Multi-dimensional exploration of the Minimum Energy Point for RISC-V subsystem in 22 nm FDSOI

Battery operated devices are highly constraint in energy budget, there is a need for energy-efficient designs. Designing circuits to operate in the Minimum Energy Point (MEP) yields great energy savings, by at least 20%, by reducing supply voltage and operating frequency. The MEP is in the near or sub-threshold region, such that the circuit becomes very sensitive to PVT variations. This thesis proposes a methodology to study and locate the MEP and its relationship with supply voltage, threshold voltage, channel length, body biasing, temperature and process variations, for a RISC-V Ibex CPU synthesized with standard cell libraries characterized for higher voltages with a commercially available 22 nm FDSOI technology. A critical path replica is converted into a ring oscillator for running SPICE simulations. The simulations are used to estimate the MEP of the CPU. Results show that HVT and UHVT libraries are unfit for operating at the MEP due to a huge gap between slow and fast corners. It is shown that SLVT provides the best timing closure. The selection of threshold voltages becomes the most important parameter, adjustments can be made later by selecting a proper channel length and body-biasing voltages.