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

Tengfei Chang,Ioana Suciu,Filip Maksimovic,Kris Pister,Xavier Vilajosana,David Burnett,Thomas Watteyne,Lydia Lee,Sahar Mesri,Osama Khan,Brad Wheeler

doi:10.3390/s20071912

Abstract

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 mm2 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.

Highlights

Low-power wireless networks are a key technology for applications ranging from industrial process monitoring to smart city and environmental monitoring
In [12], we showed a calibration algorithm to tune the oscillators on SCμM so it can send and receive frames to the OpenMote, a popular off-the-shelf IEEE802.15.4 mote built around the CC2538 chip [14]
The challenge is that SCμM has no stable time reference

Summary

Introduction

Low-power wireless networks are a key technology for applications ranging from industrial process monitoring to smart city and environmental monitoring. SCμM has a drift up to 16,000 ppm over temperature [12], three orders of magnitude higher that crystal/MEMs-based oscillators This make it extremely difficult to (1) tune the frequency to communicate on, (2) set the correct rate to modulate/demodulate, and (3) keep a good sense of time to schedule communication in a TSCH network. SCμM is a true single-chip low-power wireless mote-on-chip which combines an ARM Cortex-M0 core, and an IEEE802.15.4 radio. It measures 2×3 mm , roughly the size of a grain of rice. It features a radio timer (RFTimer) which is designed for implementing time synchronized communication protocols such as 6TiSCH (see Section 2). A frame can be loaded into the transmit buffer of the radio automatically at a specific time, without needing code to be executed on the micro-controller

Frequency Synthesis and Clock Calibration

Implementation and Experimental Results

Result

Findings

Conclusions