CMOS Design > I/O Interface Design

Schmidt Trigger

The Schmidt trigger is an inverter with two commutation points:

• V_C1 when the input rises from 0 to V_DD
• V_C2 when the input falls from V_DD to 0

The commutation point is modified by feedback transistors N1 and P1. When the input is low, the internal node is high, so N1 ties the NMOS input structure to V_DD, which shifts the commutation point to a high value. Oppositely, when the input is high, the internal node is low, so P1 ties the PMOS network to V_SS, which shifts the commutation point to a lower value V_C2.

A noise added to the clock shows the trigger benefits compared to a simple inverter. While the trigger transforms the slowly varying noisy input into a clean signal "Trigger out", the inverter switches erroneously during the input transition, which may produce an internal glitch.

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