Case study in RSFQ design: Fast pipelined parallel adder

We present a design for parallel pipelined carry-lookahead Kogge-Stone 32- and 64-bit integer adders with the traditional concurrent flow timing scheme, and the results of its gate-level logical simulation using a VHDL model, with parameters reduced from the physical-level simulation of RSFQ cells. The design uses only five different types of bit processing blocks and is easily scalable to any length of the operands. The multi-pulse logic representation together with interchanging logical polarity between pipeline stages is used to simplify the design of the blocks, which contain only two types of clocked RSFQ gates: an inverter and a D-flip-flop. Simulations show that in the absence of thermal fluctuations and random parameter spread show that the clock frequency of the adder implemented in the projected 0.8 mu m Nb-trilayer technology could be as high as 150 GHz. However, an approximate account of these factors shows that in order to achieve a 99% adder fabrication yield and a 10(-25) adder error rate the maximal frequency should be reduced to 60 GHz for 1.5% Josephson junction spread and to 52 GHz for 3% spread. Adder latency is close to 260ps for 32 bits and 320ps for 64 bits. We plan to re-design the adders to increase their speed.