Look-up Table Design for Deep Sub-threshold through Full-Supply Operation

Abstract

Field programmable gate arrays (FPGAs) are the implementation platform of choice when it comes to design flexibility. However, the high power consumption of FPGAs (which arises due to their flexible structure), make them less appealing for extreme low power applications. In this paper, we present a design of an FPGA look-up table (LUT), with the goal of seamless operation over a wide band of supply voltages. The same LUT design has the ability to operate at sub-threshold voltage when low power is required, and at higher voltages whenever faster performance is required. The results show that operating the LUT in sub-threshold mode yields a (~80×) lower power and a (~4×) lower energy than full supply voltage operation, for a 6-input LUT implemented in a 22nm predictive technology. The key drawback of sub-threshold operation is its susceptibility to process, temperature, and supply voltage (PVT) variations. This paper also presents the design and experimental results for a closed-loop adaptive body biasing mechanism to dynamically cancel global (spacial) as well as local (random) PVT variations. For the same 22nm technology, we demonstrate that the closed-loop adaptive body biasing circuits can allow the FPGA LUT to operate over an operating frequency range that spans an order of magnitude (40 MHz to 1300 MHz). We also show that the closed-loop adaptive body biasing circuits can cancel delay variations due to supply voltage changes, and reduce the effect of process variations on setup and hold times by 1.8X and 2.9× respectively. The dynamic body biasing circuits incur a 3.49% area overhead when designed to each drive a cluster of 25 LUTs.