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Recently, radio frequency identification (RFID) systems have gained popularity in manufacturing units, inventory, and logistics, as they represent an inexpensive and reliable solution for automatic identification. Moreover, RFID transponders are expected to become a key element in the ubiquitous computing scenario. Tags will likely be used to collect sensors data, enabling noninvasive environment monitoring. Low-cost passive UHF transponders are expected to play a major role in this context, due to extended read range capabilities. Within a passive tag, power harvested from the field irradiated by the reader during the communication should operate both digital control circuitry and potential sensing devices. Exploiting ultra-low power tag circuitry would provide sensing sections with higher energy, thus improving measurement performance. In this paper, the design of a novel circuit is presented, which implements the baseband processor of a UHF-RFID tag in compliance with the ISO 18000-6B protocol. Regardless of protocol selection issues, several power saving strategies are devised, both at the system and circuit levels, suitable for passive transponder implementation. Near-threshold operation has been exploited to attain ultra-low power consumption while keeping fair performance. A set of standard cells has been designed, suitable for the power-limited specific application. The proposed solution has been successfully checked by means of a physical implementation on CMOS 0.18 mum technology. Test chips have been characterized in terms of voltage and frequency operating range and power consumption figure has been extensively analyzed. Measurement results fully support the selected design approach: the baseband processor dissipates only 440 nW average power when operated at 800 kHz and 0.6 V. This extremely-low power consumption enables high-performance ubiquitous computing.