Designing Tunable Subthreshold Logic Circuits Using Adaptive Feedback Equalization

Ultralow-power subthreshold logic circuits are becoming prominent in embedded applications with limited energy budgets. Minimum energy consumption of digital logic circuits can be obtained by operating in the subthreshold regime. However, in this regime process variations can result in up to an order of magnitude variations in ION/IOFF ratios leading to timing errors, which can have a destructive effect on the functionality of the subthreshold circuits. These timing errors become more frequent in scaled technology nodes where process variations are highly prevalent. Therefore, mechanisms to mitigate these timing errors while minimizing the energy consumption are required. In this paper, we propose a tunable adaptive feedback equalizer circuit that can be used with a sequential digital logic to mitigate the process variation effects and reduce the dominant leakage energy component in the subthreshold digital logic circuits. We also present detailed energyperformance models of the adaptive feedback equalizer circuit. As part of the modeling approach, we also develop an analytical methodology to estimate the equivalent resistance of MOSFET devices in subthreshold regime. For a 64-bit adder designed in 130 nm, our proposed approach can reduce the normalized variation of the critical path delay from 16.1% to 11.4% while reducing the energy-delay product by 25.83% at minimum energy supply voltage.