Abstract
The single chip integration of a wireless sensor node would allow for cheap, low-power, dust-size devices. The key to realizing this vision is to eliminate bulky off-chip frequency references such as crystal oscillators or resonators, and their associated power-hungry circuitry. The immediate challenge of removing off-chip references is that there is no accurate on-chip frequency references, which makes it hard to tune the radio to the right frequency, and to keep an accurate sense of time. This article offers a full solution for crystal-free devices, which includes (1) initiating communication in an IEEE802.15.4 network, (2) synthesizing the 16 communication channels at startup temperature, and (3) continuously applying corrections to the inaccurate timing source to allow keeping frequency synchronization on all communication channels over a 5-55 • C temperature range. The proposed methods are accompanied by simulations and an experimental validation on the first fully-functional Single Chip Micro Mote hardware implementation. Our simulations and experimental results validate that the proposed approach achieves radio clock synchronization accuracy close to the 40 ppm limit imposed by the IEEE802.15.4 standard.
Highlights
The explosive growth of IoT is pushing the market towards cheap, low-power devices with a strong focus on miniaturization
This article introduces a mechanism to bootstrap and calibrate the on-chip Radio Frequency (RF) oscillator of a crystal-free radio to enable the normal operation of an IEEE802.15.4 Time Synchronized Channel Hopping (TSCH) network
We show through simulations and experimentation that, using our algorithms, the radio maintains communication even when undergoing temperature changes, with an accuracy close to the ±40 ppm limit imposed by the IEEE802.15.4 standard
Summary
The explosive growth of IoT is pushing the market (consumer, industrial, military) towards cheap, low-power devices with a strong focus on miniaturization. The radio offers a 10 m communication range, and draws 670 uA in reception, 1 mA when transmitting at -10 dBm. As a point of comparison, the crystal-based industry leading low-power technology (2.4 GHz, IEEE802.15.4 TSCH) is the 10 mm × 10 mm LTC5800-IPM [20], consuming 4.5 mA when receiving and 5.4 mA when transmitting at 0 dBm. Without a high accuracy time reference, it is hard to maintain compatibility with standards. To stay within the 40 ppm drift limit and maintain compliance with IEEE802.15.4, calibration algorithms need to run continuously to detect temperature changes and correct its effect on frequency drift [23]. This article introduces a mechanism to bootstrap and calibrate the on-chip Radio Frequency (RF) oscillator of a crystal-free radio to enable the normal operation of an IEEE802.15.4 TSCH network.
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