The autonomous, energy-harvesting devices need to maintain tasks with power consumption as low as possible. Scaling down the digital supply voltage into subthreshold range is one of the effective schemes. However, the signal integrity, depending on hard-to-predict inter- and intra-die process variations, restricts the voltage scalability. The minimum supply voltage varies with processes and environments, and the predefined minimum operating voltage may cause functional errors. This work therefore proposes an adaptive formulation of the minimum supply voltage. Compared with previous works, the proposed formulation enhances the effectiveness by considering noise margin and process variations. The formulation also leads to the low-overhead implementation, enabling the run-time calculation in the power-constrained devices.