6TiSCH on SCµM: Running a Synchronized Protocol Stack without Crystals
Résumé
We report the first time-synchronized protocol stack running on a crystal-free device. We 1 use an early prototype of the Single-Chip micro Mote, SCµM, a single-chip 2×3 mm 2 mote-on-a-chip, 2 which features an ARM Cortex-M0 micro-controller and an IEEE802.15.4 radio. This prototype 3 consists of an FPGA version of the micro-controller, connected to the SCµM chip which implements 4 the radio front-end. We port OpenWSN, a reference implementation of a synchronized protocol stack, 5 onto SCµM. The challenge is that SCµM has only on-chip oscillators, with no absolute time reference 6 such as a crystal. We use two calibration steps-receiving packets via the on-chip optical receiver 7 and RF transceiver-to initially calibrate the oscillators on SCµM so that it can send frames to an 8 off-the-shelf IEEE802.15.4 radio. We then use a digital trimming compensation algorithm based on 9 tick skipping to turn a 567 ppm apparent drift into a 10 ppm drift. This allows us to run a full-featured 10 standards-compliant 6TiSCH network between one SCµM and one OpenMote. This is a step towards 11 realizing the smart dust vision of ultra-small and cheap ubiquitous wireless devices.
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