I was reading about the carry-lookahead adder and about the ripple-carry adder. I saw some designs and it made me wonder about a few things.

Both questions are the same for both adders, so I'll call them Y.

  1. Is there an equations for the number of full adders in a Y adder?
  2. If we will call X the pause (t_pd) of one full adder, what is the total t_pd of the Y adder?

For a carry-lookahead adder I think we will need 32 adders for Y, is that correct? How can I calculate the t_pd?


1 Answer 1


The number of full-adders required for a carry look-ahead adder is the same as for a ripple carry adder, and in both cases it is just the number of bits to be added. The difference between these adders is how the carry signal is generated.

The total propagation delay for the ripple carry adder is essentially equal to the number of bits times the delay from carry-in to carry-out for a single full adder. It's harder to calculate the total propagation delay for a CLA adder because you have to know the delay through the CLA logic, which is not directly related to the propagation delay through a full adder cell. The CLA logic is usually implemented as 4-bit slices, so the important parameter would be the delay from carry in to carry out for the slice, multiplied by the number of slices needed.

Of course, these are rough estimates that are generally applicable when the number of bits is large (say 32 or more) and the overall delay is dominated by the carry propagation.


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