Surviving the Hair Dryer: Continuous Calibration of a Crystal-Free Mote-on-Chip

The single-chip micro-mote (SC
$\mu \text{M}$
) is a
$2\times 3$
mm
²
single-chip crystal-free mote-on-chip. SC
$\mu \text{M}$
implements the IEEE802.15.4 and BLE standards and can communicate with off-the-shelf radios compliant to those standards. SC
$\mu \text{M}$
exclusively uses on-chip oscillators, including a 2.4-GHz LC oscillator to synthesize the communication frequency, and a 2-MHz RC oscillator to clock the chip rate. The challenge is that the LC oscillator drifts at 2100 ppm over a temperature range of 45 °C, far from the 40-ppm maximum drift mandated by the IEEE802.15.4 standard. While one-shot calibration is possible, any temperature change causes IEEE802.15.4 communication to fail. This article describes a continuous calibration approach for SC
$\mu \text{M}$
to adapt the tuning of its oscillators as the temperature changes. Experimental results show that it allows SC
$\mu \text{M}$
to keep communicating with an IEEE802.15.4 radio even under the extreme condition of using a hair dryer to heat up the chip at 3 °C/min. Under these conditions, the drift of the LC oscillator stays within the ±40-ppm limit over 94% of the time. Similarly, the drift of the 2-MHz RC oscillator stays within ±1000 ppm limit 99.98% of the time.