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This paper presents a wakeup timer in 40-nm CMOS for Internet-of-Things (IoT) applications based on a bang-bang Digital-intensive Frequency-Locked Loop (DFLL). A self-biased $\Sigma \Delta $ Digitally Controlled Oscillator (DCO) is locked to an RC time constant via a feedback loop consisting of a single-bit chopped comparator and a digital loop filter, thus maximizing the use of digital circuits while keeping only the RC network and the comparator as the sole analog blocks. Analysis and behavior level simulations of the DFLL have been carried out to guide the optimization of the long-term stability and frequency accuracy of the timer. High frequency accuracy and a $10\times $ enhancement of long-term stability is achieved by the adoption of chopping to reduce the effect of comparator offset and 1/f noise and by the use of $\Sigma \Delta $ modulation to improve the DCO resolution. Such highly digitized architecture fully exploits the advantages of advanced CMOS processes, thus enabling operation down to 0.7 V and a small area (0.07 mm2). The proposed timer achieves the excellent energy efficiency (0.57 pJ/cycle at 417 kHz at 0.8-V supply) over prior art while keeping excellent on-par long-term stability (Allan deviation floor < 20 ppm) and temperature stability (33 ppm°Cat 0.8-V supply). [ABSTRACT FROM AUTHOR]

An energy-efficient fast start-up method for crystal oscillators is presented, which enables aggressive duty-cycled operation of IoT radios to minimize overall power consumption. A digitally controlled crystal oscillator using the proposed start-up technique in 90-nm CMOS is presented. Thanks to the dynamically adjusted load, the negative resistance is boosted, achieving a $13\times $ start-up time reduction and an overall power of $95\mu \text{W}$ for a 24-MHz crystal oscillator at 1 V. A fully autonomous feedback loop detects the oscillators envelop and adjusts the load capacitance at start-up. Thanks to the low-power start-up circuits, both the start-up time and the start-up energy are reduced. In addition, the robustness and versatility of the proposed method is verified by measuring quartz crystals with different frequencies and quality factors, as well as measuring against temperature, supply voltage, and load capacitance variations. [ABSTRACT FROM AUTHOR]