Chapter 6 Dynamic Voltage Scaling

The XScale microprocessors [1] were intended as a follow-on to the StrongARM microprocessors [2] developed at Digital Equipment Corp. The XScale work began in 1998 to design a microprocessor that would be embedded in high-performance “tethered,” i.e., line-powered, as well as handheld (battery-powered) system-on-chip (SOC) ICs. The ability of the processor core to operate over a wide range of supply voltages (VDD) is key to achieving both high-performance and low power consumption across such a wide application range. Using the same microprocessor core in many, diversely targeted ICs, maximizes the core development return on investment. Dynamically scaling the power supply to different voltages (VDD) to fit the application that is presently running maximizes both overall performance vs. power and energy efficiency. It was thus deemed critical to the XScale effort. Such a capability had been suggested by [3] and had been a topic of university research [4] before the XScale processor development began. Around the same time, notebook computers introduced static voltage scaling schemes, e.g., “Speed-Step,” whereby the processor power is minimized when running on battery power by using a lower VDD and clock frequency, compared to operation when powered from a wall socket. As of 2007, it is a commonly available commercial capability, and the body of academic work investigating circuits and scheduling algorithms has become quite large. with the XScale Embedded Microprocessor