Dual-threshold design of sub-threshold circuits

Dual threshold voltage (V_th) design is a common method for reducing leakage power in above-threshold circuits. This research shows that it is also effective in reducing energy per cycle of sub-threshold circuits. We first study the single-V_th design theoretically and by simulations, and find that the energy per cycle is independent of threshold voltage. However, in a dual-V_th design, the energy per cycle depends on both threshold voltage and supply voltage. We propose a framework to further reduce energy per cycle below what is possible with a single V_th. Given a nominal value for V_th, we determine an optimal supply voltage V_dd and an optimal higher V_th. Application to a 32-bit ripple carry adder shows energy saving of 29% over the single-V_th lowest energy.