , delay max(20,10) = 20

carry generation circuit C(i+1) = G(i) + C(i) P(i)     , delay = 10 + 10 = 20

to generate sum S(i+1) = C(i)  EXOR P(i)      , delay= 20

Total delay to generate carry = 20+20 = 40 ns

Total delay to generate Sum = 20 +20 +20 = 60 ns
1 votes
1 votes

This question is not formed properly because of carry generation time. Carry output is generated in 2 levels of AND, OR gate that is it will take 20 ns. but sum output time is correct because it generates using 1 XOR gate and delay of it is 20ns.

In ripple carry adder all Pi, Gi terms are computed simultaneously after that all carry terms are computed simultaneously and after that all sum terms are computed simultaneously.

so first 20ns to generate all Pi, Gi terms then 20ns to generate all carry terms then further 20ns to generate all sum terms.

So total time = 60ns.

0 votes
0 votes

3 full adder 3*30=90ns

2 OR gate 2*10=20ns

total 90+20=110ns

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