In this paper, we present an all-digital ring oscillator (RO)-based Bluetooth low-energy (BLE) transmitter (TX) for ultra-low-power radios in short range Internet-of-Things (IoT) applications. The power consumption of state-of-the-art BLE TXs has been limited by the relatively power-hungry local oscillator (LO) due to the use of LC oscillators for superior phase noise (PN) performance. This paper addresses this issue by analyzing the PN limit of a BLE TX and proposes an RO-based solution for power and cost savings. The proposed transmitter features: 1) a wideband all-digital phase-locked loop (ADPLL) featuring an fRF / 4 RO, with an embedded 5-bit TDC; 2) a 4x frequency edge combiner to generate the 2.4-GHz signal; and 3) a switch-capacitor digital PA optimized for high efficiency at low transmit power levels. These not only help reduce the power consumption and improve PN performance but also enhance the TX efficiency for short range applications. The TX is prototyped in 40-nm CMOS, occupies an active area of 0.0166 mm², and consumes 486 μW in its low-power mode, while configured as a non-connectable advertiser. The TX has been validated by wirelessly communicating beacon messages to a mobile phone.

Improving system lifetime and robustness is a key to advancing self-powered platforms for real world applications. A complete self-powered, battery-less, wearable platform requires a microwatt-power system-on-chip (SoC), operating reliably within this budget, capable of surviving long periods without charging, and recovering from power loss to its previous state. To meet these requirements, we designed a wireless sensing heterogeneous system-in-package (SiP) containing an ultra-low power (ULP) SoC, a non-volatile boot memory (NVM), and a 2.4 GHz frequency shift key (FSK) radio, all integrated with custom ULP interfaces. The SoC includes a fully integrated energy harvesting platform power manager (EH-PPM) to power the SiP and other commercial sensors. The EH-PPM is designed for small loads and powers the SoC and peripherals while drawing very low operating current. The SoC also includes a digital system data-flow for sensing applications, an analog front end for ECG signal acquisition, and a cold-boot management system (CBMS) for boot and recovery from the NVM. The CBMS enables integration of the SoC with the ULP NVM to create a wearable formfactor, self-powered system capable of recovery from power loss. The SoC also includes a radio interface tightly integrated with a compression accelerator to efficiently communicate with the FSK transmitter and reduce the FSK's transmission time. This tight integration between accelerators on the SoC and peripherals is another feature that reduces the system's power consumption by reducing the code size and number of memory accesses required to perform an operation. The SoC consumes 507 nW average power while running free-fall detection, 519 nW average power while measuring ambient temperature, and 1.02 μW during continuous ECG monitoring and post-processing.

This paper presents a top-down methodology for designing battery-less systems for the Internet-of-Things (IoT). We start by extracting features from a target IoT application and the environment in which it will be deployed. We then present strategies to translate these features into design choices that optimize the system and improve its reliability. We look into how to use these features to build the digital sub-system by determining the blocks to implement, the digital architecture, the clock rate of the system, the memory capacity, and the low power states. We also review how these features impact the choice of energy harvesting power management units.