Re: driving Spartan-3 input from 74LS TTL

I remember well that all real TTL LS devices have an effective two-
diode drop on the output.
But you can easily measure that yourself.
Then the question is whether 5 V minus 1.4 V is higher than your 3.3
V, and really higher than your 3.3 V plus a diode drop.
You can easily try it out, and measure the current flowing into the
FPGAinput when the LS output is High. Probably is it les than a few
microamps.
But then the question is about tolerances: can your 5V bw high while
your 3.3 V is low, and what about the start-up condition?
The resistor pck avoids all these headaches.
Peter Alfke

On Apr 28, 4:42 pm, Jim Granville <no.s...@xxxxxxxxxxxxxxxxxxxxxx>
wrote:
Eric Smith wrote:
I've read answer record 19146 about using a series resistor for
5V tolerance on Spartan 3 inputs.

If the signal source is a 74LS TTL signal (e.g., 74LS14), what
is the maximum Voh I can expect (over temperature, etc.)? It's not
in the specs, but I know it's not 5V. Is it above 3.3V?

I'll probably just assume 5V and use a 330 ohm resistor pack, but I'd
like to know the real-world limit.

Can you guarantee it is always going to be a 74LS14 ?
That's one important factor in skipping the limiter, and
not always under your control.

Also, what is the 5V/3.3V supply tolerances - if you know that
then you should be able to do a LS load line calc, to see
the clamp currents.
- but the answer only applies to LS14 devices...
If someone uses a HC/VHC/AHC/AC device, all that flies out
the window :)

-jg


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