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IEEE Transactions on Applied Superconductivity

16-Bit Wave-Pipelined Sparse-Tree RSFQ Adder

M. Dorojevets, Christopher Lawrence Ayala, Nobuyuki Yoshikawa, Akira Fujimaki

Published Dec 12, 2012• 35 citations
Publisher

Abstract

In this paper, we discuss the architecture, design, and testing of the first 16-bit asynchronous wave-pipelined sparse-tree superconductor rapid single flux quantum adder implemented using the ISTEC 10 kA/cm2ADP2.1 fabrication process. Compared to the Kogge-Stone adder, our parallel-prefix sparse-tree adder has better energy efficiency with significantly reduced complexity (at the expense of latency) and almost no decrease in operation frequency. The 16-bit adder core (without SFQ-to-dc and dc-to-SFQ converters) has 9941 Josephson junctions occupying an area of 8.5 mm2. It is designed for the target operation frequency of 30 GHz with the expected latency of 352 ps at the bias voltage of 2.5 mV. The adder chip was fabricated and successfully tested at low frequency for all test patterns with measured bias margins of +9.8%/-10.7%.

DOI
10.1109/tasc.2012.2233846

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In this paper, we discuss the architecture, design, and testing of the first 16-bit asynchronous wave-pipelined sparse-tree superconductor rapid single flux quantum adder implemented using the ISTEC 10 kA/cm2ADP2.1 fabrication process. Compared to the Kogge-Stone adder, our parallel-prefix sparse-tree adder has better energy efficiency with significantly reduced complexity (at the expense of latency) and almost no decrease in operation frequency. The 16-bit adder core (without SFQ-to-dc and dc-to-SFQ converters) has 9941 Josephson junctions occupying an area of 8.5 mm2. It is designed for the target operation frequency of 30 GHz with the expected latency of 352 ps at the bias voltage of 2.5 mV. The adder chip was fabricated and successfully tested at low frequency for all test patterns with measured bias margins of +9.8%/-10.7%.
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100-GHz Single-Flux-Quantum Bit-Serial Adder Based on 10-kA/cm2{\rm kA/cm}^{2}kA/cm2 Niobium Process
This paper reports the design and test results of a single-flux-quantum (SFQ) bit-serial adder, which we designed with a target-clock frequency of 100 GHz, to investigate several techniques for producing ultra-high-speed computations using SFQ circuits. The bit-serial adder was designed based on a new cell library developed for the ISTEC Advanced Process, where the critical current density and McCumber-Stewart parameters of Josephson junctions were increased to 10 kA/cm2and 2.0, respectively, to obtain higher operating frequencies. In addition, we adopted a circuit-design technique based on state transitions excluding a feedback loop in a typical bit-serial adder, and redesigned the NOR gate with the McCumber-Stewart parameter increased to 4.0 to improve performance. As a result, we experimentally obtained a sufficient dc bias margin of ±18% from low frequencies to 60 GHz, and verified the correctness of operations up to 93 GHz. We also demonstrated that the introduction of a higher bias voltage or large inductors in series with bias resistors is effective for achieving faster operation.
Abstract : The major objective of the project was to design and demonstrate operation of key components of a 30 GHz 16-bit RSFQ processor prototype implemented with the AIST/ISTEC 10 kA/cm sq. fabrication process. Our team has developed complete logical and physical designs of five RSFQ chips using the CONNECT cell library and RSFQ CAD tools developed at the Universities of Yokohama and Nagoya (Japan). The major results are the world's first successful design, fabrication, and demonstration of correct operation of a 20 GHz 8x8-bit parallel carry-save RSFQ multiplier with approximately 6K JJs, a 16-bit sparse-tree wave-pipelined RSFQ adder with approximately 10K JJs, and partial operation of an 8-bit ALU chip with approximately 9K JJs. The goal of the second phase of the project was to get detailed understanding of the performance, complexity, and energy efficiency of on-chip storage units implemented with superconductor Reciprocal Quantum Logic (RQL) using our RQL VHDL cell library tuned to the MIT Lincoln Laboratory 10 kA/cm2 248 nm process. The 8.5 GHz 1-4 Kbit 32-/64-bit multi-ported scratchpad memory, register files, write-through and write-back caches designed with RQL Non-Destructive Read-Out storage cells have the average energy consumption of 3.0-9.5 fJ/bit/operation at room temperature using the cryocooling efficiency of 0.1%.
We present a design for parallel pipelined carry-lookahead Kogge-Stone 32and 64-bit integer adders with the traditional concurrent flow timing scheme, and the results of its gate-level logical simulation using a VHDL model, with parameters reduced from the physical-level simulation of RSFQ cells. The design uses only five different types of bit processing blocks and is easily scalable to any length of the operands. The multi-pulse logic representation together with interchanging logical polarity between pipeline stages is used to simplify the design of the blocks, which contain only two types of clocked RSFQ gates: an inverter and a D-flip-flop. Simulations show that in the absence of thermal fluctuations and random parameter spread the clock frequency of the adder implemented in the projected 0.8 /spl mu/m Nb-trilayer technology could be as high as 150 GHz. However, an approximate account of these factors shows that in order to achieve a 99% adder fabrication yield and a 10/sup -25/ adder error rate the maximal frequency should be reduced to 60 GHz for 1.5% Josephson junction spread and to 52 GHz for 3% spread. Adder latency is close to 260 ps for 32 bits and 320 ps for 64 bits. We plan to re-design the adders to increase their speed.
The first single-chip superconductor FLUX-1 microprocessor prototype with a target clock frequency of 17-20 GHz has been designed in the Rapid Single Flux Quantum (RSFQ) logic and fabricated using low-temperature 4 kA/cm 2 , 1.75-μm Nb/AlOx/Nb Josephson junction technology. A FLUX-1 chip represents an 8-bit deeply pipelined microprocessor with a new parallel partitioned architecture that has been developed to tolerate interconnect delays and fill long FLUX-1 pipelines with operations. A FLUX-1 chip contains 65,759 Josephson junctions on a 10.6 mm x 13.2 mm die with flip-chip packaging. First FLUX-1 chips fabricated in 2001 are currently under testing at TRW.