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IEEE Transactions on Applied Superconductivity

Architectural and implementation challenges in designing high-performance RSFQ processors: A FLUX-1 microprocessor and beyond

M. Dorojevets, P. Bunyk

Published Jun 1, 2003• 26 citations
Publisher

Abstract

This paper discusses the major challenges and solutions in designing high-performance superconductor processors in the context of the on-going collaboration between SUNY Stony Brook, TRW, and the JPL (NASA). It presents the architecture and organization of the first 8-bit FLUX-1 RSFQ microprocessor with a target clock frequency of 20 GHz fabricated in TRW's 4 kA/cm/sup 2/, 1.75-/spl mu/m Josephson junction technology in 2001-2002. A new parallel partitioned architecture has been developed for FLUX-1 in order to fill its long processing pipelines with operations, and reduce interconnect delays. The latest version of FLUX-1, called FLUX-1R, with improved circuit design characteristics and decreased power dissipation was fabricated in July 2002. FLUX-1R contains 63,107 Josephson junctions on a 10.35 mm /spl times/10.65 mm die with flip-chip packaging. The chip is currently under testing at TRW.

DOI
10.1109/tasc.2003.813893

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This paper discusses the major challenges and solutions in designing high-performance superconductor processors in the context of the on-going collaboration between SUNY Stony Brook, TRW, and the JPL (NASA). It presents the architecture and organization of the first 8-bit FLUX-1 RSFQ microprocessor with a target clock frequency of 20 GHz fabricated in TRW's 4 kA/cm/sup 2/, 1.75-/spl mu/m Josephson junction technology in 2001-2002. A new parallel partitioned architecture has been developed for FLUX-1 in order to fill its long processing pipelines with operations, and reduce interconnect delays. The latest version of FLUX-1, called FLUX-1R, with improved circuit design characteristics and decreased power dissipation was fabricated in July 2002. FLUX-1R contains 63,107 Josephson junctions on a 10.35 mm /spl times/10.65 mm die with flip-chip packaging. The chip is currently under testing at TRW.
The first single-chip superconductor FLUX-1 microprocessor has been designed in the Rapid Single Flux Quantum (RSFQ) logic and fabricated using 4 kA/cm2, 1.75-μm Nb/AlOx/Nb Josephson junction technology as a result of the collaboration between SUNY Stony Brook and TRW, Inc. A FLUX-1 chip represents an 8-bit deeply pipelined microprocessor prototype with a target clock frequency of 17-20 GHz. A new parallel partitioned architecture has been developed in order to tolerate interconnect delays and fill long FLUX-1 processor pipelines with useful instructions. The processor includes the 16 × 32-bit pipelined instruction memory, 8 integer arithmetic-logic units interleaved with 8 registers, the branch unit, and I/O ports for 5-GHz chip-to-chip communication over Nb microstrip lines on a chip carrier. The FLUX-1 instruction set consists of ~25 arithmetic, logical, and control instructions. A FLUX-1 microprocessor chip contains 65,759 Josephson junctions on a 10.6 mm × 13.2 mm die with flip-chip packaging. First FLUX-1 chips fabricated in August 2001 are currently under testing at TRW, Inc.
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.
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', 1.75-pm Nb/AlOx/Nb Josephson junction technology. A FLUX-1 chip represents an %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-I chips fabricated in 2001 are currently under testing at TRW.
We describe the design and implementation of a single-chip microprocessor based on LTS Rapid Single-Flux-Quantum (RSFQ) technology. Two such chips are to be used in a dual-processor module, being developed by a SUNY-TRW collaboration as a spin-off of the HTMT project. Each FLUX chip represents a simple 16-bit 2-way long-instruction-word (LIW) microprocessor with a pipelined instruction memory of 30-bit instructions, decode and issue units, 8 integer ALUs interlaced with 8 registers, and input/output ports through which two FLUX chips can communicate with each other at a 7-GHz communication rate over a multi-layer MCM. The FLUX instruction set consists of /spl sim/25 instructions. High performance is reached with a scalable design featuring (1) a very high clock rate, (2) localized, regular and ultrapipelined processing in registers with very short wires, (3) instruction-level parallelism utilization with bit-level resolution of data hazards. A 16-bit implementation of FLUX processor consists of /spl sim/90,000 Josephson junctions on a /spl sim/10 mm/spl times/15 mm chip area. Our estimates show that the processor will be able to operate at clock frequencies up to 20 GHz when implemented using TRW's 4 kA/cm/sup 2/, 1.75-/spl mu/m Nb-trilayer technology.
The Flux-1 chip is an RSFQ implementation of a small general-purpose processing engine with target clock frequency of 20 GHz and over 5000 gates (over 60 K Josephson junctions) connected in an irregular pattern. The scale of this design task forced us to re-think conventional RSFQ design methodology and implement new approaches suitable for digital systems of this level of complexity and beyond. This paper presents lessons learned from the Flux-1 effort, mostly concentrating on chip physical design. Here we discuss our approach to the circuit design and verification of individual gates, gate interconnect using passive transmission lines and use of CAD tools for design automation and verification.
This paper describes our programme to develop and demonstrate ultra-high performance single flux quantum (SFQ) VLSI technology that will enable superconducting digital processors for petaFLOPS-scale computing. In the hybrid technology, multi-threaded architecture, the computational engine to power a petaFLOPS machine at affordable power will consist of 4096 SFQ multi-chip processors, with 50 to 100 GHz clock frequency and associated cryogenic RAM. We present the superconducting technology requirements, progress to date and our plan to meet these requirements. We improved SFQ Nb VLSI by two generations, to a 8 kA cm −2 , 1.25 µm junction process, incorporated new CAD tools into our methodology, demonstrated methods for recycling the bias current and data communication at speeds up to 60 Gb s −1 , both on and between chips through passive transmission lines. FLUX-1 is the most ambitious project implemented in SFQ technology to date, a prototype general-purpose 8 bit microprocessor chip. We are testing the FLUX-1 chip (5K gates, 20 GHz clock) and designing a 32 bit floating-point SFQ multiplier with vector-register memory. We report correct operation of the complete stripline-connected gate library with large bias margins, as well as several larger functional units used in FLUX-1. The next stage will be an SFQ multi-processor machine. Important challenges include further reducing chip supply current and on-chip power dissipation, developing at least 64 kbit, sub-nanosecond cryogenic RAM chips, developing thermally and electrically efficient high data rate cryogenic-to-ambient input/output technology and improving Nb VLSI to increase gate density.