Reducing clock skew variability via cross links

Abstract

Increasingly significant variational effects present a great challenge for delivering desired clock skew reliably. Nontree clock network has been recognized as a promising approach to overcome the variation problem. Existing non-tree clock routing methods are restricted to a few simple or regular structures, and often consume excessive amount of wirelength. In this paper, we suggest to construct a low cost nontree clock network by inserting cross links in a given clock tree. The effects of the link insertion on clock skew variability are analyzed. Based on the analysis, we propose two link insertion schemes that can quickly convert a clock tree to a non-tree with significantly lower skew variability and very limited wirelength increase. In these schemes, the complicated non-tree delay computation is circumvented. Further, they can be applied to the recently popular non-zero skew routing easily. Experimental results on benchmark circuits show that this approach can achieve significant skew variability reduction with less than 2% increase of wirelength.

Fast Incremental Link Insertion in Clock Networks for Skew Variability Reduction

“…According to [1], if a link is inserted between nodes u and w, the skew between u and w after link insertion is given by:q…”

“…The first term is the value of the original skew scaled by α. It has been proved in [1] that the value of α is always less than 1. The second term represents the extra skew introduced by the value of link capacitance.…”

“…The unwanted skew in the clock network is becoming a significant portion of the total clock time period, thereby limiting the speed and performance of the chip. The link-based CDNs proposed in [1][2][3] is a promising approach to reduce the skew variability in which cross-links are inserted in a given clock tree at appropriate locations. The cross links increase the correlation between the different sink delays which typically results in reduced skew variability.…”

“…The link selection schemes of [1][2][3] select the set of links to be inserted in a single step and then add all the links at the same time. Hence, we will refer to all these methods as one-step link insertion methods for the rest of this paper.…”

“…Recent works on link based non-tree CDN [1][2][3][4] propose cross-link insertion in a given clock tree to reduce skew variation. However, the current methods suffer from the drawback that they are empirical in nature, requiring the user to experiment with different parameter values.…”