Full adder

Question

Comments

This is a Ripple carry adder

Time taken by each full adder = 60 ns

Time taken in Order to ADD 4 bit using four 1-bit full adder = 4*60 = 240ns

what's the answer given ??

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Ans. 100 given

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See this

 

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100 shuld be a Answer

Answer 1

Time taken in a ripple carry adder is = (n-1)*time to propagate carry + time of a single full adder.[if carry propagate time<sum generate time]

=3*20+40

=100ns.

Answer 2

This is 1- bit rca
So time at which carry bit becomes available    
Tcarry=n*tpd(carry)
=1*20=20 ns
And time at which output sum bit becomes available Ts= (n-1)*tpdcarry+tpd(sum)
                     =0*20+40=60ns

So total propagation delay in ripple carry adder=Tcarry+Ts=20+40=60ns
60 is right answer