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Full Adder Modules for Carry Save Adder Trees

IP.com Disclosure Number: IPCOM000101853D
Original Publication Date: 1990-Sep-01
Included in the Prior Art Database: 2005-Mar-17
Document File: 3 page(s) / 64K

Publishing Venue

IBM

Related People

Weinberger, A: AUTHOR

Abstract

U.S. Patent 4,463,469 shows implementations of a Full Adder (FA) using a combination of two modules, XOR and XNOR (A+B and A+B). The FAs are combined into a Carry Save Adder (CSA) tree (shown in Fig. 1) that reduces four logical inputs to two logical outputs. Each of the three logical inputs and two logical outputs of a FA is represented as a pair of signals whose OR represents the logical signal.

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Full Adder Modules for Carry Save Adder Trees

       U.S. Patent 4,463,469 shows implementations of a Full
Adder (FA) using a combination of two modules, XOR and XNOR (A+B and
A+B).  The FAs are combined into a Carry Save Adder (CSA) tree (shown
in Fig. 1) that reduces four logical inputs to two logical outputs.
Each of the three logical inputs and two logical outputs of a FA is
represented as a pair of signals whose OR represents the logical
signal.

      In an actual implementation, it is desirable to implement the
FA with single input and output signals representing the logical
inputs and outputs.  Fig. 2 shows two types of FA modules,
implemented with single input and output signals that can accommodate
any combination of true and complement inputs necessary for combining
FAs into a CSA tree.  One type accepts three inputs of equal
polarity, and the other type of mixed polarity.  Two versions of the
first type and four versions of the second type are shown.  Only one
version of each type is needed, namely one of X1 and X2, and one of
Y1, Y2, Y3 and Y4.

      A FA module consists of two stages, each an XOR or an XNOR.
One of the two terms, which are ORed to comprise the main output (H
or SUM), is replicated via an independent emitter follower.

      An additional advantage of these modules is lower power.  Only
three emitter follower resistors are needed, compared to four in Fig.
1.  Also the function H generated in the first XOR or XNOR stage...