Electronics: What 74xx 5v logic is faster than 74"F"xx series?

Question

I have an project where I'm encountering a speed threshold of some of my 74"F"xx logic and need to go a little faster. Is AHC or another logic family faster? Can someone give me a comparison breakdown of the fastest families in the 74xx series. Unfortunately I need to stay 5v for this application. Thanks!

Answer 1

I think arbiter is not 100% correct with his ordering. HC and LS have similar speed, while S is pretty fast, even more than AC (ACT being the TTL-interface equivalent of AC, and consequently a little slower). I would order the families like this (faster families on top):

FAST (F) and AS
S
AC
AHC
LS and HC
74

Answering one of your questions, as you see in the list, AHC is not faster than F. Nevertheless, AS is, but they are more or less the same in regards of speed while F is better in terms of power, so AS is closer to obsolesence and I wouldn't recommend it for new designs.

There is a family faster than F called FASTr (of course), but it has only a few devices available, mostly line drivers and such. Probably the same happens with BCT (it does have few devices but I'm not sure if it is faster) or some of the other newer technologies.

I think F and AS are the faster families of general-purpose discrete logic (edit: I forgot about ECL, is it still around?). Arbiter has a point when he recommends moving to programmable logic if F is not fast enough (as long as your circuit is fairly complex, because if it is just a buffer or latch, consider FASTr). It is not only that modern programmable logic devices provide better delays than their discrete equivalents, but also that discrete circuits with speeds higher than those of F components would not be easy to connect reliably.

Answer 2

Roughly:

74ACT < 74ALS < 74S < 74LS < 74HC < 74

However, if you really want more speed, use PLDs or FPGAs to avoid the I/O delays that are inherent in the 74xx discrete device logic. Even a 22V10 will kick the pants off the equivalent circuits built from discrete NANDs, NORS, FFs, etc.

Answer 3

74LSxx series are high speed switching