Power Evaluation of a Handheld Computer

The computing requirements of battery-powered systems are increasing faster than battery capacity. Because large, heavy batteries are unacceptable for handheld computers, low-power design and power management are crucial for these systems. One of the main obstacles limiting these techniques is the lack of a detailed analysis of system power under representative workloads. With the understanding provided by such an analysis, device designers can improve the power consumption of next-generation systems and optimally manage the fixed amount of energy provided by the battery. This article presents a comprehensive study of a handheld device’s power consumption. We measured the total power as well as the power dissipated by six subcircuits. Using this information and running the same benchmarks on different configurations of the same hardware, we divided power consumption into nine domains, such as the processor core, dynamic RAM (DRAM), liquid crystal display (LCD), speaker, and power supply. As the subject of our study, we used the Itsy pocket computer, developed at Compaq’s Western Research Laboratory. The Itsy project’s aim was to develop a flexible research platform for pocket computing. Itsy developers built several versions of the hardware, all based on the StrongARM SA-1100 processor. Our study concentrates on Itsy version 2.4, a complete handheld computer measuring 118 mm × 65 mm × 16 mm and weighing only 130 grams. Because one of the Itsy project’s goals was to support research in power evaluation and management, several features in the system make it an ideal platform for our study. Although it is easy to measure a system’s power with expensive, highly sophisticated instruments, moderate-cost instruments (accessible in any commercial or academic environment) usually suffice. For meaningful results, however, an understanding of the limitations of the chosen instruments and methodology is necessary. Our study devotes special attention to this issue. (See the “Related work” sidebar for a summary of other studies.)