[sdiy] CMOS chip questions

[David G. Dixon]

As usual, excellent responses!  As for the diode-resistor logic, I'm only
using three diodes for this purpose (two to sense a binary 5, and one to
sense a binary 8) so chaining is not an issue.  Also, speed isn't an issue
either.  I could use just about anything, but I do like the low power
consumption of 74HC, particularly since my 5V supply is coming from 78L05 in
that itty bitty TO-92 package.  I don't really want to overheat that little
guy!

> > 74HC is usually fine.  I prefer 74HCT since its inputs are TTL
> compatible.
> > If you need something faster, use 74AC.  The problem with diode logic (I
> > guess you're using diode resistor logic?) is that you can't really chain
> > stuff together.  The speed depends on your choice of diodes and
> resistors
> > as well as whatever you're driving.  I believe TTL circuits prefer pull-
> up
> > resistors rather than pull-down (doesn't matter with CMOS), so this may
> > limit your options in some cases.
> >
> I'll agree totally here, 74HC instead of CD4xxx is usually just fine
> unless
> you really need the devices to withstand 15V Logic levels. And in the
> 74HC4xxx
> series you'll get the very same pinout as the original CD4xxx devices, so
> that is a non-issue as well. With most small-scale suppliers, 74HC has
> better
> availability than CD4xxx anyways. When it comes to single gates, there are
> such on the market as well. Okay, all i've seen are SMT devices, but when
> you
> need to save real estate, i'd go for that anyways. It ain't no black
> magic. A
> bit pricey in terms of $$/gate compared to the usual 4-gates-in-a-case
> stuff,
> but a lot easier to work with considering all those design questions
> popping
> up when working with discrete Diode-resistor logic. And: if you should
> happen
> to not use some gate in a case, you'll most likely be better of with
> spending
> a couple resistors and tieing those inputs to high or low.
> Regarding 74HC vs. 74AHC vs. 74AC in most cases you're safe going for
> 74HC.
> Coz, when you actually need the additional speed of 74AC and AHC compared
> to
> plain 74HC, you'll know anyways. In all those cases with SDIY i've ever
> come
> across, 74HC had plenty of speed and still loads of margin. It's not like
> clock jitters of half a nanosecond would kill us here.
> 
> HTH,
> Florian
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