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ACM Great Lakes Symposium on VLSI (GLSVLSI)

Compound Logic Gates for Pipeline Depth Minimization in Single Flux Quantum Integrated Systems

Rassul Bairamkulov, Giovanni De Micheli

Published May 31, 2023• 6 citations

Abstract

Superconductive electronics is a promising candidate for supplementing or replacing existing CMOS VLSI systems. Rapid single-flux quantum (RSFQ) is one of the most advanced superconductive technologies operating at tens of gigahertz while reducing the operating power by up to three orders of magnitude as compared to conventional semiconductor systems. Achieving VLSI complexity of RSFQ integrated systems, however remains an elusive task due to fundamental differences between RSFQ and CMOS technologies. Most RSFQ logic gates, such as NOT and XOR are sequential in nature. Compared to CMOS, the number of logical pipeline stages is prohibitively large, greatly complicating the design of complex systems. Furthermore, additional circuitry, such as splitters and path balancing flip flops, constitute a major overhead. In this paper, a gate compounding technique is presented to maximize the functionality achievable within a single clock cycle. The logic gates are decomposed into primitives that can be efficiently combined to evaluate complex expressions in a single clock cycle. Structures generated by gate compounding are not sensitive to signal arrival time, simplifying the system design process. The expressive power of SFQ logic is increased, allowing any two-input truth table to be implemented within a single clock cycle. A 4-bit carry lookahead adder (CLA) is implemented using compound gates, demonstrating smaller area, pipeline depth, and clock tree size.

DOI: 10.1145/3583781.3590287

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Superconductive electronics is a promising candidate for supplementing or replacing existing CMOS VLSI systems. Rapid single-flux quantum (RSFQ) is one of the most advanced superconductive technologies operating at tens of gigahertz while reducing the operating power by up to three orders of magnitude as compared to conventional semiconductor systems. Achieving VLSI complexity of RSFQ integrated systems, however remains an elusive task due to fundamental differences between RSFQ and CMOS technologies. Most RSFQ logic gates, such as NOT and XOR are sequential in nature. Compared to CMOS, the number of logical pipeline stages is prohibitively large, greatly complicating the design of complex systems. Furthermore, additional circuitry, such as splitters and path balancing flip flops, constitute a major overhead. In this paper, a gate compounding technique is presented to maximize the functionality achievable within a single clock cycle. The logic gates are decomposed into primitives that can be efficiently combined to evaluate complex expressions in a single clock cycle. Structures generated by gate compounding are not sensitive to signal arrival time, simplifying the system design process. The expressive power of SFQ logic is increased, allowing any two-input truth table to be implemented within a single clock cycle. A 4-bit carry lookahead adder (CLA) is implemented using compound gates, demonstrating smaller area, pipeline depth, and clock tree size.

Rapid single-flux quantum (RSFQ) is one of the most advanced cryogenic superconductive electronics technologies. With orders of magnitude smaller power dissipation, RSFQ is an attractive technology for cloud computing, aerospace electronics, and high-speed interfacing with quantum computing systems. Technological challenges however greatly complicate the realization of VLSI-complexity RSFQ systems. For example, gate-level pipelining in SFQ systems incurs a significant area overhead due to the need for path balancing. This issue is particularly detrimental to SFQ systems due to the limited layout density of RSFQ systems.

Among the major issues in modern large-scale rapid single-flux quantum (RSFQ) circuits are the complexity of the clock network, tight timing tolerances, poor applicability of existing CMOS-based design algorithms, and extremely deep pipelines, which reduce the effective clock frequency. In this article, asynchronous dynamic single-flux quantum majority gates are proposed to solve some of these problems. The proposed logic gates exhibit high bias margins and do not require significant area or a large number of Josephson junctions as compared to existing RSFQ logic gates. These gates exhibit a tradeoff among the input skew tolerance, clock frequency, and bias margins. Asynchronous logic gates greatly reduce the complexity of the clock network in large-scale RSFQ circuits, thereby alleviating certain timing issues and reducing the required bias currents. Furthermore, asynchronous logic allows existing design algorithms to utilize CMOS approaches for synthesis, verification, and testability. The adoption of majority logic in complex RSFQ circuits also reduces the pipeline depth, enabling higher clock speeds in very large scale integration RSFQ circuits.

An architecture of rapid single-flux-quantum logic circuits using clockless gates is proposed. Compared with a circuit composed of only clocked gates, a circuit using clockless gates includes fewer clocked gates, and therefore, a smaller clock distribution network. A circuit in the proposed architecture is constructed from pipeline stages. Each pipeline stage consists of a column of clocked gates and a succeeding block of clockless gates. The function of a pipeline register of a stage is implemented by the latching function of the clocked gates in the column. The combinational logic of a stage is implemented by the logic function of the clocked gates in the column and that of the clockless gates in the block. We designed a two-stage-pipelined 4-bit carry-lookahead-adder using clockless gates as a design example of the proposed architecture. It had reductions of the count of clocked gates and the area by 33 and 13%, respectively, compared with that composed of only clocked gates. The 4-bit adder in the proposed architecture was fabricated and demonstrated with the maximum clock frequency of 21.4 GHz and the bias margin of ±16%.

Rapid Single Flux Quantum (RSFQ) logic is a digital circuit technology that in recent years has presented itself as an alternative to semiconductors in the application of ultra high speed, very low power applications. The optimal timing of digital circuits operating at hundreds of Gigahertz is still a complex problem for both RSFQ and semiconductor technologies. The fact that most RSFQ gates require a clock signal to function makes this even more complex. Various RSFQ liming schemes have been adapted from semiconductor design methodologies, and some have been designed specifically for RSFQ. Currently, synchronous clocking schemes outperform other schemes, but with the scale of RSFQ circuits ever increasing, the proper use of timing schemes are becoming more crucial. This paper describes a new asynchronous self-timing scheme where the details of clock distribution and clocking are built into the logic gates. Tests were done on the newly developed asynchronous logic-gales and a asynchronous full adder was implemented and tested.

Rapid single flux quantum (RSFQ) logic is a digital circuit technology that in recent years has presented itself as an alternative to semiconductors in the application of ultra high speed, very low power applications. The optimal timing of digital circuits operating at hundreds of GHz is still a complex problem for both RSFQ and semiconductor technologies. The fact that most RSFQ gates require a clock signal to function makes this even more complex. Various RSFQ timing schemes have been adapted from semiconductor design methodologies, and some have been designed specifically for RSFQ. Currently, synchronous clocking schemes outperform other schemes, but with the scale of RSFQ circuits ever increasing, the proper use of timing schemes are becoming more crucial. This paper describes a new asynchronous self-timing scheme where the details of clock distribution and clocking are built into the logic gates. Tests were done on the newly developed asynchronous logic gates and an asynchronous full adder was implemented and tested

Design and manufacturing of superconductive electronics have been evolving over the past three decades with significant progress made in related fields. Rapid single flux quantum (RSFQ) logic circuits have become popular among superconductive logic families and its energy-efficient variants (ERSFQ and eSFQ) have shown promise as an ultra lowpower and high-speed circuit fabric. SFQ circuits have been demonstrated at tens of GHz with an energy consumption of an attojoule per gate. There are many differences between SFQ and conventional CMOS circuits. SFQ logic circuits are based on the manipulation of the quantized magnetic flux pulses. Most of the logic gates are sequential in nature requiring the clock to be distributed to every logic gate. SFQ logic gates have no gain and hence splitters are needed to drive multiple fanouts. Design and successful demonstration of a controllable superconducting switch and a compact reliable memory element have evaded researchers so far. This paper starts by describing key differences between SFQ logic and conventional CMOS and concludes by listing key challenges that must be overcome to achieve the very large scale integration of SFQ circuits and make the demonstration of a superconductive CPU a reality.