Steininger, A., Maier, J., & Najvirt, R. (2016). The Metastable Behavior of a Schmitt-Trigger. In 2016 22nd IEEE International Symposium on Asynchronous Circuits and Systems (ASYNC). IEEE. https://doi.org/10.1109/ASYNC.2016.19
Schmitt-Trigger circuits are the method of choice for converting general signal shapes into clean, well-behaved digital ones. In this context these circuits are often used for metastability handling, as well. However, like any other positive feedback circuit, a Schmitt-Trigger can become metastable itself. Therefore, its own metastable behavior must be well understood; in particular the conditions that may cause its metastability.<br /><br />In this paper we will build on existing results from Marino to show that (a) a monotonic input signal can cause late transitions but never leads to a non-digital voltage at the Schmitt-Trigger output, and (b) a non-monotonic input can pin the Schmitt-Trigger output to a constant voltage at any desired (also non-digital) level for an arbitrary duration. In fact, the output can even be driven to any waveform within the dynamic limits of the system. We will base our analysis on a mathematical model of a Schmitt-Trigger’s dynamic behavior and perform SPICE simulations to support our theory and confirm its validity for modern CMOS implementations. Furthermore, we will discuss several use cases of a Schmitt-Trigger in the light of our results.
The final publication is available via <a href="https://doi.org/10.1109/ASYNC.2016.19" target="_blank">https://doi.org/10.1109/ASYNC.2016.19</a>.