Energy-speed exploration for very-wide range of dynamic V-F scaling

Minimum-energy operation of digital CMOS circuits is commonly associated to the sub-VT regime, carrying huge performance and variability penalties. This paper shows that it is possible to achieve 8x higher energy-efficiency with a very-wide range of dynamic voltage-frequency scaling, from nominal voltages down to the lower boundary of near-VT operation. The cell-library is exercised in a 65nm commercial PDK and targets near-VT operation, mitigating the variability effects without compromising the design in terms of area and energy at strong inversion. The set of cells allows a maximum of 2-stacked transistors, and includes master-slave registers. We report results for medium complexity designs which include a 25kgates notch filter, a 20kgates 8051 compatible core, and 4-combinational/4-sequential ISCAS benchmark circuits. In this work the maximum frequency attainable at each supply for a wide variation of voltage is studied from 150mV up to nominal voltage (1.2V). The sub-VT operation is shown to hold the minimum energy-point at roughly 0.29V, which represents a 2x energy-saving compared to the near-VT regime. Although energy-efficiency peaks in sub-VT for the circuits studied, we also show that in this ultra-low Vdd the circuit timing and power suffer from substantially increased variability impact and a 30x performance drawback, with respect to near-VT.