Fewer G4FETs than conventional transistors would be needed to implement multiplexers.

Generic structures have been conceived for multiplexer blocks to be implemented in field-programmable gate arrays (FPGAs) based on four-gate field-effect transistors (G4FETs). This concept is a contribution to the continuing development of digital logic circuits based on G4FETs and serves as a further demonstration that logic circuits based on G4FETs could be more efficient (in the sense that they could contain fewer transistors), relative to functionally equivalent logic circuits based on conventional transistors.

A Four-to-One Multiplexer is a special case of a 2n-to-1 multiplexer, which can perform a variety of logic functions on 2n binary data inputs (x0,...x2n –1), and n control (selection) inputs (c0,...cn–1). In this case, n = 2. The combination of the control inputs can be interpreted as a binary integer, c, in the range of 0 to 2n – 1.
Results in this line of development at earlier stages were summarized in two previous NASA Tech Briefs articles: “G4FETs as Universal and Programmable Logic Gates” (NPO-41698), Vol. 31, No. 7 (July 2007), page 44, and “Efficient G4FET-Based Logic Circuits” (NPO-44407), Vol. 32, No. 1 (January 2008), page 38 . As described in the first-mentioned previous article, a G4FET can be made to function as a three-input NOT-majority gate, which has been shown to be a universal and programmable logic gate. The universality and programmability could be exploited to design logic circuits containing fewer components than are required for conventional transistor-based circuits performing the same logic functions. The second-mentioned previous article reported results of a comparative study of NOT-majority-gate (G4FET)-based logic-circuit designs and equivalent NOR- and NAND-gate-based designs utilizing conventional transistors. [NOT gates (inverters) were also included, as needed, in both the G4FET-and the NOR- and NAND-based designs.] In most of the cases studied, fewer logic gates (and, hence, fewer transistors), were required in the G4FET-based designs.

There are two popular categories of FPGA block structures or architectures: one based on multiplexers, the other based on lookup tables. In standard multiplexer-based architectures, the basic building block is a treelike configuration of multiplexers, with possibly a few additional logic gates such as ANDs or ORs. Interconnections are realized by means of programmable switches that may connect the input terminals of a block to output terminals of other blocks, may bridge together some of the inputs, or may connect some of the input terminals to signal sources representing constant logical levels 0 or 1.

The left part of the figure depicts a four-to-one G4FET-based multiplexer tree; the right part of the figure depicts a functionally equivalent four-to-one multiplexer based on conventional transistors. The G4FET version would contains 54 transistors; the conventional version contains 70 transistors.

This work was done by Farrokh Vatan and Amir Fijany of Caltech for NASA’s Jet Propulsion Laboratory.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:

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Refer to NPO-44735, volume and number of this NASA Tech Briefs issue, and the page number.

This Brief includes a Technical Support Package (TSP).

Efficient Multiplexer FPGA Block Structures Based on G4FETs (reference NPO-44735) is currently available for download from the TSP library.

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