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

Area of Science:

• Embedded Systems
• Wireless Sensor Networks
• Low-Power Radio Communication

Background:

• Crystal-free devices lack absolute time references, posing challenges for time-synchronized protocols.
• The Single-Chip micro Mote (SCµM) integrates an ARM Cortex-M0 microcontroller and IEEE802.15.4 radio on a single chip.
• Existing synchronized protocol stacks require stable time references, typically from external crystals.

Purpose of the Study:

• To demonstrate the first time-synchronized protocol stack on a crystal-free mote.
• To adapt and implement the OpenWSN protocol stack on the SCµM prototype.
• To overcome the limitations of on-chip oscillators for precise timing in wireless networks.

Main Methods:

• Utilized an FPGA-based microcontroller connected to the SCµM chip's radio front-end.
• Developed a two-step calibration process using optical and RF receivers to initially synchronize on-chip oscillators.
• Implemented a digital trimming compensation algorithm with tick skipping to mitigate oscillator drift.

Main Results:

• Successfully calibrated SCµM's on-chip oscillators without an external crystal.
• Reduced apparent oscillator drift from 567 ppm to 10 ppm using digital compensation.
• Established a standards-compliant 6TiSCH network between the SCµM prototype and an OpenMote device.

Conclusions:

• Time synchronization is achievable on crystal-free devices, paving the way for miniaturized wireless nodes.
• The developed calibration and compensation techniques are effective for low-power, integrated wireless systems.
• This work represents a significant step towards the realization of smart dust and ubiquitous computing.