A Novel Clock Tree Aware Placement Methodology for Single Flux Quantum (SFQ) Logic Circuits

Abstract

In a single-flux-quantum (SFQ) circuit, almost all cells need to receive the clock signal which incurs a high clock routing overhead. Besides, the clock tree of an SFQ circuit requires the insertion of a clock splitter cell at every tree branching point which renders the conventional design flow of placement followed by clock tree synthesis ineffective to obtain a high quality clock tree with low clock skew. To address these issues, we propose a two-stage global placement methodology and a placement refinement algorithm after placement legalization. Our two-stage global placement methodology first applies a conventional global placement algorithm to place the cells in the given SFQ circuit evenly, which is followed by clock tree synthesis and clock splitter insertion, and then performs a second stage of global placement to re-place both the original cells and clock splitters at the same time. In the second global placement stage, the look-ahead legalization technique is used to spread out the original cells and the clock splitters, and the clock tree is re-synthesized several times to obtain an optimized clock tree topology such that there are little overlaps of the clock splitters with the original circuit cells. In addition, the total wirelength of data signals and clock signal is optimized concurrently. After legalizing the placement of all cells, our placement refinement method can be run to further reduce the clock skew. Compared with the previous state-of-the-art work, on average we can reduce the total half-perimeter wirelength and clock skew by 9% and 31%. respectively.