Dynamic Supply and Threshold Voltage Scaling towards Runtime Energy Optimization over a Wide Operating Performance Region

This paper proposes a runtime voltage-scaling method that optimizes the supply voltage (Vdd) and the threshold voltage (Vth) under a given delay constraint. This paper refers to the optimal voltage pair as a Minimum Energy Point (MEP). This paper firstly proposes a closed-form continuous function that determines the MEP over a wide operating performance region ranging from the above-threshold region down to the subthreshold region. The MEP dynamically fluctuates depending on the operating condition determined by a given delay constraint, an activity factor and a circuit temperature. In order to track the MEP, this paper proposes a voltage scaling technique enabling to set Vdd and Vth to near the MEP without iteratively tuning the voltages based on the proposed function. Existing MEP tracking techniques iteratively tune Vdd, which may not be suitable in terms of (1) the hardware design cost for generating a number of VddS and (2) the MEP tracking time. Measurement results based on a 32-bit RISC processor fabricated in a 65-nm process technology shows that the proposed method estimates the MEP within a 5% energy error in comparison with the actual MEP operation.