Clock skew minimization during FPGA placement

Unlike traditional ASIC technologies, the geometric structures of clock trees in a field-programmable gate array (FPGA) are usually fixed and cannot be changed for different circuit designs. Furthermore, the clock pins are connected to the clock trees via programmable switches. As a result, the load capacitances of a clock tree may be changed, depending on the utilization and distribution of logic modules in an FPGA. It is possible to minimize clock skew by carefully distributing the load capacitances or, equivalently, the logic modules used for the circuit design implementation. In this paper we present an algorithm for selecting logic modules used for circuit placement such that the clock skew is minimized. The algorithm can be applied to a variety of clock tree architectures, including those used in the major commercial FPGA's. The algorithm can also be extended to handle buffered clock trees and multiple clock trees Experimental results show that the algorithm can reduce clock skews significantly as compared with the traditional placement algorithms which do not consider clock skew minimization.