A Schmitt-trigger input has a hysteresis (VH) between the positive-going threshold voltage (VP) and the negative-going threshold voltage (VN). Therefore, even if an input signal is noisy while transitioning from Low to High, it does not become High unless it crosses the VP threshold. Also, once the input signal becomes High, it remains High if it does not fall below VN.
In this way, an input signal with a low slew rate (i.e., a signal with high input rise and fall times) is less likely to cause a chattering problem.
When a positive-going signal is applied to the input, a p-channel CMOS MOSFET at the output turns on at VP, causing its output value to switch. When a negative-going signal is applied to the input, an n-channel CMOS MOSFET at the output turns on at VN, causing its output value to switch. This means that the output is guaranteed not to make a logic transition while the input voltage is in the hysteresis (VH) range.
However, if large noise causes the input signal to cross the threshold repeatedly, a false output might appear.
Also, since neither the p-channel nor the n-channel MOSFET is completely off around the middle of the hysteresis range, tiny shoot-through current flows, increasing the supply and GND currents (ICC and IGND).
It is therefore not recommended to apply an excessively slowly changing signal to a Schmitt-trigger input.