8-Bit Asynchronous Sparse-Tree Superconductor RSFQ Arithmetic-Logic Unit With a Rich Set of Operations

This paper describes the design and testing of an 8-bit asynchronous wave-pipelined sparse-tree Rapid Single Flux Quantum (RSFQ) Arithmetic Logic Unit (ALU). Compared to previously developed RSFQ ALUs, this unit features an extensive set of 8 arithmetic and 12 logical operations. The execution of ALU operations consists of two steps. First, when necessary, one or both operands are inverted, and then operations are performed on these pre-processed data. Unlike the RSFQ Kogge-Stone-based designs, our parallel-prefix sparse-tree ALU has significantly reduced circuit complexity while maintaining robust operational margins at high frequency. An 8-bit ALU has been implemented with the International Superconductivity Technology Center (ISTEC) 10 kA/cm² 1.0 μm 9-metal ADP2.1 fabrication process as a joint effort between Stony Brook University, Yokohama National University, and Nagoya University. Using the CONNECT cell library and SFQ CAD tools developed at Nagoya and Yokohama, the Stony Brook team has developed a complete logical and physical design of the ALU chip. The 8-bit ALU core (without SFQ-to-dc and dc-to-SFQ converters) consists of 8832 Josephson junctions with an area of 7.2 mm². Simulations show that the ALU can operate at the maximum rate of 42 GHz. It has the latency of 374 ps at a bias voltage of 2.5 mV. The chip was fabricated and tested at low frequency in 2012. Testing results showed malfunctioning of some gates but despite these shortcomings we still verified several ALU operations with the measured DC bias voltage margins of ±1.8%.