20.11 A Crystal-Less BodyID with an Asynchronous Clockless Leakage-Powered Wake-Up Receiver and Over-the-Channel Clock Recovery

Energy-constraint wireless transceivers with wake-up receivers (WuRX) traditionally rely on an always-running clock (Fig. 20.11.1) for sampling incoming data and digital correlation, necessitating local oscillator calibration through a reference, such as crystal oscillator. This consumes unnecessary power during idle periods and adds significant size and cost to the system. Removing the always-running clock can reduce system power consumption to leakage levels during idle, requiring innovative approaches to implement the wake-up functionality while maintaining low latency. The absence of the external clock reference can enhance energy efficiency and system integration but requires innovation in clock synchronization methods. Recent advances in the solid-state community targeting ultra-low-power (ULP) wearables have achieved notable energy efficiency and latency [1]–[19]. However, limitations remain in having an always-running clock and ULP clock generation unit needing external reference, creating a research need for ULP yet low-latency solutions. [1] demonstrates a ULP WuRX with 17nW, albeit with a higher latency of 2ms due to the ULP clocks operating at 4kHz. [2] presents a ULP WuRX with 200us latency but at the cost of 220uW power consumption, restricting its impact on battery-less nodes. [3] showcases a battery-less wearable node with a combined peak power of only 13.96uW, but it relies on a power-hungry external crystal for clocking, negating the energy efficiency gains. While [4]–[6] offer ULP crystal-less designs, they either depend on external reference voltages for calibration or utilize a forwarded clock through power or amplitude shift keying (ASK) signals, which limit their applicability across different use cases.