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.

中文翻译：

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施密特触发器的亚稳态行为

施密特触发器电路是将一般信号形状转换为干净、性能良好的数字信号的首选方法。在这种情况下，这些电路也经常用于亚稳态处理。然而，与任何其他正反馈电路一样，施密特触发器本身可以变为亚稳态。因此，必须很好地了解其自身的亚稳态行为；特别是可能导致其亚稳态的条件。在本文中，我们将基于 Marino 的现有结果来证明 (a) 单调输入信号会导致延迟转换，但绝不会导致施密特触发器输出端出现非数字电压，以及 (b) 非单调输入可以在任意持续时间内将施密特触发器输出固定到任何所需（也是非数字）电平的恒定电压。实际上，输出甚至可以驱动到系统动态限制内的任何波形。我们将基于施密特触发器动态行为的数学模型进行分析，并执行 SPICE 仿真以支持我们的理论并确认其对现代 CMOS 实现的有效性。此外，我们将根据我们的结果讨论施密特触发器的几个用例。