Temperature Calibration on a Crystal-Free Mote

We present a method to calibrate a crystal-free wireless system on chip for use as an IoT temperature sensor between 0°C and 100°C. Using two clocks on the mote that would already be running during radio operation as a regular IoT node—a timer similar to a 32kHz crystal and another 2MHz chipping clock for the radio transmitter—we create a temperature sensor by finding a linear relationship between the ambient temperature and the ratio of these two clock frequencies. We show that a simple two-point temperature calibration is sufficient to find this linear model, and by averaging over a few temperature measurements, we observe a temperature error of less than 2°C. We can then use the mote as a tiny wireless temperature sensor by using the temperature estimate to calibrate the radio frequency oscillator.