Electronics > Projects, Designs, and Technical Stuff
Detailed logic family differences? LS vs HC vs AH etc.
Zero999:
--- Quote from: gorplop on February 17, 2020, 02:23:16 pm ---So would a 74HCU04 as a ring oscillator made from 3 inverters have more predictable frequency?
--- End quote ---
Only if you put an RC circuit between each stage to slow the propagation delays down to a known time constant. If you just connect three gates together, the frequency would be no more predictable.
The problem with the 74HC04 is, in a design such as a crystal oscillator, when all that's required is an amplifier, the parasitic ring oscillator can select the frequency, rather than the crystal. The 74HCU04 only has one stage, thus less gain and no ring oscillator, so it should oscillate at the crystal's resonant frequency.
David Hess:
A 2 or more input gate will be more representative of how input structures vary between logic families because multiple inputs are handled in different ways.
Older CMOS databooks include transistor level schematics.
T3sl4co1l:
It's been my experience that inputs within a gate, tend to share ESD structures, so there's a small amount of hFE between inputs. The hFE to one or the other supply is usually much more significant, though (enough to cause latchup above, say, 100mA in 74HC family -- coincidentally, these are rated "free of latchup below 100mA" :) ).
I don't happen to have any >2 input gates handy, so I don't know how it varies between adjacent or distant inputs. Whole gates seem to be separate enough they have no measurable interaction.
This isn't useful for the most part, but a case where it might be of interest is a low power, gated RC oscillator: you might be using a schmitt NAND with an RC feedback, and the other input to enable/disable it. If the /EN pin is subject to current injection on the order of 1mA, the other pin will experience leakage currents on the order of 30uA, skewing timing.
Another consequence is, injected current is actually shunted to the opposite rail, as much as it's clamped. The ESD diodes are actually emitters of parasitic BJTs, and with hFE ~ 1 they effectively cascode about half the current to the opposite supply. (Or was this more like 0.3, I forget; I've got the models on my website.) So injected current isn't just shunted to the respective supply, it's actually increasing total chip dissipation!
Tim
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