Temperature-Insensitive Dual- $V_{\rm th}$ Synthesis for Nanometer CMOS Technologies Under Inverse Temperature Dependence

With the scaling of CMOS technologies, the gap between nominal supply voltage and threshold voltage has decreased significantly. This trend is further amplified in low-power nanometer libraries, which feature cells with identical size and functionality, but different threshold voltages. As a consequence, different cells may have different delay behaviors as the temperature varies within a circuit. For instance, cells with low-threshold devices may experience an increase in delay when temperature increases, whereas cells using high-threshold devices may experience the opposite behavior. The latter effect, also known as inverse temperature dependence (ITD), poses new challenges to circuit designers. Besides making timing analysis more difficult, ITD has important and unforeseeable consequences for power-aware logic synthesis. This paper describes the impact that ITD may have on the design of nanometer circuits. We also provide a threshold voltage assignment algorithm for dual threshold voltage synthesis, which guarantees temperature-insensitive operation of the circuits, together with a significant reduction of both leakage and total power consumption. Experiments performed on a set of standard benchmarks show timing compliance at any operating temperature, and an average leakage reduction around 28% compared to circuits synthesized with a standard synthesis flow that does not take ITD into account. We also apply our proposed synthesis algorithm to a realistic case study consisting of a 32-bit, IEEE-754 floating point unit.