On the Construction of Optimal Time Adders

Abstract

In this paper we present the design of a novel optimal time adder: the conditional carry adder. In order to perform addition a tree-like combination of multiplexer cells is used in the carry computation part. We show that, for the complete conditional carry adder, this results in an overall computation time which seems to be substantially shorter than for any other known (optimal time) adder (e.g. carry look ahead adders ([5]) or conditional sum adders ([12])). The second part of this paper contains a uniform approach to the computation of the carry function resulting in seven different classes of optimal time adders. It is shown that the conditional carry adder and the carry look ahead adder are representatives of two different classes. While section 1 defines the conditional carry adder and proposes a realization which is very time efficient, section 2 provides the possibility to compare this choice with other possible realizations and to choose a different design depending e.g. on specific properties of a given technology.