Any 74HCTxxx recognizes 3.3V CMOS signals at its inputs when powered
with 5V.
The magic here is on the "T" that stands for TTL, that is it recognizes
level '1' from 2V and up.
They work only when powered with 5V.

A much better solution is the SN74LVC8T245 or SN74LVCH8T245. They
support both Vcca and Vccb from 1.8V to 5.5V and any of the power
supplies can be shut down or powered up in any sequence or timing.
They can even be powered partially.


Isaac



Em 26/8/2012 12:07, peter green escreveu:
> I want to take a group of 3.3V logic signals, buffer them and output=20
> them as either 3.3V or 5V logic signals at the board users selection.
>
> I first thought of using a normal logic chip but I can't seem to find a=20
> family that has 3.3V compatible inputs when running off 5V and is=20
> specced to run off 3.3V and I'd rather users didn't have to swap chips=20
> to change the output voltage.
>
> I found a chip from TI that can do such conversions, the SN74LVCC3245=20
> but it seems to have painful power up requirements (according to Ti Vcca=
=20
> must come up BEFORE vccb). I guess I could put in a RC circuit or=20
> something to slow down the coming up of Vccb but it seems a messy solouti=
on.
>
> Any thoughts/advice on this.

--=20
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