Question

In a 4 stage carry look adder the c3 will be available after ???​

Answer 1

Answer:

Motivation behind Carry Look-Ahead Adder :

In ripple carry adders, for each adder block, the two bits that are to be added are available instantly. However, each adder block waits for the carry to arrive from its previous block. So, it is not possible to generate the sum and carry of any block until the input carry is known. The

i^{th} block waits for the

i-1^{th} block to produce its carry. So there will be a considerable time delay which is carry propagation delay.

Consider the above 4-bit ripple carry adder. The sum

S_{4} is produced by the corresponding full adder as soon as the input signals are applied to it. But the carry input

C_{4} is not available on its final steady state value until carry

C_{3} is available at its steady state value. Similarly

C_{3} depends on

C_{2} and

C_{2} on

C_{1}. Therefore, though the carry must propagate to all the stages in order that output

S_{3} and carry

C_{4} settle their final steady-state value.