20 Apr
2006
20 Apr
'06
12:34 a.m.
Tony Duell schrieb:
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,
Sure you can do this nowadays. I was talking about 20 years ago.
Today, the 9368 is still available from several surplus chip vendors -
albeit for prices that makes one believe the chip was not silicon but
carbon (i.e. diamond).
Realistically, today one wouldn't even consider GALs for it - Noone
still makes counters with the classical 7490+7475+7447 triple. Multiplex
the whole stuff with a cheap PIC controller, or for special purposes,
use something like the CA3161/CA3162 which are also still available,
surprisingly. Heck, I recently even encountered a 8279, also an
interesting gadget from the past.
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
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.
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
This is not really straight forward, and probably not even resource
saving, is it? A diode is maybe 2 Eurocent, but in the past where such
tricks where necessary, a 7447 and a 7442 and some diode matrix summed
up to much more. But I remember I once have seen an application note for
this, maybe in the old TTL cookbook (?).
Holger