Fast simulation framework for subthreshold circuits

Subthreshold voltage operation, which can be considered an extreme case of voltage scaling, can greatly reduce the power consumption of circuits. This is beneficial in embedded applications that must run off of batteries and scavenged energy. Subthreshold operation has been proven to be very effective by several successful prototypes in the recent years, yet there is no fast and effective way for designers to estimate power and delay of a design operating in the subthreshold region. Traditional gate-level simulation tools are not set up to perform timing and power analysis in the subthreshold region and transistor level simulations are very time consuming due to the accuracy required to measure the very low levels of current. This paper presents a simulation framework that can accurately characterize a circuit from nominal voltage, all the way down into the subthreshold region. This framework uses the nominal frequency and power of a target circuit and a normalized ring oscillator curve to characterize the circuit at lower voltages. The contribution of this paper is a detailed analysis of this framework in the presence of a variety of design parameters such as bus lengths, transistor widths, etc. The simulation framework is extremely quick and accurate across a wide variety of circuits.