Evaluation and Reduction of Power Consumption in the Navigator Wearable Computer

The increasing density of integrated circuits has made it possible to design computers which are small and lightweight enough to be mobile. Mobile computers have constraints that their desktop predecessors do not, especially in the domains of size, weight, and power. The power consumed by a mobile computer is a key constraint, because the power determines the battery capacity required, which is a major factor in the minimum volume and weight of the system. Power management can be performed at several levels, from the transistor level in hardware up to the application level in software. Designers of fully custom mobile computers can build in power controls at each level, resulting in a wide range of options for power management. Designers of semicustom mobile computers, using both off-the-shelf and custom components, often have fewer power management options open to them, depending upon the number of power management controls available in the off-the-shelf components. The Navigator 1 wearable computer was a semicustom design. This paper discusses the evaluation of subsystem power consumption, describes possible modifications for reducing power consumption, and classifies the modifications along three axes. Four modifications were implemented, resulting in a factor of 2.9 increase in battery life while the system is idle. Results are also presented for a simple equation to predict battery life based upon user activity.