The Single Chip Micro Mote (SCµM) is a crystalfree single-chip mote that brings us one step closer to the Smart Dust vision, in particular as it can communicate with off-theshelf IEEE802.15.4 and Bluetooth Low Energy devices. However, before it can be part of such networks, the crystal-free SCµM chip needs to be able to accurately tune its communication frequency to synchronize to the network. This is a challenge since its on-board RC and LC-based resonating circuits have a drift rate that can be 3 orders of magnitude worse than crystalbased oscillators typically used in today's radios. This article introduces QuickCal, a solution that allows a SCµM chip to selfcalibrate against off-the-shelf devices dedicated to assisting with its calibration. We show that a SCµM chip can self-calibrate against this QuickCal Box in fewer than 3 min. We further validate that, once it has self-calibrated, a SCµM chip can reliably communicate with off-the-shelf IEEE802.15.4 devices. Finally, we demonstrate a heterogeneous network -composed of a SCµM chip and an OpenMote device -implementing a full 6TiSCH Industrial IoT protocol stack, which uses time synchronization and channel hopping. This is the first time that a crystalfree radio is participating in a channel-hopping enabled TSCH network.
SCµM is a 2×3×0.3 mm 3 system-on-chip that contains an ARM Cortex-M0 and a 2.4 GHz IEEE802.15.4 radio. This paper describes the two-step calibration routine needed to run a full 6TiSCH stack on SCµM. It is, to the best of our knowledge, the first time a fully standards-compliant protocol stack runs on a crystal-free radio, such that it can participate in a network with off-the-shelf radios.
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 (SCµM) is a 2×3×0.3 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 SCµM 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 SCµM 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 SCµM 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.
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