6 Jan
2012
6 Jan
'12
2:10 a.m.
There was one bit of the desing that handn't been
completed, It came
down to producing a logic signal from 3 other logic signals [...]
I glanced at the problem and said 'That's
trivial. I can do it in one
IC'. One of the others's replied 'A PROM? A PAL? are they fast
enough? Do we have the programmer for them'. 'No', I said, 'Just a
plain TTL chip with no programming needed'.
OK, what was the chip I suggested?
Without knowing which of the 256 possible three-input functions
functions was needed, we can't more than guess. It could have been
anything from a '08 or '21 if the function was simple AND or '32 if OR
or '86 if XOR, through something like a '00 for ~C | (A & B), all the
way up to a '151, which allows generating whatever function you like by
strapping the data inputs correctly.
Depending on the design, the dynamic behaviour may matter (for some
choices, the output may glitch on certain input transitions even though
both stable states give the same output), and if this were a case like
that it certainly could affect your choice.
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