Schmitt trigger circuits are often used as input circuits to guard against noise-induced false switching. Schmitt triggers are characterized by having hysteresis in their voltage transfer curve. The hysteresis insures that small fluctuations on the rising or falling edge of the input signal do not induce a false switching event.

A non-inverting Schmitt trigger circuit is shown. PMOS transistors and NMOS transistors are used respectively as weak pull-up and pull-down devices that are controlled by the output voltage Vout through the feedback connection. If Vin is high the output of the 1st inverter is high. So that Vout is low from the 2nc inverter. This biases PMOS and NMOS off. If we increases Vin, the output node of NOT1 is held high by PMOS, which delays the switching.

A bi-directional pad provides circuitry for both input and output signals. The input circuits are identical to those described above. Output drivers should be capable of tri-state operation so that they do not interfere with incoming signals. An example is shown in figure. The output circuit uses large driver FETs that are controlled by the NAND and NOR logic network. The gates are considered part of a scaled driver chain since the FET capacitances will be large. The enable signal En is the tri-state control. When En =0 the output circuit is in a Hi-Z state and the pad can be used for inputs. When En=1 the output circuit acts as a non-inverting buffer for the data input. Care must be taken to insure that the output signal is used by the input circuit.