PBA Based Optimization for Slew Propagation

Abstract

 When we move towards the nanometer designs, the interconnections between the semiconductor devices. At this level the performance of the design is affected by the metal interconnects due to induced noise. So, in nanometer designs all these affects should also be considered. Propagation of slew in design is an important aspect while performing the static timing analysis (STA) of a design. Slew has a direct impact on delay of a timing path and could make the design pass or fail the timing closure. In conflicts of slew propagation, there are two approaches to move forward. The first approach is graph-based static timing analysis. In a graph-based approach, the worst case delays are taken into consideration by taking into the account the worst case slews (slow slews) along the timing paths, for setup analysis. While in case of hold analysis, the best case delays are taken into consideration by taking into the account the best case slews (fast slews) along the timing paths. The second approach is path-based static timing analysis. In this approach, the actual delays are taken into consideration by taking into account the actual slews along the timing paths, for setup as well as hold analysis. In path-based static timing analysis, delay is computed of the timing path so as to obtain the actual delay values. Such delay calculation takes some extra amount of time. The propagation of slew at various slew merging points in a design have been observed with the help of timing reports. When a path based approach is applied on a design, a significant improvement in the TNS (Total Negative Slack) and WNS (Worst Negative Slack) could be seen in comparison to the graph based approach. When such analysis is done on various designs, a significant improvement was observed. With this path based approach, the area required to implement the design reduced significantly. This is obtained by obtaining the number of cells added during optimization, which comes out to be less in the case of path based approach. Thus, path based approach not only reduce the TNS and WNS, but also helps in congestion reduction in a design by decreasing the area used for implementation of design.