Assuming that you can still find something that slow, or can
have one custom-designed (not cheap -- I know whereof I speak
since custom I/O design is what I do here....)
Lacking a supply of 74LS devices or a budget for full-custom
silicon, you're pretty much stuck with the native edge rates
for standard silicon. Assuming that your product is behind
the curve but not too far (say 350-500nm technology, the
stuff that likes 3.3v) then you can figure on worst-case (slow)
edge rates in the 300-600ps range and fast-case ones about
60% of that.
IOW, 300ps or faster.
-- D. C. Sessions email@example.com
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