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This paper proposes a method for generating RSFQ logic circuits utilizing special RSFQ gates in logic synthesis. It is suited for the standard technology mapping flow used in logic synthesis tools and enables generating RSFQ circuits utilizing confluence buffers (CBs) and resettable DFFs (RDFFs), to reduce the number of clocked gates by reduction of logic depth. A library of supergates, each consisting of several gates, is used to compose a resultant circuit in the technology mapping. In this paper, a library containing supergates including those special gates is generated and is utilized to design circuits with the gates. First, a generation of a supergate library is proposed to introduce CBs and RDFFs, which realize logic OR without clock pulses and a fused function of logic AND and NOT, respectively. The generated library contains supergates including the special gates and they are used to compose the resultant circuits with the special gates. A mapping approach is also proposed. It distinguishes non-clocked gates such as CBs and clocked gates. It takes the number of inserted DFFs for path balancing into account to reduce the number of clocked gates. The method was implemented into the state-of-the-art academic EDA tool, i.e., ABC, and was evaluated. Evaluation results show that the number of clocked gates is reduced by about 30% from the original ABC with path-balancing DFFs.

We have demonstrated the execution of several programs stored in an instruction memory by a rapid single-flux-quantum bit-serial 8-bit microprocessor named CORE e2h. We employed a minimal instruction set architecture composed of 13 instructions based on the reduced instruction set computer. We integrated a 128-bit instruction memory and a 128-bit data memory with an arithmetic logic unit, two registers, a program counter, an instruction register, and a controller unit on a single chip. The bit-serial operation was performed by an on-chip clock generator, while the system clocks are provided from room-temperature electronics. The CORE e2h was made up of 11 000 Nb/AlOx/Nb Josephson junctions and fabricated with the Advanced Industrial Science and Technology 10-kA/cm2, 9-Nb-layer process. We obtained the correct results for all the executed programs, including the integer division, the two kinds of summation algorithms, the Euclidean algorithm, and finding the greatest divisor. These test programs contained a single or nested double loop, and the maximum number of executed instructions was 205. We confirmed the stable operation with the DC bias margins of around 10% at 50-GHz for different test programs. The measured electric power consumption was 2.5 mW at 4.2 K, and the estimated computing power was 500 × 106 instructions/s.

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 $\pm$ 16%.

The conversion method of netlists consisting of conventional logic gates to RSFQ circuits is proposed. It treats netlists for CMOS circuits as the design entry and converts them to the netlists of RSFQ circuits, considering the reduction of the number of clocked gates, including D flip-flops (DFFs) for path balancing. It utilizes two kinds of special RSFQ gates for the reduction. One is a confluence buffer that can be utilized for realizing logic- or . The other is a small resettable DFF which can be utilized as an NIMPLY gate with the tuning of the order of pulse arrivals. The detection methods of replaceable gates with those two kinds of gates are proposed. An automatic test pattern generation tool is utilized for detecting replaceable or gates to speed up the previously proposed detection method. To minimize the number of clocked gates considering the cost of replacements, the selection problem of gates for replacements with those special RSFQ gates and the assignment of a logic level for each gate are formulated as an instance of integer linear programming. The proposed conversion method has been evaluated with ISCAS85 circuits, a 32-bit ALU, and a 64-bit ALU. The evaluation results show that the number of clocked gates and the latency in the cycles of circuits is reduced by about 10%. The delay time of the converted circuits has been estimated and the reduction of delay overhead is discussed.

An automated layout design flow for rapid single-flux-quantum circuits is proposed in this article. In order to realize small circuit area and low latency, both Josephson transmission lines (JTLs) and passive transmission lines (PTLs) are used for interconnects. Placement and routing are performed considering effective use of JTLs and PTLs. Logic cells are grouped into clusters, and JTLs are used inside clusters. PTLs are used among clusters. The placement process is composed of three steps: first, cell clustering, second, cell placement and JTL routing in each cluster, and third, cluster placement. After the placement process, routing among clusters are performed using PTLs. We applied the proposed design flow to sample circuits with several hundreds of gates. Though the circuit area is not fully optimized, the latency of the circuits designed by the proposed design flow are lower than those of the circuits designed manually.