Ultra-low power wireless sensor nodes system consumes significant amount of energy if the nodes want to communicate with each other. To reduce average power consumption, communication modules will only be operational from time to time and remain in sleep for the most of the time. If the communication timing is not synchronized between each nodes, some nodes have to keep the communication module active longer, which will quickly dominate the energy budget and shorten the battery life. To avoid this, a stable frequency source is essential for design of each nodes.
The quartz crystal oscillator is known for its excellent frequency stability. However, with a conventional circuit, the power required to maintain the oscillation is too high to be used under stringent energy budget of the ultra-low power wireless sensor nodes system.
The project focus on three sources of power consumption: driver, oscillation amplitude, and peripheral power.
Instead of applying square wave to the driver, pulse injection scheme was incorporated to inject energy to crystal only when it is needed, while eliminating static power consumption. The pulse amplitude is larger than the supply voltage from which driver is running from, thereby maximizing transconductance during injection. The output driver and the oscillation is confined within the smallest supply voltage pair to minimize energy loss per each oscillation cycle. The peripheral circuit such as DLL is operated at near threshold level to remove any short-circuit current. These different supply voltage is internally generated using switched capacitor network.
The circuit was implemented in 0.18um CMOS process using triple well option. Total area including the capacitor for switched capacitor network is 0.3mm^2.
The lowest power consumption of 5.58nW was observed at room temperature. Supply voltage dependence of operating frequency is 17.6ppm/V. The circuit was confirmed to operate from -20 degrees Celsius to +80 degrees Celsius and stays within frequency specification boundary of the quartz crystal. As can be seen from the Allan deviation plot, the circuit maintains long-term stability performance of the quartz crystal, and therefore can be used for wireless sensor network system.
Dongmin Yoon, Dennis Sylvester, David Blaauw, “A 5.58nW 32.768kHz DLL-Assisted XO for Real Time Clocks in Wireless Sensing Applications,” IEEE International Solid-State Circuits Conference (ISSCC), February 2012 ©IEEE