A pausible clock with crystal oscillator accuracy

Pausible clocking is an efficient means of establishing communication between different timing domains without suffering from the risk of metastable upsets, as seen with synchronizer based solutions. On the downside, pausible clocks are usually ring-oscillator based and hence exhibit relatively bad accuracy and stability. In this paper we extend an existing solution to pausible clocking that allows to synchronize the ring oscillator to a high precision reference like a crystal clock. In that basic solution, the generated clock will run out of sync with the reference during a synchronization phase whenever it is paused for longer than a clock period. Our goal here is to avoid irregular periods of the generated clock and to keep it in sync with the reference at all possible times. We achieve this by (a) decoupling the communication interface in a way that allows a data transfer without actually pausing the clock, and (b) carefully aligning the signal timings. This finally yields a clock source that combines the benefits of being stable like a crystal clock and allowing hazard-free communication, like a ring-oscillator based pausible clock.