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International Symposium on Memory Systems (MEMSYS)

Demonstration of superconducting memory with passive transmission line-based reads

Randy Posey, Randall Burnett, Quentin Herr, Donald L. Miller

Published Sep 30, 2019• 1 citation
Publisher

Abstract

Reciprocal Quantum Logic (RQL) is a superconducting logic family that operates at significantly lower power than conventional CMOS. RQL gate-based memories have been demonstrated, but have limited scaling due to clocked Josephson transmission line (JTL) signal propagation. Here we present a form of superconducting memory that utilizes an RQL write-path in conjunction with a faster, RF transmission-line-based read path, known as passive random access memory (PRAM).

DOI
10.1145/3357526.3357577

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Reciprocal Quantum Logic (RQL) is a superconducting logic family that operates at significantly lower power than conventional CMOS. RQL gate-based memories have been demonstrated, but have limited scaling due to clocked Josephson transmission line (JTL) signal propagation. Here we present a form of superconducting memory that utilizes an RQL write-path in conjunction with a faster, RF transmission-line-based read path, known as passive random access memory (PRAM).
New superconductor single flux quantum (SFQ) technology, such as Reciprocal Quantum Logic (RQL), is currently considered one of the promising candidates for highperformance energy-efficient computing. This paper presents our work on the design and detailed energy efficiency analysis of three types of 32- and 64-bit RQL multi-ported pipelined local storage structures (13 total), namely 1) random access memory (RAM) and register files, 2) direct-mapped write-through and write-back caches, and 3) first-in-first-out (FIFO) buffers. Our layout-aware cell-level design process uses a VHDL RQL cell library developed at the Ultra High Speed Computing Laboratory at Stony Brook University (SBU). The SBU VHDL RQL cell library specifies the dynamic and standby energy consumption, gate delays, a number of Josephson junctions (JJs) per cell, and approximate sizes of individual cells based on the parameters of the 248 nm 100 μA/μm210 Nb metal layer SFQ fabrication process currently under development at the MIT Lincoln Laboratory. Gate and wire delays as well as clock skew are taken into account during digital circuit simulation done with Mentor Graphics CAD tools. After completing a physical chip layout, the circuit models need to be updated and re-simulated to include the effects of parasitic inductances and actual wire lengths on signal propagation delays. To meet both performance and energy efficiency targets, the RQL storage structures were designed with RQL non-destructive read-out single-bit storage cells. We chose a relatively moderate clock frequency of 8.5 GHz for all storage units to keep their read latencies in the range of 1- 3 cycles. The most complex design in terms of JJs is a tripleported 4 Kbit 64x64-bit register file with 253,918 JJs and its read access latency of 338 ps. The highest energy consumption in terms of energy/operation/bit (~9.5 aJ at 4.2 K) is for a write hit in a 2 Kbit 32-bit wide write-back cache. The average energy consumption of the RQL storage designs varies from ~1.6 aJ/operation/bit for a small 4x32-bit FIFO to 7.3 aJ/operation/bit for the 2 Kbit write-back cache at 4.2 K. Given the cryocooler efficiency of 0.1%, this means the energy consumption of ~1.6-7.3 fJ/operation/bit at room temperature. The physical implementation of the RQL storage units will become feasible upon the development of the target MIT fabrication process and CAD tools for VLSI RQL chip design in 2015-2016.
This paper gives a newfangled dimension to superconducting logic which is the successor of CMOS logic and other superconducting quantum logic. Reciprocal Quantum logic (RQL) is proposed to achieve the less memory capability. RQL is a modified version of Rapid Single Flux Quantum (RSFQ). We designed and analyzed different RQL logic like half adder, full adder, counter, and Carry Save Adder (CSA) and we achieved an increase in clock speed compared to RSFQ logic by 20%. The designs are made of AndOr and AnotB gate operation and analyzed using WRSPICE software.
Complimentary Medal-Oxide Semiconductor (CMOS) technology is expected to soon reach its fundamental limits of operation. The fundamental speed limit of about 4 GHz has already effectively been sidestepped by parallelization. This increases raw processing power but does nothing to improve power dissipation or latency. One approach for increasing computing performance involves using superconducting digital logic circuits. In this thesis I describe a new kind of superconducting logic, invented by Quentin Herr at Northrop Grumman, which uses reciprocal pairs of quantized single magnetic flux pulses to encode classical bits. In Reciprocal Quantum Logic (RQL) the data is encoded in integer units of the magnetic flux quantum. RQL gates operate without the bias resistors of previous superconducting logic families and dissipate several orders of magnitude less power. I demonstrate the basic operation of key RQL gates (AndOr, AnotB, Set/Reset) and show their self-resetting properties. Together, these gates form a universal logic set and provide memory capabilities. Experiments measuring the bit error rate of the AndOr gate extrapolated a minimum BER of 10−480 and a BER of 10−44 with 30% margins on flux biasing. I describe an analytic timing model for RQL gates which demonstrates the self-correcting timing features. From this model I derive equations for the timing behavior and operating limits. Using this timing model I ran simulations to determine correction factions for more accurate predictions at higher frequencies. Using these results, I also develop Very High Speed Integrated Circuit (VHSIC) Hardware Description Language (VHDL) models to describe the combinational logic of RQL gates. To test the timing predictions of the timing model, I performed three experiments on Nb/AlOx/Nb circuits at 4.2 K. The first measured the time of output. The second measured the operating margins of the circuit. The third measured the maximum frequency of operation for RQL circuits. Together, these three experiments showed quantitative agreement with the model for the timing output, qualitative agreement with the limits of operation, and a projected speed limit of 50 GHz for the Hypres 4.5 kA/cm2 process. To power RQL circuits I describe a new design for power splitters and combiners which minimize standing waves. I describe a new kind of Wilkinson power splitter which required numerical optimization but proved to be adequate. I experimentally tested two new designs of the power splitter. Both showed less than 10% variation in standing waves between power splitter and combiner, making it adequate for RQL circuits. I also compared these results with the S-parameters of the power network, which also indicated that the design was adequate for RQL circuits. Finally, I tested an 8-bit Kogge-Stone architecture carry-look ahead adder designed using VHDL models. The adder contained 815 Josephson junctions and was fully functional at 6.21 GHz with a latency of 1.25 clock cycles. The adder produced the correct logical output, had a measured optimal operating point within 8% of the optimal simulated operating point, and measured power margins of 1 dB. It operated best at the designed clock amplitude of 0.88 Ic and dissipated 0.570 mW of power.
New superconductor single flux quantum logics with no static power dissipation in bias resistors, such as Reciprocal Quantum Logic (RQL), offer opportunities to create energy-efficient superconductor processors operating at high frequencies with ultra-low power consumption. This paper discusses the results of our work on the cell-level design and analysis of a benchmark set of 32-/64-bit RQL processor integer and floating-point units such as adders, multipliers, an arithmetic-logic unit, and an array shifter, as well as small 1-4 Kbit RQL on-chip storage components such as register files, on-chip memory, and the top level caches. Our layout-aware design process includes the complete cell-level design and approximate physical layout of the circuits followed by the VHDL simulation, verification, and energy profiling using our RQL VHDL cell library tuned to the future MIT Lincoln Laboratory 10 kA/cm2 248 nm process with 10 Nb metal layers and the minimum JJ critical current of 38 μA. Our designs have the energy efficiency of ~1.0 single-precision TFLOPS/W and '0.5 double-precision TFLOPS/W for floating-point units, and ~1-24 TOPS/W for 32-bit integer units at room temperature using the cryocooling efficiency of 0.1 % (1000 W/W). The 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%. While these results are very promising, more work is needed to evaluate the contribution of the energy costs of instruction scheduling and off-chip main memory access to the energy efficiency of RQL computing across a whole system.
Efficiency of high-performance computing is on high demand with both speed and energy efficiency. Reciprocal Quantum Logic (RQL) is one of the technology which will produce high speed and zero static power dissipation. RQL uses AC power supply as input rather than DC input. RQL has three set of basic gates. Series of reciprocal transmission lines are placed in between each gate to avoid loss of power and to achieve high speed. Analytical model of Bit-Level Architecture are done through RQL. Major drawback of reciprocal Quantum Logic is area, because of lack in proper power supply. To achieve proper power supply we need to use splitters which will occupy large area. Distributed arithmetic uses vector- vector multiplication one is constant and other is signed variable and each word performs as a binary number, they rearranged and mixed to form distributed system. Distributed arithmetic is widely used in convolution and high performance computational devices.