Innovative practices session 7C: Self-calibration & trimming

Critical Path Monitors (CPM) are a way of modeling the frequency response of a microprocessor to voltage, environment, workload, and other operating point changes. When coupled with a frequency controller, the CPM gives the microprocessor the ability to adjust its frequency to match the current operating environment. This allows for more efficient designs since voltage and frequency margins required to compensate for voltage droops, di/dt events, temperature changes, and other noise events are no longer needed. Calibration is key to functional Critical Path Monitors. Calibration compensates for process variation and pulls the CPM in-line with the hardware it is controlling. In this talk the CPM, frequency control loop, and calibration methodology of the Power7+ microprocessor is described. The CPM models frequency response well enough, after calibration, to allow for a 22% margin reduction. Our measurements demonstrate the value of the CPM for modeling frequency response that can be applied to DVFS microprocessors with the potential to reduce development and test times and to make systems more resilient.