A system includes a frequency-locked loop (FLL) circuit, a sensor-hub circuit and a processor. The FLL circuit is used to generate a low-frequency clock. The sensor-hub circuit is coupled to a number of sensors and is configured to periodically poll the sensors during polling periods and to detect sensor activities. The processor is coupled to the sensor-hub circuit and can process sensor signals from one or more active sensors. The processor is off during polling periods and is turned on when a sensor activity is detected. The polling periods are based on the low-frequency clock generated by the FLL circuit.