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IEEE Transactions on Applied Superconductivity

Adder-accumulator cells in RSFQ logic

S.S. Martinet, DARREN K. BROCK, M. J. Feldman, Mark F. Bocko

Published Jun 1, 1995• 5 citations
Publisher

Abstract

We are in the process of designing a Finite Impulse Response (FIR) filter for use in a Digital Signal Processing (DSP) system based entirely on Rapid Single Flux Quantum (RSFQ) logic. One aspect of this project involves the development of the arithmetic unit of the filter, in this case an adder-accumulator multiplier. This article describes two cells can perform the function of accumulated addition in the adder-accumulator multiplier. We have fabricated both cells and tested them at low speed.>

DOI
10.1109/77.403224

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IEEE Transactions on Applied Superconductivity•Jun 1, 1995•28 citations
We are in the process of designing a Finite Impulse Response (FIR) filter for use in a Digital Signal Processing (DSP) system based entirely on Rapid Single Flux Quantum (RSFQ) logic. One aspect of this project involves the development of the arithmetic unit of the filter, in this case an adder-accumulator multiplier. This article describes two cells can perform the function of accumulated addition in the adder-accumulator multiplier. We have fabricated both cells and tested them at low speed.>
Abstract: We have developed and experimentally evaluated at high-speed, a complete set of arithmetic circuits (multiply, add, and accumulate) for high performance digital signal processing (DSP). These circuits take advantage of the unique features of the Rapid Single-Flux Quantum (RSFQ) logic/memory family, including fusion of logic and memory functions at the gate level, pulse representation of clock and data, and the ability to maintain inter cell propagation delays using Josephson transmission lines (JTLs). The circuits developed have been successfully used in the implementation of a serial radix 2 butterfly, a decimation digital filter, and of an arithmetic unit for digital beam forming. The 8×8-bit RSFQ multiplier uses a two- level parallel carry- save reduction tree that significantly reduces the multiplier latency. The 80-GHz carry-save reduction is implemented with asynchronous data-driven wave pipelined [4:2] compressors built with toggle flip-flop cells.
We have designed, fabricated, and tested a rapid single flux quantum (RSFQ) 1 bit arithmetic logic unit (ALU) block. The circuit consists of three DC current driven SFQ switches and a half-adder. We successfully tested the half-adder cell at clock frequency up to 20 GHz. The switches were commutating output ports of the half-adder to produce AND, OR, XOR, or ADD functions. For a high-speed test, we attached two switches at the input ports of the half-adder to control the high-speed input data by low-frequency pattern generators. The output in this measurement was an eye-diagram. Using this set-up, the circuit was successfully tested up to 20 GHz. The chip was fabricated using a standard HYPRES 1 kA cm −2 Nb Josephson junction fabrication process.
Residue number system (RNS) arithmetic has a promising role for fault-tolerant high throughput superconducting single flux quantum (SFQ) circuits for digital signal processing (DSP) applications. We have designed one of the basic computational blocks used in DSP circuits, one-decimal-digit RNS adder. A new design for its main component, the single-modulus adder, has been developed. It combines simple and robust RSFQ elementary cells, both combinational and sequential. The central units are a circular shift register, a code converter, and the clock control circuitry. Our mod5 adder employs 195 Josephson junctions, consumes 50 μW of power, and occupies an area of less than 2 mm2. Chips were fabricated at HYPRES, Inc. using 1 kA/cm2 low-Tc Niobium technology. The mod5 adder was successfully tested at low speed, and gave experimental bias margins of ±26%.
As part of the effort to develop a superconductive microprocessor, we constructed a rapid single flux quantum (RSFQ) 4-bit arithmetic and logic unit (ALU) in a pipelined structure. To speed up the circuit, we used a forward clocking scheme. The 4-bit ALU consisted of ten RSFQ Half Adders, four 2 /spl times/ 2 switches, and several D flip-flops. By commutating output ports of the half adder, we were able to produce AND, OR, XOR, and ADD functions. The 4-bit ALU was fabricated using the Korea Photonics Technology Institute's ten-level 1.0 kA/cm/sup 2/ Nb process. The size of the ALU was 3.0 mm /spl times/ 1.5 mm, fitting in a 5 mm /spl times/ 5 mm chip. The fabricated 1-bit ALU block was successfully tested at up to a 40 GHz clock frequency. The complete 4-bit ALU operated correctly at up to a 5 GHz clock frequency.
We have designed and RSFQ multiplier-accumulator, the central component of our decimation digital filter. The circuit consists of 38 synchronous RSFQ cells of six types arranged into a rectangular systolic array fed by one parallel input and one serial input. Timing is based on counter-flow clock distribution scheme with simulated maximum clock frequency of 11 GHz. The circuit, fabricated at Hypres, Inc., contains 1100 Josephson junctions, has power consumption less than 0.2 mW, and area less than 2.5 mm/sup 2/. The multiplier-accumulator has been tested at low frequency demonstrating full functionality and stable operation over a 24 hour testing period. This four-bit multiplier accumulator is one of the largest reported RSFQ circuits verified experimentally to date.
We have designed a Rapid Single-Flux-Quantum (RSFQ) bit-serial real-time pipeline multiplier for digital signal processing (DSP) applications. A single-bit module of this multiplier consists of 96 Josephson junctions and uses a B-flip-flop-based carry-save adder (CSA). For HYPRES' standard 1-kA/cm/sup 2/ Nb process with 3.5 /spl mu/m-diameter Josephson junctions the module occupies all area of 350/spl times/600 /spl mu/m/sup 2/. Simulations show that the circuit should dissipate 28 /spl mu/W of power at 2.6 mV dc supply voltage and operate at frequencies of up to 25 GHz. We have successfully tested all cells of the module and verified correct operation of a simplified version of the module at low frequencies. According to numerical simulations, the speed of the multiplier is limited by the CSA. In order to overcome this bottleneck we have developed a concept of a fast carry-save pipeline adder based on XOR gates which uses an RSFQ-specific algorithm for carry bit calculation.>