The increasing complexity of modern rapid single flux quantum (RSFQ) circuits has made the issue of multiple fanout of growing importance. Most RSFQ gates can only drive a single output. Splitter gates can however distribute an SFQ pulse to multiple fanout. To drive N SFQ gates, N-1 splitters with a fanout of two are required. Large splitter trees are often used in high speed VLSI complexity SFQ systems. These splitters require significant area and increase the path delay. In this paper, three area and power efficient splitter topologies for large scale RSFQ circuits are introduced. These SFQ splitters are an active splitter tree with fewer JJs, a passive splitter, and a multi-output active splitter. A methodology is presented for determining when to use passive or active splitters. Tradeoffs among the number of JJs, bias current of each stage, and delay are reported along with a margin analysis. The proposed splitters greatly reduce the required bias currents and delay of large scale RSFQ circuits by enabling multiple fanout. The methodologies and techniques are applicable to automated layout and clock tree synthesis for large scale SFQ integrated circuits.
The increasing complexity of modern rapid single flux quantum (RSFQ) circuits has made the issue of multiple fanout of growing importance. Most RSFQ gates can only drive a single output. Splitter gates can however distribute an SFQ pulse to multiple fanout. To drive N SFQ gates, N-1 splitters with a fanout of two are required. Large splitter trees are often used in high speed VLSI complexity SFQ systems. These splitters require significant area and increase the path delay. In this paper, three area and power efficient splitter topologies for large scale RSFQ circuits are introduced. These SFQ splitters are an active splitter tree with fewer JJs, a passive splitter, and a multi-output active splitter. A methodology is presented for determining when to use passive or active splitters. Tradeoffs among the number of JJs, bias current of each stage, and delay are reported along with a margin analysis. The proposed splitters greatly reduce the required bias currents and delay of large scale RSFQ circuits by enabling multiple fanout. The methodologies and techniques are applicable to automated layout and clock tree synthesis for large scale SFQ integrated circuits.
Rapid Single Flux Quantum (RSFQ) logic cells have traditionally been limited to driving one fanout cell because of the difficulty in distributing the single flux quantum pulse to multiple fanouts. This paper presents a method to modify the standard RSFQ cells at their input/output interfaces to other cells in order to support a multiple-fanout drive capability. This capability is especially useful for clock distribution in RSFQ logic. This is because RSFQ logic is requires the clock signal to be provided to every logic gate. This is why this paper focuses on the clock signal driving more than one cell without the use of splitters. The potential tradeoff is in lower margins for the cells. However, by careful design of the RSFQ cells, the yield is not compromised by our proposed technique.
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.
High current requirements in single flux quantum (SFQ) circuits pose scalability challenges, contributing to issues such as high thermal loads, static power dissipation, and increased sensitivity to flux trapping under non-ideal shielding or cool down conditions. AC biasing, successfully used in other superconductive logic families, has been integrated into rapid single flux quantum (RSFQ) circuits and typically includes AC-to-DC converters, introducing area and complexity overhead. This work proposes an alternative method for applying AC biasing to existing RSFQ circuits using inductively coupled bias junctions, eliminating the need for AC-to-DC converters. This approach also combines resistive DC biasing with inductive AC biasing, reducing static power dissipation. The proposed approach targets clock splitters, which are significant contributors to on-chip current demand, achieving an average 94% reduction in splitter current and a 62% reduction in overall system current with a Josephson junction overhead of 6.5%. The technique is compatible with conventional RSFQ system architectures and logic families while significantly enhancing energy efficiency and scalability.
The increasing complexity of modern rapid single flux quantum (RSFQ) circuits has made on-chip signal routing, an issue of growing importance. In this paper, several methods for routing large-scale RSFQ circuits are described, and a process is presented for determining when to use passive microstrip transmission lines (PTL) and active Josephson transmission lines (JTL). The effect of the size of the JTL inductor and Josephson junctions on the length of a JTL chain for a target delay is also discussed. The dependence of the JTL inductance on the physical layout is evaluated, and the effects of the primary PTL parameters on delay are characterized. A novel PTL driver and receiver configuration is also proposed. Tradeoffs among the number of JJs, inductance, and length of a PTL stripline in the receiver and driver circuits are reported. The energy dissipation is evaluated for two different interconnects. A tradeoff between the proposed PTL circuits and an optimized JTL in terms of energy dissipation and delay is discussed. Guidelines for choosing the optimal element values are determined, and a simulated bias margin of ±29% for the bias current of the proposed receiver operating at 20 GHz in a 10 kA/cm2technology for a 1-mm transmission line is achieved. Summarizing, guidelines and design tradeoffs appropriate for automated layout and synthesis are provided for driving long interconnect in SFQ VLSI circuits.
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.
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.