A Temperature-Compensated BLE Beacon and 802.15.4-to-BLE Translator on a Crystal-Free Mote

Crystal-free radios have the potential to revolutionize the IoT: due to their single-chip nature, they are both very cheap (no external components required) and very small (the size of a grain of rice). The Single-Chip Micro Mote ( $\text{SC}\mu \mathrm{M}$ ) is a $2\times 3\times 0.3\ \text{mm}^{3}$ crystal-free chip that can communicate with off-the-shelf transceivers over Bluetooth Low Energy (BLE) or IEEE 802.15.4. Setting its communication frequency is challenging because the crystal-free chip can rely only on internal oscillating circuits, which are very susceptible to temperature. Without compensation, a $\text{SC}\mu \mathrm{M}$ chip can no longer communicate with an off-the-shelf BLE receiver if the temperature changes by more than 1.25 °C. This paper introduces a two-step temperature compensation method, allowing $\text{SC}\mu\mathrm{M}$ to successfully send BLE frames over a 20 °C temperature range. After performing initial calibration during optical bootloading, we use an open-loop linear model to estimate the ambient temperature and continuously tune the mote's local oscillator (LO) frequency as the temperature changes. We show how the mote can use the intermediate frequency of 802.15.4 frames it receives from nearby off-the-shelf transceivers as a frequency reference to adjust its LO frequency. This compensation method enables $\text{SC}\mu \mathrm{M}$ to operate as a tiny BLE beacon, a BLE temperature sensor (for retail or medical applications), or a 802.15.4-to-BLE translation device.