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IEEE Transactions on Applied Superconductivity

A Layout Design Flow for RSFQ Circuits Based on Cell Clustering and Mixed Wiring of JTLs and PTLs

Takashi Dejima, Kazuyoshi Takagi, Naofumi Takagi

Published Aug 7, 2020• 2 citations
Publisher

Abstract

An automated layout design flow for rapid single-flux-quantum circuits is proposed in this article. In order to realize small circuit area and low latency, both Josephson transmission lines (JTLs) and passive transmission lines (PTLs) are used for interconnects. Placement and routing are performed considering effective use of JTLs and PTLs. Logic cells are grouped into clusters, and JTLs are used inside clusters. PTLs are used among clusters. The placement process is composed of three steps: first, cell clustering, second, cell placement and JTL routing in each cluster, and third, cluster placement. After the placement process, routing among clusters are performed using PTLs. We applied the proposed design flow to sample circuits with several hundreds of gates. Though the circuit area is not fully optimized, the latency of the circuits designed by the proposed design flow are lower than those of the circuits designed manually.

DOI
10.1109/tasc.2020.3014928

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International Symposium on Signals, Circuits and Systems•Oct 4, 2006•2 citations
An automated layout design flow for rapid single-flux-quantum circuits is proposed in this article. In order to realize small circuit area and low latency, both Josephson transmission lines (JTLs) and passive transmission lines (PTLs) are used for interconnects. Placement and routing are performed considering effective use of JTLs and PTLs. Logic cells are grouped into clusters, and JTLs are used inside clusters. PTLs are used among clusters. The placement process is composed of three steps: first, cell clustering, second, cell placement and JTL routing in each cluster, and third, cluster placement. After the placement process, routing among clusters are performed using PTLs. We applied the proposed design flow to sample circuits with several hundreds of gates. Though the circuit area is not fully optimized, the latency of the circuits designed by the proposed design flow are lower than those of the circuits designed manually.
The introduction of passive-transmission-line (PTL) wiring to large-scale SFQ logic circuits offers several advantages over traditional Josephson-transmission-line (JTL) wiring, such as smaller wiring area, less delay, lower power consumption, and more freedom in routing. PTL wiring can drastically change the style of designing SFQ logic circuits. We propose a new methodology for designing large-scale SFQ circuits using PTL wiring. Aiming at generalizing SFQ circuit design, we chose synchronous clocking. We developed a placer that was suitable for SFQ circuit design to assist us with the new design methodology. The placer first synthesizes an H-tree clock distribution network, placing SFQ pulse splitter cells at each branch. It then places logic cells to minimize the maximum length of data PTL wires, aiming at higher operating speeds. Finally, a router completes the PTL wiring. To evaluate the design methodology we designed an 8-bit general purpose RISC processor. Within twelve hours, we obtained a 15-mm-square SFQ circuit that could operate up to 27.6 GHz. We adopted an advanced Nb process, which was characterized by a critical current density of 10 kA/cm2, two 5-mum-wide PTL wiring layers, and a minimum cell size of 30 mum times 30 mum. The circuit consisted of about 20,000 logic cells, which approximated more than 400,000 Josephson junctions including splitters to distribute the clock signal.
The increasing complexity of modern rapid single flux quantum (RSFQ) circuits has made on-chip signal routing, an issue of growing importance. In this paper, several methods for routing large-scale RSFQ circuits are described, and a process is presented for determining when to use passive microstrip transmission lines (PTL) and active Josephson transmission lines (JTL). The effect of the size of the JTL inductor and Josephson junctions on the length of a JTL chain for a target delay is also discussed. The dependence of the JTL inductance on the physical layout is evaluated, and the effects of the primary PTL parameters on delay are characterized. A novel PTL driver and receiver configuration is also proposed. Tradeoffs among the number of JJs, inductance, and length of a PTL stripline in the receiver and driver circuits are reported. The energy dissipation is evaluated for two different interconnects. A tradeoff between the proposed PTL circuits and an optimized JTL in terms of energy dissipation and delay is discussed. Guidelines for choosing the optimal element values are determined, and a simulated bias margin of ±29% for the bias current of the proposed receiver operating at 20 GHz in a 10 kA/cm2technology for a 1-mm transmission line is achieved. Summarizing, guidelines and design tradeoffs appropriate for automated layout and synthesis are provided for driving long interconnect in SFQ VLSI circuits.
In single-flux-quantum (SFQ) circuits, the delay produced by the Josephson transmission line (JTL) is comparable with or larger than the one produced by the logic cells. Therefore, it is difficult to find the routes that satisfy the timing constraints in a large circuit manually. To overcome this obstacle, we propose a two-step automatic JTL routing technology that performs coarse and fine timing adjustments. First, an automatic router draws dummy wires within coarse timing constraints, and then the dummy wires are replaced with JTL cells. Fine timing adjustments are done in the latter step. Some JTL cells in clock and data paths are replaced with faster JTL cells so that clock signals always arrive earlier than data signals at clocked-gates. Two example circuits were designed using the automatic JTL routing technology. One is composed of nearly 600 Josephson junctions. It was experimentally tested up to 35 GHz with on-chip test components. The other is composed of about 4000 Josephson junctions. After the fine timing adjustment, logic simulation showed that it can operate at 20 GHz. We also experimentally confirmed its correct operations at low speed.
We have developed a method of designing single-flux-quantum (SFQ) logic circuits with passive gate-to-gate interconnections. Based on our method, we designed a 2/spl times/2 switch in which all the interconnections are implemented with passive transmission lines (PTLs) while short Josephson transmission line (JTL) segments are used only to adjust the signal timings. Compared with an identical switch using JTL interconnections, the switch using PTL interconnections has 45% fewer wiring junctions and requires 48% less wiring power current. The switch operated at 40 GHz with a bias margin of /spl plusmn/9.5%.
We developed a design approach for interface circuits to connect Single Flux Quantum (SFQ) cells by using passive transmission lines (PTL's). In the approach, an interface circuit between a PTL and JTL is optimized to obtain a standard interface circuit, and then, modifications are made to previously designed SFQ cells and the standard interface circuit to connect the SFQ cells by using PTL's. The key point is the use of approximately the same interface circuit with every SFQ cell to maintain the matching condition between the interface circuit and the PTL's. Based on this approach, we designed an interface circuit and a test circuit composed of two D-flip-flops connected using 2-mm-long PTL's via the interface circuits. The impedance of the PTL was 2 /spl Omega/. We achieved high-speed operation of the test circuit up to 35 GHz with a bias margin of -15/+30%.
An important precondition for the successful realization of any high-speed complex electronic device are the efficient (in terms of speed and signal degradation) long-distance interconnects. The two techniques for long-distance on-chip signal propagation, used by the modern rapid single flux quantum (RSFQ) designers - via Josephson transmission lines (JTLs) and via passive transmission lines (PTLs) - are investigated here. Novel design techniques are presented, which significantly improve the JTLs and PTLs performance as long-distance SFQ pulse transmitters within the digital RSFQ circuits.