Optimizing Phase-Scheduling with Throughput Trade-offs in AQFP Digital Circuits

Abstract: Adiabatic Quantum-Flux-Parametron (AQFP) logic is a promising emerging superconducting technology for ultra-low power digital circuits, offering orders of magnitude lower power consumption than CMOS. However, AQFP scalability is challenged by excessive buffer overhead due to path balancing technology constraints. Addressing this, recent AQFP works have proposed design solutions to reduce path balancing overhead using phase-skipping and phase-alignment. Phase-skipping is a circuit-level technique that allows data transfer between AQFP gates clocked with non-consecutive clock phases. In contrast, phase-alignment is an architectural approach involving repeating input patterns to allow data transfer between AQFP gates across multiples of full clock cycles. While both techniques individually mitigate the area overhead of path-balancing, they have not yet been jointly explored. In this work, we present the first clock phase scheduling algorithm that combines phase-skipping and phase-alignment. We first present a minimum area method that on average, achieves a 25% area reduction compared to phase-skipping alone and a 11% reduction compared to phase-alignment. We then extend the method to enforce a target throughput, enabling efficient area-performance trade-offs. With our throughput constrained optimization, we achieve on average 6.8% area savings with a 2.62x increased throughput compared to the state-of-the-art phase-aligned method.