Construction of an RSFQ 4-bit ALU with half adder cells

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 × 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/cm² Nb process. The size of the ALU was 3.0 mm × 1.5 mm, fitting in a 5 mm × 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.