AVR-Chat

Well, that was an interesting reply.

5V is NOT the high level for TTL. Never was.
It's not the high level for CMOS either.

5V is the VCC level for TTL though.

Each logic family determines it's low and high thresholds.
TTL levels are well defined here: 
http://media.iet.hist.no/lab3ee/avgrensa/74xx.htm

Then there's the more modern LSTTL
http://media.iet.hist.no/lab3ee/avgrensa/74lsxx.htm

Good comparison here:
http://www.interfacebus.com/voltage_threshold.html


The why of it, is that this is how the inputs of the gates are 
designed, and the difference between the high and low threshold gains 
you noise immunity as you increase it, but it costs you power, since 
you spend more in resistive dissipation, and charging/discharging 
gate capacitances.