QuickCal: Assisted Calibration for Crystal-Free Micromotes

• Published in: IEEE internet of things journal Vol. 8; no. 3; pp. 1846 - 1858
• Main Authors: Chang, Tengfei, Watteyne, Thomas, Maksimovic, Filip, Wheeler, Brad, Burnett, David C., Yuan, Titan, Vilajosana, Xavier, Pister, Kristofer S. J.
• Format: Journal Article
• Language: English
• Published: Piscataway IEEE 01.02.2021; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: 6TiSCH; Bluetooth; Calibration; crystal-free; Crystals; Drift rate; Industrial applications; Internet of Things; Oscillators; Protocols; single-chip micro mote (SC<italic xmlns:ali="http://www.niso.org/schemas/ali/1.0/" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">μ M); single-chip mote; Synchronization; Time synchronization
• ISSN: 2327-4662; 2327-4662
• DOI: 10.1109/JIOT.2020.3015725

The single-chip micro mote (SC<inline-formula> <tex-math notation="LaTeX">\mu \text{M} </tex-math></inline-formula>) is a crystal-free single-chip mote that brings us one step closer to the Smart Dust vision, in particular, as it can communicate with off-the-shelf IEEE802.15.4 and Bluetooth low energy devices. However, before it can be part of such networks, the crystal-free SC<inline-formula> <tex-math notation="LaTeX">\mu \text{M} </tex-math></inline-formula> chip needs to be able to accurately tune its communication frequency to synchronize to the network. This is a challenge since its onboard RC and LC-based resonating circuits have a drift rate that can be three orders of magnitude worse than crystal-based oscillators typically used in today's radios. This article introduces QuickCal, a solution that allows an SC<inline-formula> <tex-math notation="LaTeX">\mu \text{M} </tex-math></inline-formula> chip to self-calibrate against off-the-shelf devices dedicated to assisting with its calibration. We show that an SC<inline-formula> <tex-math notation="LaTeX">\mu \text{M} </tex-math></inline-formula> chip can self-calibrate against this QuickCal Box in fewer than 3 min. We further validate that once it has self-calibrated, an SC<inline-formula> <tex-math notation="LaTeX">\mu \text{M} </tex-math></inline-formula> chip can reliably communicate with off-the-shelf IEEE802.15.4 devices. Finally, we demonstrate a heterogeneous network-composed of an SC<inline-formula> <tex-math notation="LaTeX">\mu \text{M} </tex-math></inline-formula> 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 crystal-free radio is participating in a channel-hopping-enabled TSCH network.