Most activity monitors use an accelerometer and gyroscope sensors to characterize the wearer's physical activity. The monitor measures the motion by polling an accelerometer or gyroscope sensor or both every 20-30 ms and frequent polling affects the battery life of a wearable device. One of the key features of a commercial daily-activity monitoring device is longer battery life so that the user can keep track of his or her activity for a week or so without recharging the battery of the monitoring device. Many low-power approaches for a step-counting system use either a polling-based algorithm or an interrupt-based algorithm. In this paper, we propose a novel approach that uses the tap interrupt of an accelerometer to count steps while consuming low power. We compared the accuracy of step counting and measured system-level power consumption to a periodic sensor-reading algorithm. Our tap interrupt approach shows a battery lifetime that is 175% longer than that of a 30 ms polling method without gyroscope. The battery lifetime can be extended up to 863% with a gyroscope by putting both the processor and the gyroscope into sleep state during the majority of operation time
