16 Apr
2002
16 Apr
'02
4:52 p.m.
Richard Erlacher wrote:
Carry logic found in adders can use up a lot of terms if one is not
careful.
OK, but
that's still not enough to generate _all_ possible functions.
I'm getting really curious what sort of function one could NEED that requires
a set of more than 64 products of those 42 inputs, though you can certainly
use more than that by combining sums if you do need. I think that's why this argument has persisted for
so long. With programmable
devices, I can build what I want. With TTL SSI/MSI/LSI you can build what you
want. We could switch roles, but the fact remains, each approach has
advantages. I just believe that the advantages of the SSI/MSI/LSI approach
are diminshing, while the programmable logic approach continues to expand.
But the
programmable logic setup is still a mess. I have yet find a
CLEAN hardware description language that is portable, simple and free.
If you were ever to want to investigate, thoroughly,
at least as thoroughly as
you could by building such a thing, a system that required such a complicated
logic function, with as many as 56 ORs of products with 1..20 ANDed inputs,
you would likely start with a simulator and not with hardware.
I tend to work bottom up. I start with REAL hardware and build up.
You'd then
write a top-level functinoal simulation program and then test it with a sample
program. (snip)
Testing is good. How ever it still needs to fit in the hardware.
Doesn't this seem less spainful than (a) finding a
set of bare wire-wrap
boards, (b) installing sockets, (c) working out a large schematic design, (d)
acquiring the long list of parts, only some of which you'll already have,(e)
integrating the various modules you fabricate and test separately, (f) finding
the problems and going on only when there don't appear to be any more (else go
to (C)...) (g) and then, finally, going to the step at which you'd have
arrived without ever fiddling with any hardware with the former approach? You
don't really EVER have to implement anything in programmable logic unless you
want to, but you can use all the tools to support a development in
SSI/MSI/LSI.
But debuging TTL is visible. You put a logic probe on the output and
test data in. I favor switches and lights.It is not that I don't like
modern software , I as design should be able to have the final say in
exactly how things are connected even if this means shooting myself in
the foot now and then.
--
Ben Franchuk - Dawn * 12/24 bit cpu *
www.jetnet.ab.ca/users/bfranchuk/index.html