Dwayne Reid wrote...

>VIH is about 1.4V for a non-ST input when Vdd=5V.  Vf for a red LED is
>about 1.7V, Vf for green and yellow LEDs is about 2.1V.  Note: VIH drops as
>Vdd drops - that is why this measurement is possible.
>
>I'll be the first to admit that I haven't tried my suggestion yet but it
>*should* work.

I've no doubt it might work for a one-off, or for a short run of
units.  But for any more than that, I wouldn't want to rely on
it: I've had too much experience over the years with designs that
developed production troubles because the original designer
counted on parts performing better than specified.

What it comes down to is, do you trust that this 1.4V threshold
(which you measured on a large number of units, I assume) is
going to hold from unit to unit, and from production lot to
production lot, over a long period of time?  Or do you use the
PIC specifications as your guide?

That's what I prefer to do; and the specs say that at Vdd = 4V,
which is the voltage level Kieren wants to detect, the PIC's TTL
input threshold voltage could be anywhere within a 3:1 range,
from 0.6V to 1.8V.  That's a pretty wide spread.

DD

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