US 5,847,984 A · Combination propagate and generate carry lookahead gate

Abstract

Carry lookahead adders reduce the number of logic levels required to sum two numbers. A pure carry lookahead adder, however, requires circuits with large fan-outs and fan-ins making it impractical to build for a large number of bits. Carry lookahead tree adders use a plurality of small carry lookahead adders to build a complete adder. A binary carry lookahead adder is a typical implementation of a carry lookahead tree adder. Each stage of the binary carry lookahead adder generates a propagate output and generate output from propagate and generate inputs and calculates a carry output from a carry input. A modified generate circuit is introduced which reduces the propagation delay of the generate circuit. A modified propagate circuit is introduced that ensures the inputs to the modified propagate circuit do not create a short circuit. Additionally, a combination propagate and generate circuit is introduced which reduces the number of transistors required to implement the propagate and generate circuits. Still further, a quadrature carry lookahead tree adder is discussed and a combination propagate and generate circuit for the quadrature tree adder is presented.

Claims20

Dependent claims

A combined generate and propagate circuit in a carry lookahead adder, comprising:

a generate output node;
a propagate output node;
a first pair of switches comprising a first switch coupled between a first potential and said propagate output node and a second switch coupled between said first potential and said generate output node;
a second pair of switches comprising a third switch coupled between a second potential and said propagate output node, and a fourth switch coupled between said second potential and said generate output node;
a third pair of switches comprising a fifth switch coupled between a third node and said propagate output node, and a sixth switch coupled between said third node and said generate output node;
a fourth pair of switches comprising a seventh switch coupled between a fourth node and said propagate output node, and an eighth switch coupled between said fourth node and said generate output node;
a ninth switch operably coupled between said first potential and said third node;
and
a tenth switch operably coupled between said second potential and said fourth node.

6 more elements

A combined generate and propagate circuit in a carry lookahead adder, comprising:

a generate output node;
a propagate output node;
a first switch coupled between a third node and said propagate output node;
a second switch coupled between a fourth node and said propagate output node;
a third switch coupled between said third node and said generate output node;
a fourth switch coupled between said fourth node and said generate output node;
a fifth switch coupled between a fifth node and said propagate output node;
a sixth switch coupled between a sixth node and said propagate output node;
a seventh switch coupled between said fifth node and said generate output node;
a eight switch coupled between said sixth node and said generate output node;
a ninth switch coupled between a seventh node and said propagate output node;
a tenth switch coupled between a eighth node and said propagate output node;
a eleventh switch coupled between said seventh node and said generate output node;
a twelfth switch coupled between said eighth node and said generate output node;
a thirteenth switch coupled between a first potential and said propagate output node;
a fourteenth switch coupled between a second potential and said propagate output node;
a fifteenth switch coupled between said first potential and said generate output node;
a sixteenth switch coupled between said second potential and said generate output node;
a seventeenth switch operably coupled between said third node and said fifth node;
and
a eighteenth switch operably coupled between said fourth node and said sixth node;
a nineteenth switch operably coupled between said fifth node and said seventh node;
a twentieth switch operably coupled between said sixth node and said eighth node;
a twenty-first switch operably coupled between said seventh node and said first potential;
a twenty-second switch operably coupled between said eighth node and said second potential.

22 more elements

Dependencies are inferred from the claim text.

Combination propagate and generate carry lookahead gate

Life of this patent

Combination propagate and generate carry lookahead gate

Life of this patent

Abstract

Claims20