Sensitivity Based Link Insertion for Variation Tolerant Clock Network Synthesis

Joon-Sung Yang1,  Anand Rajaram2,  Ningyu Shi1,  Jian Chen1,  David Z. Pan1
1University of Texas at Austin, 2University of Texas at Austin/Texas Instruments


Abstract: Clock distribution is one of the most important and 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 [16]. However, most of the existing methods like [15] 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 [6] considers the delay variation directly, but it is very slow even for small clock trees. In this paper, we 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 is most susceptible to variation effects by evaluating the skew sensitivity to variations. Another key feature of our algorithm is that it is compatible with any higher order delay model/variation model, unlike the existing algorithms. We verify the effectiveness of our algorithm using HSPICE based Monte Carlo simulations on a set of standard benchmarks. Experimental results show that our algorithm is very fast and achieves better skew variability reduction while utilizing considerably lesser routing resources compared with existing methods.