8080 Assembler-Text Editor (ATE)
Tony Duell
ard at p850ug1.demon.co.uk
Wed Apr 19 16:40:14 CDT 2006
> > Some do. I recall feeling "giddy" when I had a design with
> The 9368 has proper hex decoding of the pseudo tetrades - but those days
> it was almost impossible to locate, compared to the common 7447 types.
> Eventually, I programmed 74188s for that purpose, after a short period
These days you can do it with a small GAL (like a 16V8), but it's not
quite as obvious as it sounds. There are only 8 product terms per output,
and IIRC some segments are on for more than 8 of the input combinations.
What you do is that if there are 8 or fewer input combinations that turn
on a given segment, you program the product terms for those combinations,
if there are 8 or more, you program the product terms for the input
combinations where that segment is _off_, and then invert the signal in
the output logic macrocell.
> (as a pupil) where I made a diode matrix decoder based on a 74154 -
> this soldered well on breadboards.
Another trick was to use the 7447 for 0-7 (or 0-9, depending on how you
felt), decode the other 8 or 6 inputs using a 1-of-n decoder (like a
7442), and re-encode those with a diode matrix, blank the outputs of the
7447 for those input combinations (either using the blanking input, or by
forcing the data inptus to 1111) and finally logically OR the outputs of the
diode matrix and the 7447
-tony
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