How to Synchronize a Pausible Clock to a Reference

Pausible clocking is an efficient means for metastability free communication in heterochronous GALS systems. State-of-the-art implementations are based on ring oscillators, which suffer from bad frequency stability and accuracy. Building a stable pausible clock by simply switching a crystal based clock source, on the other hand, has been shown to incur metastability issues, thus rendering pausible clocking no better than a conventional synchronizer approach. In this paper we propose a solution that provides the stability and accuracy of a crystal based clock while still being free from metastable upsets. It is based on a ring oscillator that can be paused and resumed just like in the traditional approach, but is synchronized with a crystal clock during its stable phases of operation. We will present an appropriate circuit implementation and carefully analyze its operation. We will give evidence that the circuit switches on and off as fast as the traditional one, and that the clock output does not suffer from metastable voltage or spurious pulses at any time.