Each cell could implement a Phase Locked Loop that locks to one of its neighbors. This would decentralize the clock while the system remains self-referentially synchronous. You would still need a reference signal to start the system's initial frequency and phase, but then I think you can remove the reference and the system would continue in sync.<p><a href="http://en.wikipedia.org/wiki/Phase-locked_loop" rel="nofollow">http://en.wikipedia.org/wiki/Phase-locked_loop</a>