# Difference between TTL I/O type vs Schmitt trigger I/O type

While reading microcontroller data sheet i came across i/o types like

i) TTL

ii) schmitt trigger.

I came to know that TTL i/o logic means 0v-0.8v -logic state 0, 2-Vcc means logic state 1 (as per wiki [1] http://en.wikipedia.org/wiki/Digital_signal#Logic_voltage_levels ).

my questions are

i) what will be the logic state when my input voltage is 1-2 in TTL in i/o type

ii) as per definition schmitt trigger is a circuit with positive feedback, but i didnt understood how it works, advantage of using schmitt trigger type input over TTL

• Suppose the input is low and the output is low. There is some specified level for the input voltage such that the output is guaranteed to be high when the input is above that voltage. This voltage level is called $V_{IH}$ and let's say it's 2.4V. Once the input has risen above that level and the output changes to a high voltage (a logic 1) then internal positive feedback causes $V_{IH}$ to be reduced, let's say to 2.0V. This means that any noise riding on the input signal that is less than 0.4V cannot cause the output to switch back to a 0. – Joe Hass Jul 20 '13 at 19:12