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IEEE Transactions on Applied Superconductivity

Single flux quantum circuits for 2.5 Gbps data switching

A. Hodge Worsham, A. H. Miklich, Donald L. Miller, Joonhee Kang, John X. Przybysz

Published Jun 1, 1997• 15 citations
Publisher

Abstract

We have designed and measured single flux quantum (SFQ) circuits for data switching at 2.5 Gbps. A fully functional 2/spl times/2 switching node is presented. The node can be used to form a larger switching fabric (4/spl times/4, 8/spl times/8, etc.). The node accepts data streams at either or both inputs, routing the data based on address bits present in the data stream. This approach allows the data to be self-routing and the switch to be self-synchronous. In addition, the switch can route data correctly on an active input line if the other is idle.

DOI
10.1109/77.621741

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We have designed and measured single flux quantum (SFQ) circuits for data switching at 2.5 Gbps. A fully functional 2/spl times/2 switching node is presented. The node can be used to form a larger switching fabric (4/spl times/4, 8/spl times/8, etc.). The node accepts data streams at either or both inputs, routing the data based on address bits present in the data stream. This approach allows the data to be self-routing and the switch to be self-synchronous. In addition, the switch can route data correctly on an active input line if the other is idle.
A crossbar switch has been designed using Single Flux Quantum (SFQ) gates exclusively for all internal functions. A 4/spl times/4 prototype has been fabricated in our Nb process foundry with J/sub c/ of 2000 A/cm/sup 2/. We report on the design and performance of the switch and of an individual crosspoint element at high data rates (/spl ges/1 Gbps). A novel design of a double-edge-triggered dc/SFQ converter is discussed. The requirements for output amplification and on-chip versus off-chip amplifier issues will be presented.
We have been developing a single-flux-quantum (SFQ) cross-bar switch, which is a main component of a network packet switch. We think that a network switch is an application in which the high speed of SFQ technology would be advantageous. Anticipating general and large-scale SFQ logic circuit design, we used the cell-based design method and the CONNECT standard SFQ cell library. The two-input and two-output cross-bar switch, a core switch component, consists of 13 logic cells connected by Josephson-transmission-line (JTL) cells. Because of the large size of JTL cells and the large delay in them, timing adjustment becomes more difficult as the operating speed and circuit size increase. After using a commercially available automatic router to find appropriate routes efficiently, we used a static timing analyzer for fine timing adjustment. Timing violations were fixed by changing JTL path delays using the tools we developed. The target operating frequency of the switch was 40 GHz, which corresponds to a clock period of 25 ps. Careful timing adjustment was necessary to ensure correct operations at such a high speed. The test chip was fabricated by using an NEC standard Nb process. The circuit, including on-chip test circuitry, was composed of about 1500 Josephson junctions. We confirmed its correct operations up to 50 GHz with a bias margin of /spl plusmn/20%.
A new SFQ data communication switch has been designed and tested. Complete operation of a 4/spl times/4 switch circuit has been demonstrated with address decoding at low-speed. Transmission through a given path in the switch has been demonstrated for data rates up to 4 Gb/s. Circuit simulations show operation of the switch cells up to 30 Gb/s. The circuit was fabricated using HYPRES's standard 1 kA/cm/sup 2/ niobium process. The switch has a crossbar architecture with an RF SQUID based switch cell at each crosspoint. The address is decoded by means of RSFQ shift registers which are integrated into the switch matrix. The design enables high bit-rate, low crosstalk, non-blocking architecture, NRZ or RZ data format, and self routing of variable length data packets.
We evaluate the performance of a single flux quantum (SFQ) circuit and discuss its possible system-level application. First, we describe an optical-electrical-SFQ hybrid router system that is divided into three functions: an SFQ packet-routing function, a control function, and an optical transmission function. For the packet-switching core, we have developed a packet-switching architecture based on an internal speedup time-shifted Batcher Banyan approach. Our simulated results show that this architecture should provide good scalability and a low packet-loss rate. Additionally, we have fabricated an asynchronous pulse arbitration circuit. This circuit is a key component in our packet-switching system. The circuit synchronizes two asynchronous incoming signals without any global clock signals. It consists of a clock generator based on a ring oscillator and data latches. By measuring the average output voltage of the Josephson junction, we confirmed the output signal control by the clock at over 60 Gbps throughput.
We demonstrated the 4 /spl times/ 4-cross/bar-40 Gbps/ch switch which is functionally compatible with a commercial semiconductor one in order to evaluate the current SFQ circuit technology quantitatively in both a operating speed and power consumption. According to our numerical simulations, the SFQ switch has one or two orders of magnitude higher throughput per a single channel than that of the semiconductor, and has much smaller exchanging times between routing paths. In an actual test based on the NEC Nb-2.5 kA/cm/sup 2/ junction technology, correct operation using an on-chip test system made up of 4700 Josephson junctions was confirmed up to 37 Gbps/ch which is about 14 times faster than the semiconductor of 2.7 Gbps/ch speed. We also confirmed that SFQ circuits consumed 1/800 power compared with those of semiconductor. The reason why data rates obtained experimentally did not reach the designed speed of 40 Gbps/ch is due to both the timing error and the influence of magnetic fields caused by large bias currents exceeding 500 mA. These results of quantitative evaluation show that the SFQ technology has much greater potentials than the semiconductor one in both the operating speed and the power consumption, which means it is very effective to apply the SFQ technology to digital applications.