Fundamental concepts of power and energy measurement with the computer-aided-design tools

Computer-aided design (CAD) tools are frequently employed to verify the design objectives before the fabrication of an integrated circuit. Circuit simulators typically provide built-in functions to measure the power consumption. However, the accuracy of the measured power is mostly overlooked since the approximations and the methodology used by the existing built-in power estimation tools are not well documented. A blind-trust in the CAD tools may lead to gross errors in the power estimation. A generic methodology to accurately measure the power and energy consumption with circuit simulators is described in this paper. An equation to calculate the device power consumption based on the different current conduction paths in a MOSFET is presented. An expression for the total power consumption of a complex circuit is also derived by explicitly considering the different circuit terminals including the inputs, the outputs, and the body-contacts. Results indicate that the built-in functions of commercial circuit simulators can produce significant power measurement errors in a 65nm CMOS technology. For deeply scaled nano-CMOS circuits producing significant leakage, a conscious power measurement with the proposed explicit methodology is recommended for an accurate pre-fabrication circuit characterization.