  Battery-free computational RFID platforms, such as WISP (Wireless
Identification and Sensing Platform), are emerging intermittently powered
devices designed for replacing existing battery-powered sensor networks. As
their applications become increasingly complex, we anticipate that
synchronization (among others) to appear as one of crucial building blocks for
collaborative and coordinated actions. With this paper we aim at providing
initial observations regarding the synchronization of intermittently powered
systems. In particular, we design and implement the first and very initial
synchronization protocol for the WISP platform that provides explicit
synchronization among individual WISPs that reside inside the communication
range of a common RFID reader. Evaluations in our testbed showed that with our
mechanism a synchronization error of approximately 1.5 milliseconds can be
ensured between the RFID reader and a WISP tag.
