Low-Energy Clock Generation for Internet-of-Things Applications
Aggressively duty-cycling the operation of a system between ON and OFF states has proven to be the most effective way to reduce the average power consumption in energy-constrained applications such as Internet-of-Things (IoT). Since these devices are either battery-powered with a very small form factor or rely on energy harvesting where small amount of ambient energy is captured to power up the device, ultra-low power consumption is crucial in enabling all the advantages that applications such as IoT and biomedical implantable/wearable devices are expected to produce. However, the amount of power saving in a duty-cycled system is usually constrained by two main factors: 1) the start-up time of the system; and 2) the OFF-state power consumption. The system start-up time is usually limited by the long start-up time of its reference oscillator, typically a high-Q, MHz-range crystal oscillator that usually takes several milliseconds to turn on. The OFF-state (sleep) power consumption, on the other hand, is dominated by the sleep timer that is an always-ON 32KHz crystal oscillator. Sleep timer synchronizes the transmitting and receiving bursts which requires a very accurate oscillation frequency to minimize the guard band in order to reduce the active energy consumption. To address the mentioned challenges, we have developed circuit techniques and architectures that enable low-power clock generation.
To kick-start high-Q oscillators, such as crystal and/or MEMS-based reference oscillators, pre-energization of the resonator through injecting energy for a precise duration is proposed. A universal analysis for energy injection into high-Q resonators is developed and used to calculate the optimal injection duration essential to obtain a minimum start-up time. The proposed method ameliorates the sensitivity of the start-up time to the matching between the injection and resonance frequencies. To further relax the frequency accuracy requirement of the injection oscillator, energy injection with a dynamically-adjusted injection duration is presented. Measurement results from a 65nm CMOS IC show that the proposed technique reduces the start-up time of multiple tested crystal oscillators to about 100-120 number of oscillation cycles. The measured start-up time using the proposed precisely-timed energy injection method is 15 times faster than the best case reported in the literature while consuming the lowest start-up energy of ~12nJ.
In order to reduce the sleep power consumption, an ultra-low power sleep timer that is based on a DC-only sustaining amplifier is presented. New oscillator architecture is proposed that enables sub-nW power consumption in a 32KHz crystal oscillator by amplifying the oscillation signal at DC, instead of amplifying it at the oscillation frequency. Measurement results of 20 different 65nm standard CMOS dies show an average power consumption of 0.55nW drawn from a 0.5V supply at room temperature for a 32KHz crystal oscillator. The measured long-term stability of the sleep timer indicated by the Allan deviation floor is 14ppb. The proposed oscillator architecture does not require any calibration schemes or multiple supply domains, unlike most prior art.