From Computer History Wiki
Jump to: navigation, search


My digital design friends tell me there's a distinction between latches and flip-flops. Larsbrinkhoff (talk)

Your friends are right. Wikipedia uses the confusing 'a flip-flop or latch is..', which is easy to interpret to mean that 'latch' is just another word for 'flip-flop', which is not what is meant. There is a distinction, although there's overlap (b/c there are different types of latches). And when I design something I think of latches and flip-flops as entirely different things (e.g. flip-flops to divide or fix clock signals, latches to buffer/hold data). (Here's a web page which talks about flip-flops vs latches): Tor (talk) 09:35, 12 March 2018 (CET)
Thanks. Then maybe the redirect from Latch to Flip-flop isn't entirely appropriate. Although, it might be better than nothing. Larsbrinkhoff (talk)
In current terminology, the two indeed refer to sets of things which do not overlap. However, after thinking about it (which I hadn't done before :-), I still think it might be appropriate to cover them in one article:
  • They are both devices which use feedback to store information
  • Separate articles would therefore have a lot of duplication
  • Historically (and this is a history wiki :-), the terms were not so carefully delineated; e.g. in Pfister's book (he's the guy who came up with the whole SR/D/JK/T nomenclature), he talks about "R-S flip-flops"
I will definitely try and make clearer (than it already is) that in contemporary terminology, the two terms refer to disjoint sets. See what you think... Jnc (talk) 14:15, 13 March 2018 (CET)
Also, the division into flops/latches is not necessarily recent. I looked at the TI TTL data-book, from 1976, and they make the latch/flop distinction there. Jnc (talk) 18:10, 13 March 2018 (CET)
I talked to another friend, and he was vehement that they were the same thing. So clearly it's unclear. :-) Larsbrinkhoff (talk)
Yup. In fact, I just recently had confusion talking to my partner on the QSIC project about state storage devices - as part of clearing it up, he introduced me to the 'transparent' terminology.
But I had a bit of hard time compressing it all into short text. The issue was S-R device, which doesn't have a 'data' input, or a clock, and is not (strictly speaking) 'transparent'! It's too primitive! And the J-K flop, which doesn't have a data input, didn't help! Eventually I resorted to saying 'flops are now so-and-so, everything else is latches'! Jnc (talk) 18:10, 13 March 2018 (CET)
Looks pretty good now! Tor (talk) 09:24, 15 March 2018 (CET)
Alas, in reading about various kinds of flip-flops in the TI TTL reference book, I discovered things are even more complex than I realized! See, e.g. the 7473/6/8 and 74LS73/6/8 - which have different clocking! So I've now added a section which attempts to give a general, systematic framework for understanding flops/latches: when they look at the inputs, etc.
I hope I've made things better; the text may be more complex now, but alas the reality is complex, and I think I have captured things in a way which makes the complexity easier to grasp. But I'd be interested in feedback! Jnc (talk) 09:52, 15 March 2018 (CET)