Exascale computing and its associated increasingly massive amounts of data require increasingly efficient computing platforms; and semiconductor technology is struggling to keep up. Meanwhile, superconducting electronics (SCE) is increasingly gaining interest in both industry and academia as a serious contender to replace silicon for supercomputing. In particular, nonresistively biased single-flux quantum (SFQ) circuits have a theoretical potential of three orders of magnitude reduction in power accompanied with one order of magnitude of higher speed, and adiabatic quantum-flux-parametron (AQFP) circuits possess a theoretical potential of six orders of magnitude reduction in power when compared to the state-of-the-art semiconductor circuits. Unfortunately, the SCE community lacks a reliable design flow. Rather than creating a complete design flow from scratch, it is often more efficient to reuse well-established semiconductor tool flows developed by EDA companies that have benefited from research and development over the past half-century. One of the components of this flow is the hardware description language (HDL) modeling of gates for behavioral and functional simulation of large netlists. The previous HDL models proposed for SCE lack modularity are not compatible with existing CMOS standards, and are overly complex. This article presents SystemVerilog models for SFQ and AQFP gates and the circuits that interface between them. Our models are compatible with standard delay format and commercial simulators. They offer a generalizable modular debugging platform, are elegant and conceptual, and can capture many circuit parameters, environmental effects, and advanced SCE phenomena.
Exascale computing and its associated increasingly massive amounts of data require increasingly efficient computing platforms; and semiconductor technology is struggling to keep up. Meanwhile, superconducting electronics (SCE) is increasingly gaining interest in both industry and academia as a serious contender to replace silicon for supercomputing. In particular, nonresistively biased single-flux quantum (SFQ) circuits have a theoretical potential of three orders of magnitude reduction in power accompanied with one order of magnitude of higher speed, and adiabatic quantum-flux-parametron (AQFP) circuits possess a theoretical potential of six orders of magnitude reduction in power when compared to the state-of-the-art semiconductor circuits. Unfortunately, the SCE community lacks a reliable design flow. Rather than creating a complete design flow from scratch, it is often more efficient to reuse well-established semiconductor tool flows developed by EDA companies that have benefited from research and development over the past half-century. One of the components of this flow is the hardware description language (HDL) modeling of gates for behavioral and functional simulation of large netlists. The previous HDL models proposed for SCE lack modularity are not compatible with existing CMOS standards, and are overly complex. This article presents SystemVerilog models for SFQ and AQFP gates and the circuits that interface between them. Our models are compatible with standard delay format and commercial simulators. They offer a generalizable modular debugging platform, are elegant and conceptual, and can capture many circuit parameters, environmental effects, and advanced SCE phenomena.
Superconducting electronics provide us with cryogenic digital circuits that can rival established technologies in performance and energy consumption. Today, the lack of tools for the design of large-scale integrated superconducting circuits is a major obstacle to their deployment. Few research institutions and companies have contributed to making such tools available. This review focuses on methods, algorithms, and open-source design tools for logic synthesis of superconducting circuits in two major families: single-flux quantum (SFQ) circuits and adiabatic quantum flux parametron (AQFP).
Josephson junction-based superconducting logic families have been proposed to implement analog and digital signals, which can achieve low energy dissipation and ultra-fast switching speed. There are two representative technologies: DC-biased RSFQ (rapid single flux quantum) technology and its variants that achieve a verified speed of 370 Ghz, and AC-biased AQFP (adiabatic quantum-flux-parametron) that achieves an energy dissipation near quantum limits. Despite extraordinary characteristics of the superconducting logic families, many technical challenges remain, including the choice of circuit fabrics and architectures that utilize the SFQ technology and the development of effective design automation methodologies and tools. This paper presents our work on developing design flows and tools for DC- and AC-biased SFQ circuits, leveraging unique characteristics and design requirements of the SFQ logic families. More precisely, physical design algorithms, including placement, clock tree routing, and signal routing algorithms targeting RSFQ circuits are presented first. Next, a majority/minority gate-based automatic synthesis framework targeting AQFP logic circuits is described. Finally, experimental results to demonstrate the efficacy of the proposed framework and tools are presented.
Josephson Junction (JJ) based superconductor logic families have been proposed and implemented to process analog and digital signals [1] for its low energy dissipation and ultrafast switching speed. Thanks to its construction of resistance-less wires and ultrafast switches, it can operate at clock frequencies of several tens of gigahertz and even hundreds of thousands of times as energy efficient as its CMOS counterparts. It has been perceived to be an important candidate to replace stateof-the-art CMOS due to the superior potential in operation speed and energy efficiency, as recognized by the U.S. IARPA C3 and SuperTools Programs and Japan MEXT-JSPS Project. The design and fabrication of superconducting circuits have already been established [2]-[4]. In addition, a prototype superconducting microprocessor "Core 1" has been demonstrated in 2004 [3], which is able to execute instructions at a high clock frequency of several tens of gigahertz, and with extremely low-power dissipation. These achievements make superconducting electronics highly promising for future high-performance computing applications. As one of the most matured superconducting technology, the Rapid-Single-Flux-Quantum (RSFQ) technology is proposed by K. Likharev, O. Mukhanoc, V. Semenov in 1985 [1]. Despite its capability to be operated at an ultra-high speed of hundreds of GHz while maintaining extremely low switching energy (10^-19 J), it suffers from an increasing static power due to on-chip resistors that are required for constant DC bias supply for the main RSFQ circuit. Numerous methods have been proposed to resolve the static power dissipation problem of RSFQ, including low-voltage RSFQ (LV-RSFQ) [5], reciprocal quantum logic (RQL) [6], LRbiased RSFQ [7] and energy-efficient single-flux quantum (eSFQ) [8]. The Adiabatic Quantum-Flux-Parametron (AQFP) technology, on the other hand, uses AC bias/excitation currents as both multiphase clock signal and power supply [9] to mitigate the power consumption overhead of DC bias while operating at a frequency of few GHz. Consequently, AQFP is remarkably energy efficient compared to RSFQ, albeit operating at a lower frequency. The energy-delay-product (EDP) of the AQFP circuits fabricated using processes such as the AIST standard process 2 (STP2) and the MIT-LL SFQ process [10], [11], is at least 200 times smaller than those of the other energy-efficient superconductor logics and is only three orders of magnitude larger than the quantum limit [9]. Physical testing results of an AQFP 8-bit carry-look-ahead adder and large scale circuits consisting up-to 10,000 AQFP logic gates have demonstrated the AQFP being a promising technology that is robust against circuit parameter variations [12]. Despite the high application potential of AQFP in VLSI circuits, a systematic, automatic synthesis framework for AQFP is imminent. There are two features of AQFP that restrict conventional CMOS synthesis methods being directly applied on AQFP. In spite of And-Or-Inverter(AOI) based representation, which conventional CMOS circuits highly relies on, AQFP circuits prefer majority gates. In fact, its two inputs AND and OR gates are also built with three inputs majority gate with one input being constant. In addition, given its clock-synchronized data propagation nature, AQFP technology requires all inputs to any gate having equal delay. In order to meet this balanced timing requirement, splitters and buffers need to be inserted to the circuit. As a matter of fact, some circuit size can be doubled even with optimum amount of buffers and splitters inserted. The buffer and splitter insertion method can have a huge impact on the overall resource consumption. As the design complexity increases, an unoptimized buffer and splitter insertion method could result in huge amount of unnecessary buffers and splitters added. In addition to a complete synthesis framework, an Integrated Development Environment (IDE) for AQFP design is also lacking. It is imminent to have an IDE for AQFP integrating tools that offer schematic and layout editor, simulation and analysis for better and more efficient AQFP design flow. In this paper, we propose a complete design tool for AQFP design including an Integrated Development Environment (IDE), a complete majority based synthesis framework and a buffer and splitter insertion framework. we propose a majority gates synthesis framework for AQFP circuits that is capable of converting any AOI netlist to its corresponding MAJ netlist by mapping all feasible three-input sub-netlists to corresponding MAJ based implementations. In addition, we also propose an automated buffer and splitter insertion method that is capable of adding the optimum amount of buffers and splitters to any given gate-level netlist. The proposed method can find the minimum amount of buffers and splitters to inserted to achieve equal delay with any library limitation on the size of splitters. Experimental results suggest that the proposed methods can deliver very optimized results. The majority conversion tool can reduce circuit size by an average of 16:47% and delay by 30:21% in average. The buffer and splitter insertion tool only introduces an average of 14:24% overhead in size and 4:70% delay with splitter fanout size limited to 4 compared to an unachievable ideal results with no limitations on splitter fan-out size. Overall, the balanced design in majority gates can be reduced by 23:85% and 29:54% in size and delay compared to its AND/OR/Inverter implemented counterpart.
Special Section on Progress & Trend of Superconductor-based ComputersIt is our great pleasure to have a Special Section entitled "Progress & Trend of Superconductor-based Computers" in IEICE Transactions on Electronics.Superconducting devices based on Josephson junctions exhibit low power consumption and fast switching.Superconducting transmission lines are free of dispersion up to several hundred GHz.Unique logic circuits such as fast single flux quantum (SFQ) circuits and ultralow power adiabatic quantum-flux parametron (AQFP) logic circuits were proposed and have been improved.These circuits have been well developed based on not only their theoretical advantage but also reliable design and fabrication technology of superconducting integrated circuits.Recently, strong interest on quantum computing motivates the development of superconducting quantum-bits, control and readout circuits, and their integration.In this Special Section, seven invited papers and a contributed one describe these topics: two papers for SFQ, four for AQFP, and two for key devices for quantum circuits.Finally, I would like to appreciate activities of all authors, reviewers, and members in editorial committee for completing this special section.
Recent advances in fabrication complexity and density-especially for the Institute of Advanced Industrial Science and Technology (AIST) advanced process 2 (ADP2) process and the MIT Lincoln Lab (MITLL) SFQ5ee process and beyond-allow the implementation of a superconducting field-programmable gate array (SFPGA) with sufficient complexity to host a number of different programmed functions. We discuss design choices for an all single flux quantum SFPGA targeting these modern processes, making heavy use of passive transmission line (PTL) layers they provide. Various configurations for lookup tables, basic logic elements and timing schemes are shown, and a viable switch block implementation is discussed. Academic FPGA electronic design automation (EDA) tools are repurposed for mapping Verilog hardware description language (HDL) functions to the chosen SFPGA design, whilst HDL and electrical simulations are used to show its correct operation. We show simulations for programming and the operation of an SFPGA, and discuss die size requirements for practical implementation.
Circuit level simulation is too slow to be used for verification of function and timing of large RSFQ circuits. The alternative, known from semiconductor digital circuit design, is simulating at the logic (gate) instead of the circuit (transistor or junction) level. Using a hardware description language (HDL) such as Verilog, it is possible to write functional model of each of the RSFQ basic gates. A large RSFQ circuit composed of hundreds gates and thousands Josephson junctions can then be simulated using standard semiconductor industry CAD tools. We have developed a library of Verilog models for over 15 basic RSFQ gates. We describe in detail our model for the DRO RSFQ cell. We show how this model can be generalized for other more complex cells. Our library has been verified by employing it in the design of timing for three large RSFQ circuits.