Beyond Ideal DVFS Through Ultra-Fine Grain Vdd-Hopping

International audience ; DVFS is the de-facto standard for low energy Multi-Processor SoCs. It is based on the simple, yet efficient principle of lowering the supply voltage (Vdd) to the minimum threshold that satisfies the frequency constraint ($$f_{\mathrm {clk}}$$) required by the actual workload. An ideal-DVFS deals with the availability of on-chip high resolution voltage regulators that can deliver the supply voltage with a fine step resolution, a design option that is too costly.While previous research focused on alternative solutions that can achieve, or at least get close to, the efficiency of ideal-DVFS while using a discrete set of supply voltages, this work introduces Ultra-Fine Grain Vdd-Hopping (FINE-VH), a practical methodology that brings DVFS beyond its theoretical limit.FINE-VH leverages the working principle of Vdd-Hopping applied within-the-core by means of a layout-assisted, level-shifter free, dynamic dual-Vdd control strategy in which leakage currents are minimized through an optimal timing-driven poly-bias assignment procedure. We propose a dedicated back-end flow that guarantees design convergence with minimum area/delay overhead for a cutting-edge industrial Fully-Depleted SOI (FDSOI) CMOS technology at 28 nm.Experimental results demonstrate FINE-VH allows substantial power savings w.r.t. coarse-grain (i) ideal-DVFS, (ii) Vdd-Hopping, (iii) Vdd-Dithering, when applied on the design of a RISC-V architecture. A quantitative analysis provides an accurate assessment of both savings and overheads while exploring different design options and different voltage settings.