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.

I. Introduction

As the abstraction of a system-wide phase-accurate synchronous clock faces its limits in today's complex systems-on-chip, GALS (globally asynchronous locally synchronous) architectures [1] have become the de-facto standard. Here a reliable and efficient data transfer between the individual clock domains is crucial. Among the available solutions, pausible clocking has attracted much interest [2], [3], [4], [5], [6], [7], [8], as it allows an efficient data transfer that is completely free from metastable upsets. This is not only conceptually appealing, it also saves the need for (multi-stage) synchronizers that cause performance penalties in other approaches. Virtually all present implementations of a pausible clock are based on a ring oscillator, whose frequency can neither be accurately designed (only calibrated), nor is it stable, as it is determined by circuit delays which get more uncertain with every new technology node. This calls for a solution that is based on a stable and accurate crystal clock source. However, as shown in [9] gating a crystal clock incurs the same metastability issues as a synchronizer-based data transfer and thus spoils the key advantage of using a pausible clocking scheme.