SummaryAll IoT devices require a clock source for computation and communication. Crystal oscillators have been the standard clock source when an accurate, low power reference frequency is needed, but the growing IoT market demands improvements in five areas. These are reduced phase noise for higher data rate communication, faster startup time for duty-cycled systems, smaller physical size for further integration, improved environmental robustness, and resistance to tampering. Reference clocks built from MEMS resonators, including bulk acoustic wave (BAW) resonators, are uniquely suited to meet these requirements.