04/01/1993 09:00 AM
Computer Engineering
We have proposed an efficient measure to reduce the clock skew by assigning the clock network with variable branch widths. This measure has long been used for ``H\'\' clock tree. This paper formulates the optimal sizing of a general clock network as an optimization problem which minimizes the clock skew in a feasible set of widths. This feasible set of branch widths is decided by the process technology and routing resources. The skew minimization problem is turned into a least-squares estimation problem, and a modified Gauss-Marquardt\'s method is then used to determine the optimal widths of clock branches. This optimization method combines the best features of the methods based on Taylor series and methods based on gradients. An efficient algorithm is also proposed that assigns the good initial widths especially for a clock tree which let the later optimization process converge much more quickly. Our method is very flexible and can handle the general clock network including loops. The clock network can exhibit distributed RC and lossy transmission line behaviors. The method employs a scattering- parameters based delay macromodel to evaluate the timing of the clock network during the optimization process. The major objective of our sizing method is to minimize the skew, but as a by-product that the largest path delay is also reduced.
