  Accurate and energy-efficient clock synchronization is an enabler for many
applications of Wireless Sensor Networks. A fine-grained synchronization is
beneficial both at the system level, for example to favor deterministic radio
protocols, and at the application level, when network-wide event timestamping
is required. However, there is a tradeoff between the resolution of a WSN
node's timekeeping device and its energy consumption. The Virtual
High-resolution Timer (VHT) is an innovative solution, that was proposed to
overcome this tradeoff. It combines a high-resolution oscillator to a low-power
one, turning off the former when not needed. In this paper we improve VHT by
first identifying the jitter of the low-power oscillator as the current limit
to the technique, and then proposing an enhanced solution that synchronizes the
fast and the slow clock, rejecting the said jitter. The improved VHT is also
less demanding than the original technique in terms of hardware resources.
Experimental results show the achieved advantages in terms of accuracy.
