6TiSCH on SCµM: Running a Synchronized Protocol Stack without Crystals

• Published in: Sensors (Basel, Switzerland) Vol. 20; no. 7
• Main Authors: Chang, Tengfei, Watteyne, Thomas, Wheeler, Brad, Maksimovic, Filip, Khan, Osama, Mesri, Sahar, Lee, Lydia, Suciu, Ioana, Burnett, David, Vilajosana, Xavier, Pister, Kris
• Format: Journal Article
• Language: English
• Published: Switzerland MDPI 30.03.2020
• Subjects: Computer Science; Embedded Systems; Networking and Internet Architecture; crystal-free; 6TiSCH; smart dust; SCµM; Smart dust; Crystal-free
• ISSN: 1424-8220; 1424-8220
• DOI: 10.3390/s20071912

We report the first time-synchronized protocol stack running on a crystal-free device. We use an early prototype of the Single-Chip micro Mote, SCµM, a single-chip 2×3 mm 2 mote-on-a-chip, which features an ARM Cortex-M0 micro-controller and an IEEE802.15.4 radio. This prototype consists of an FPGA version of the micro-controller, connected to the SCµM chip which implements the radio front-end. We port OpenWSN, a reference implementation of a synchronized protocol stack, onto SCµM. The challenge is that SCµM has only on-chip oscillators, with no absolute time reference such as a crystal. We use two calibration steps - receiving packets via the on-chip optical receiver and RF transceiver - to initially calibrate the oscillators on SCµM so that it can send frames to an off-the-shelf IEEE802.15.4 radio. We then use a digital trimming compensation algorithm based on tick skipping to turn a 567 ppm apparent drift into a 10 ppm drift. This allows us to run a full-featured standards-compliant 6TiSCH network between one SCµM and one OpenMote. This is a step towards realizing the smart dust vision of ultra-small and cheap ubiquitous wireless devices.