ISRO | ISRO CS 2020 | Question 8

In a 8-bit ripple carry adder using identical full adders, each full adder takes 34 ns for computing sum. If the time taken for 8-bit addition is 90 ns, find time taken by each full adder to find carry.
(A) 6 ns
(B) 7 ns
(C) 10 ns
(D) 8 ns


Answer: (D)

Explanation: Given,

n = 8 Ripple Carry Adder
SUM delay from each Full Adder Sdelay = 34 ns
CARRY delay from each Full Adder Cdelay = ?
Overall delay = T = 90 ns 

Now,

T = Sdelay + (n – 1)Cdelay
90 = 34 + 7 × Cdelay
Cdelay = 8 ns 

So, option (D) is correct.

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