  This paper presents a fully-integrated CMOS temperature sensor for
densely-distributed thermal monitoring in systems on chip supporting dynamic
voltage and frequency scaling. The sensor front-end exploits a sub-threshold
PMOS-based circuit to convert the local temperature into two biasing currents.
These are then used to define two oscillation frequencies, whose ratio is
proportional to absolute-temperature. Finally, the sensor back-end translates
such frequency ratio into the digital temperature code. Thanks to its
low-complexity architecture, the proposed design achieves a very compact
footprint along with low-power consumption and high accuracy in a wide
temperature range. Moreover, thanks to a simple embedded line regulation
mechanism, our sensor supports voltage-scalability. The design was prototyped
in a 180nm CMOS technology with a 0{\deg}C $-$ 100{\deg}C temperature detection
range, a very wide supply voltage operating range from 0.6V up to 1.8V and very
small silicon area occupation of just 0.021$mm^2$. Experimental measurements
performed on 20 test chips have shown very competitive figures of merit,
including a resolution of 0.24{\deg}C, an inaccuracy of $\pm$1.4{\deg}C, a
sampling rate of about 1.5kHz and an energy per conversion of 1.06nJ at
30{\deg}C.
