  This article presents the first keyword spotting (KWS) IC which uses a
ring-oscillator-based time-domain processing technique for its analog feature
extractor (FEx). Its extensive usage of time-encoding schemes allows the analog
audio signal to be processed in a fully time-domain manner except for the
voltage-to-time conversion stage of the analog front-end. Benefiting from
fundamental building blocks based on digital logic gates, it offers a better
technology scalability compared to conventional voltage-domain designs.
Fabricated in a 65 nm CMOS process, the prototyped KWS IC occupies 2.03mm$^{2}$
and dissipates 23 $\mu$W power consumption including analog FEx and digital
neural network classifier. The 16-channel time-domain FEx achieves 54.89 dB
dynamic range for 16 ms frame shift size while consuming 9.3 $\mu$W. The
measurement result verifies that the proposed IC performs a 12-class KWS task
on the Google Speech Command Dataset (GSCD) with >86% accuracy and 12.4 ms
latency.
