Observations of the Effect of Temperature upon Stability of Resource-constrained, Embedded Clocks

摘要

Wireless Sensor Networks (WSNs) are being increasingly sought after for critical applications such as security and health care. This is leading to an increased need to understand and quantify the reliability and stability of WSN-based systems when deployed in real environments. Timing synchronization is a vital capability for many WSN functions such as robust communications and data combining. This paper presents measured results of the frequency output of a WSN node clock. The clock incorporates several layers of software and hardware components, which need to function robustly together. We show how counting errors propagate through these layers to affect the resulting output counter frequency. We also observe that there is a strong relationship between the standard deviation in clock frequency and temperature variation; with the standard deviation varying between 2 × 10~(-5) Hz and 10 × 10~(-5) Hz (the standard deviation in the mean of a 10 minute rolling window of frequency measurements) as the ambient temperature crosses a threshold of approximately 23.5 degrees Celsius. Quantification of the stability of a clock output is an important factor in any subsequent synchronization, affecting decisions such as the employment of hibernation for energy saving, sensor sample rates and time division multiple access communications. As wireless sensing systems become increasingly complex, both in terms of capability and requirements, it will be increasingly important to optimize every element. When synchronizing clocks it is important to make judgments about the optimum intervals over which to carry out synchronization and to be able to estimate the accuracy achieved. The observations presented here have highlighted that temperature is an important factor in determining both of these.