Alphabetic trees-theory and applications in layout-driven logic synthesis

Abstract

Routing plays an important role in determining the total circuit area and circuit performance and hence must be addressed as early as possible during the design process. In this paper, an effective routing-driven approach for technology-dependent logic synthesis, which relies on alphabetic tree construction, is presented. Alphabetic trees are trees generated under the restriction that the initial order on the leaf nodes is maintained while not introducing any internal edge crossing. First, a mechanism for generating all alphabetic trees on a given number of leaf nodes is presented. Next, the number of such trees is calculated under different height and degree restriction and used to derive upper bounds on the complexity of alphabetic tree optimization problem. A classification of tree cost functions, for which alphabetic trees can be generated in polynomial time, is also proposed. Specifically, alphabetic tree optimization algorithms are applied to generate optimal alphabetic fan-out trees. For fan-out optimization, we obtained 14% improvement in chip area at the cost of 1% loss in performance.