  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. 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.
