Sensitivity Based Link Insertion for Variation Tolerant Clock Network Synthesis

Clock distribution is one of the key limiting factors in any high speed, sub-100nm VLSI design. Unwanted clock skews, caused by variation effects like manufacturing variations, power-ground noise etc., consume increasing proportion of the clock cycle. Thus, reducing the clock skew variations is one of the most important objectives of any high-speed clock distribution methodology. Inserting cross-links in a given clock tree is one way to reduce unwanted clock skew variations. However, most of the existing methods use empirical methods and do not use delay/skew variation information to select the links to be inserted. This can result in ineffective links being inserted. The work of (Lam, et al., 2005) considers the delay variation directly, but it is very slow even for small clock trees. In this paper, the authors propose a fast link insertion algorithm that considers the delay variation information directly during link selection process. Our algorithm inserts links only in the parts of the clock tree that are most susceptible to variation effects by evaluating the skew sensitivity to variations. Another key feature of the algorithm is that it is compatible with any higher order delay model/variation model, unlike the existing algorithms. The authors verify the effectiveness of our algorithm using HSPICE based Monte Carlo simulations on a set of standard benchmarks