Good idea SS, but it would seem that adding a page to a pdf is as difficult as a hip replacement.
I don't think the aircraft analogy quite holds up there, me old hipster. I think it struggled to take off
Yes, it will certainly 'take off' at 0.8 x Vdd for St and TTL.
However, the Hippy Airways TTL-star will 'take off' at a relativley slower speed. People may wonder why. They may question their calculations or, indeed, their sanity. They may waste hours/days checking and posting.
For anyone vaguely interested, the 12F683 Data Sheet, for example, says a 'High' is >2.0V input for TTL input @Vdd=4.5 to 5.5V.
So, if you designed your threshold circuit based on 0.8 x Vdd you will be miles out!
Your plane will have taken off ages ago.
Or if you are wondering why a 3V digital o/p can 'talk' to some PICAXE i/p pins (@ 5V) and not others then you will have to post a question and wait hours for the answer.
Indeed, based on the Manual, a 3V or 3V3 digital device O/P can NEVER talk to a PICAXE I/P (@ Vdd=5V) and that's plain cobblers.
Never mind. Let's move on as I can see an allergic reaction brewing.
Bottom line to EVERYONE:-
If you want more precise electrical figures then cross-reference your PICAXE pin allocation against the actual Microchip PIC Data Sheet pinout and look it up for yourself.