Clock skew verification in the presence of IR-drop in the power distribution network

Clocks are perhaps the most important circuits in high-speed digital systems. The design of clock circuitry and the quality of clock signals directly impact the performance of a very large scale integrated chip. Clock skew verification requires high accuracy and is typically performed using circuit simulators. However in high-performance deep-submicrometer digital circuits, clocks are running at higher frequencies and are driving more gates than ever, thus presenting a higher current load on the power distribution network with the potential for substantial power grid voltage (IR)-drop. This IR-drop affects the clock timing and must be taken into account in the verification process. Since IR-drop is a full-chip phenomenon, the use of standard circuit simulation on both the clock circuitry and the power-grid is not practical. In this paper, we present a new methodology for the verification of clock delay and skew. An iterative technique is presented for clock simulation in the presence of full-chip dynamic IR-drop. The effect of IR-drop on the timing of clock signals is quantified on a small example, and demonstrated on a large chip.