GaBuZoMeu wrote:
Propagation delay isn't much different, but what's about the rise and fall times? I have no DB at hands...
Every time I look into this kind of thing, I'm reminded of how much information there is and that wading through it all is not a quick thing.
My book on 74LS is not very complete; but on 74HC, it looks like it couldn't be much slower and still meet the propagation delays (PD's) for the simplest ones, according to my National Semiconductor books. On the 74HC00 for example, it says typical PD is 8ns while typical rise and fall time is 7ns, both at 6V. It's 9 and 8ns @ 4.5V. The PD is measured to the half way point of the output swing, not 90%, but it does also include half the input swing which is about the same rate in the testing. Don't ask me why they don't specify it at 5V.
I spot-checked a few 74AC's to compare, and the PD's were anywhere from about twice as fast as the equivalent 74HC to nearly five times as fast! There seemed to be the least difference in the simple ones like 74xx00, while the more complex ones like 74xx5xx showed greater differences. For the 74AC, the 18-page "descriptions and family characteristics" article in the front of the book shows the rise and fall times bottoming out at around 1ns, give or take, depending on lot, temperature, etc., which is seven times as fast as the 74HC numbers given above. And as BDD said, the drive strength of 74AC is much greater than 74HC.
Regarding 74HC versus 74LS, the HC has slightly shorter (ie, faster) PD's than 74LS for most parts, with just a few exceptions.
In virtually all cases, I would recommend 74HC above 74LS. The only strength LS has over HC that I can think of is that it's less vulnerable to damage from static discharge; but with reasonable handling, I seem to have never damaged an HC (or any other CMOS part) with static.