Adaptive Voltage Frequency Scaling Using Critical Path Accumulator Implemented in 28nm CPU

A high bandwidth critical path accumulator (1 sample/4GHz) capable of providing accurate timing margin information is reported. We present an adaptive voltage mechanism using these critical path accumulators that improves upon existing approaches by: (1) enabling replica paths to function as a statistical sample of the full set of Fmax limiting paths resulting in improved tracking, and (2) explicit disambiguation of the voltage impact on delay from the intrinsic circuit speed by coupling path margin assessments with a voltage reading from the integrated power supply monitors. This scheme is implemented in 28nm leading generation CPU core and is shown to track Fmax accurately with a standard deviation <;2% (~1 FO2 delay) across a large range of process, voltage and temperature. Core power is reduced to the tune of 7-20% (at same performance) across various processor states.