On Sun, 11 Nov 2007 10:04:22 -0600
Terry Hancock <[EMAIL PROTECTED]> wrote:
> JB suggested off-list that I should mention that inserted letters in
> 7400-series chips are usually not significant for our purposes.
>
> For example a 74LS138 is just a 74138 that uses "Low-power Schottky"
> implementation while a 74HC138 is one that uses "High-power CMOS".
Little correction here, HC stands for High Speed CMOS (in contrast
to the slow 4000 family of CMOS logic chips). I haven't seen standard
74 devices (ie those with no codes) in ages, and no S or LS devices
in years (ok, i haven't been looking for those either). HC is still
around, but only used by hobyists. Most popular these days are
AC (advanced CMOS) and ACT (advanced CMOS with TTL compatible output).
Main reason is speed and because AC/ACT can 1 to 1 replace HC/HCTs
in most cases (unless you depend on a certain delay behavior, but
then you have a bad design anyways ;-)
> Some
> of them can't be used together for impedence-matching/power-consumption
> reasons, but they are identical at the logic and pinout level
The incompatibilities are in the voltage levels for high/low only.
CMOS uses level relative to Vcc, while TTL has fixed (and incompatible
positioned) levels. Though nothing that could not be fixed with a simple
pull up resistor.
> -- which
> is (AFAIK) all the Verilog will encode anyway.
Where does Verilog come into the game with 74s?
> Letters at the beginning are probably manufacturer codes (I'm not
> certain, though).
Yes, they are.
> Letters at the end usually indicate package codes. When the 7400s first
> came out, they were all in standard 0.3" wide, 0.1" pin-pitch
> through-hole DIP packages. But many of these are for various "small
> outline" / "surface mount" packages which are a lot more compact. I'm
> discovering that there is a bewildering array of these, though.
Never trust the package codes of any device without samples.
Especially not 74s and even less if your distributor uses
"equivalent" types from a different manufacturer.
Oh yes, and check the codes printed on the packages that
you actually got the right chips. For some reason a lot
of distributors seem to have troubles to label 74s in
small SMD packages correctly.
> Anyway, they should still have the same arrangement of pins.
Nope. Only if they are still 14 pin. But these days a lot of
the chips are single gates, which have a pseudo standard
(ie most use the same pinout, but not all).
(side note: most of the 74s with higher pin counts and
more complex functionality were replaced by CPLDs and FPGAs
and thus died out)
Attila Kinali
--
Linux ist... wenn man einfache Dinge auch mit einer kryptischen
post-fix Sprache loesen kann
-- Daniel Hottinger
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