Hardware Assisted Clock Synchronization for Real-Time Sensor Networks
Time synchronization in wireless sensor networks is important for event ordering and efficient communication scheduling. In this paper, we introduce an external hardwarebased clock tuning circuit that can be used to improve synchronization and significantly reduce clock drift over long periods oftime without waking up the host MCU. This is accomplished through two main hardware sub-systems. First, we improve upon the circuit presented in  that synchronizes clocks using the ambient magnetic fields emitted from power lines. The new circuit uses an electric field front-end as opposed to the original magnetic-field sensor, which makes the design more compact, lower-power, lower-cost, exhibit less jitter and improves robustness to noise generated by nearby appliances. Second, we present a low-cost hardware tuning circuit that can be used to continuously trim a micro-controller's low-powerclock at runtime. Most time synchronization approaches require a CPU to periodically adjust internal counters to accommodate for clock drift. Periodic discrete updates can introduce interpolation errors as compared to continuous update approaches and they require the CPU to expend energy during these wake up periods. Our hardware-based external clock tuning circuit allows the main CPU to remain in a deep-sleep mode for extended periods while an external circuit compensates for clock drift. We show that our new synchronization circuit consumes 60% less power than the original design and is able to correct clock drift rates to within 0.01 ppm without power hungry and expensive precision clocks.