Thermal sensing is one of the most commonly desired features in IoT devices to monitor either environmental or system/chip conditions. An accurate temperature sensor usually requires carefully calibrated, high-accuracy ADCs, which prevents their use in ultra-low-power sensor nodes. In some sensing architectures, a highly accurate timing reference can replace the ADC . With this in mind, we observe that almost all IoT systems incorporate a high-accuracy timing source (real-time clock, RTC) for time synchronization, data recording, and radio communication. Typically, these RTCs employ crystal or MEMS-based oscillators, or RC oscillators in extremely small form factor devices. We therefore propose to build a temperature sensor that uses a system’s core RTC as a timing reference to minimize power/area overhead. We also show a fully integrated sensor that includes an RC-based timing circuit for systems that might not have a timing source.