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IEEE International Midwest Symposium on Circuits and Systems (MWSCAS)

Superconducting Shuttle-flux Shift Buffer for Race Logic

Meriam Gay Bautista, Patricia Gonzalez-Guerrero, Darren Lyles, George Michelogiannakis

Published Aug 9, 2021• 4 citations
Publisher

Abstract

This paper presents a superconducting magnetically coupled shift buffer based on a shuttle-flux shift register that stores single flux quantum (SFQ) pulses. The shift register has a DC bias operating margin of ±34% at 10 GHz, the power dissipation of 3.6µW at the clock bias, and 38% fewer Josephson junctions for 1024 stages compared to DFF-based shift registers. Clock bias is independent of the input pulse sequence through inductive coupling with a minimal current drive. Based on our shift register, we add two non-destructive readouts (NDRO) cells to construct a buffer that can inexpensively store the temporal information of race logic (RL) pulses or a pulse train for spiking neural networks. These applications motivate deep shift registers with many shifting intervals, which our shift register can implement with only a pair of coupled inductors and one JJ per shifting interval.

DOI
10.1109/mwscas47672.2021.9531899

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This paper presents a superconducting magnetically coupled shift buffer based on a shuttle-flux shift register that stores single flux quantum (SFQ) pulses. The shift register has a DC bias operating margin of ±34% at 10 GHz, the power dissipation of 3.6µW at the clock bias, and 38% fewer Josephson junctions for 1024 stages compared to DFF-based shift registers. Clock bias is independent of the input pulse sequence through inductive coupling with a minimal current drive. Based on our shift register, we add two non-destructive readouts (NDRO) cells to construct a buffer that can inexpensively store the temporal information of race logic (RL) pulses or a pulse train for spiking neural networks. These applications motivate deep shift registers with many shifting intervals, which our shift register can implement with only a pair of coupled inductors and one JJ per shifting interval.
This paper presents a superconducting, magnetically-coupled, shuttle-flux shift register (SF-SR) that stores single flux quantum (SFQ) pulses. This shift register has a DC bias operating margin of ±34% at 10 GHz, with a power dissipation of3.6 μW3.6~\mu W3.6 μWand 38% fewer Josephson junctions (JJs) when scaled up to multiple stages compared to a data flip-flop (DFF) based shift register. The clock input is inductively coupled and is independent from the data input. We then present three applications for our SF-SR. In the first application, we add two non-destructive readout (NDRO) cells to construct a buffer that temporarily stores the temporal information of a series of race logic (RL) pulses. The second application is a pseudo-random number generator based on a linear function shift register (LFSR). The third application is N parallel SF-SRs that can act similar to a deserializer or instead can emulate a single SF-SR of N times higher clock frequency. These three applications motivate deep shift registers with many shifting intervals, which our SF-SR can implement with fewer JJs and lower power consumption compared to DFF-based shift registers.
Testing on a single-flux-quantum (SFQ) shift register fabricated with a SFQ Nb-AlO/sub x/Nb Josephson junction process indicates that a stable flux quantum can be stored in a rapid SFQ (RSFQ) shift register stage and be shifted by SFQ pulses. Design simulations showed that logic errors can occur in SFQ shift registers even though the circuit passes the average voltage test. Three-junction-per-stage shift registers had an advantage over the two-junction-per-stage shift register in handling data with successive 1's and in incorporating resistors to increase tolerance of trapped magnetic flux. Input and output buffers for data and clock pulses were added to get an RSFQ shift register to obtain correct performance. The highest error-free operation speed for the data with successive 1's is about half of the speed for propagation of a lone data 1. Error-free operation of SFQ shift registers with three junctions per stage could be obtained up to speeds of 30-60 GHz with Josephson junctions having I/sub c/R/sub n/ (critical-current-normal-resistance product) values of 600 mu V.>
Superconducting single flux quantum (SFQ) circuits have been shown to offer huge advantages that are ideal for high performance computing applications. These circuits can operate at very high frequencies (tens of GHz) but consume very low power (from nanowatts to microwatts per gate). Energy-efficient SFQ (ERSFQ) circuits, evolved from traditional rapid SFQ (RSFQ) circuits, were invented with the goal of elimination of static power dissipation. When compared with RSFQ circuits, ERSFQ circuits require a new biasing network that includes biasing Josephson junction(s), biasing inductor(s), and feeding Josephson transmission line(s). Some general suggestions on how to design the network have been offered, but a detailed analysis and optimization scenario for the network is still to be provided. In this paper, we first optimize the previously reported DC-biased two-Josephson junction per bit RSFQ shift register circuit, which can be used to provide a more robust, energy-efficient, and high memory capacity storage unit. The simulated power dissipation of 16-bit RSFQ shift register is 0.45 μW per bit when operating at 21.7 GHz. We then convert the RSFQ-based shift register into an ERSFQ structure and investigate the effects of the ERSFQ bias network on the margins of the ERSFQ circuit. Finally, we propose a more comprehensive optimization method for the ERSFQ biasing network.
Experimental superconducting shift registers consisting of on-chip clock generators, clock regeneration and distribution circuits, shift register elements, and readout circuits are designed using rapid single flux quantum logic/memory (RSFQ) gates. A 7-b shift register has been tested to 12 GHz and a 17-b to 21 GHz using external triggering clocks with relative delay measurements. Testing with internal clocks generated from Josephson oscillations shows a potential high-speed operation of 45 GHz for the 7-b and 30 GHz for the 17-b shift registers. Two types of magnetically coupled readout gates are discussed. The chips are fabricated using a Nb/AlO/sub /x/Nb Josephson-junction process at a critical-current density of 1000 A/cm/sup 2/. The power dissipation per bit is 3 mu W.>
Experimental superconducting shift registers con- sisting of on-chip clock generators, clock regeneration and dis- tribution circuits, shift register elements, and readout circuits are designed using rapid single flux quantum logic / memory (RSFQ) gates. A 7-b shift register has been tested to 12 GHz and a 17-b to 21 GHz using external triggering clocks with relative delay measurements. Testing with internal clocks generated from Josephson oscillations shows a potential high-speed opera- tion of 45 GHz for the 7-b and 30 GHz for the 17-b shift registers. Two types of magnetically coupled readout gates are discussed. The chips are fabricated using a Nb/AIO,/Nb Josephson-junction process at a critical-current density of 1000 A / cm2. The power dissipation per bit is 3 p.W.
A 12 bit Flux Shuttle shift register with a new write and readout gate has been simulated and fabricated using Nb/Al/sub 2/O/sub 3//Nb Josephson junctions. Write, shift and read operations have been tested successfully at 4.2 K. Drive currents are independent from input bit sequence. Although measurements were disturbed by trapped flux, minimum margins are /spl Delta/I=/spl plusmn/10%. The power dissipation of a shift register cell is 9 nW/GHz. Total power losses caused by terminating resistors are 70 /spl mu/W. The minimum line width may be scaled down to 0.5 /spl mu/m, because power losses per unit area of long Flux Shuttle shift registers are extremely low.