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74LVX245 -- what the heck?!

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ebastler:
Here's a trap for young players -- and, as I just demonstrated, not-so-young ones too...

The 74LVX logic series' common trait is "low voltage operation, 5V tolerant inputs". They run from a single 3.3V supply, produce the expected full swing output (0 to 3.3V), and accept input levels up to 5V without complaining. So they are convenient to interface 3.3V circuits, say FPGAs, to legacy 5V logic environments. There's the usual set of 74 series logic gates etc., which provide 5V tolerance on all their inputs.

And then there's the 74LVX245, an octal bus transceiver... According to this older ST Micro datasheet, it behaves just like all its siblings: https://www.st.com/resource/en/datasheet/CD00001340.pdf. But today, only ON Semiconductor (ex Fairchild) makes the LVX series, and theirs are like this: https://www.mouser.de/datasheet/2/308/MC74LVX245-D-1811198.pdf

Notice that little "application note" on the front page?

--- Quote ---A Parasitic Diode is Formed between the Bus and VCC Terminals. Therefore, the LVX245 cannot be Used to Interface 5.0 V to 3.0 V Systems Directly.

--- End quote ---

Cannot?! What the heck -- isn't that the whole reason to exist for this logic family?! Inspection of the "recommended operating conditions" reveals that only the two control lines are 5V tolerant on this particular chip, but the 8-bit bus is not. Guess what -- it's the bus I wanted to interface to the outside world...  :palm:  From personal experience I can now confirm that the chip does indeed not handle 5V on those inputs well, but will let them raise the 3.3V supply voltage -- thank you very much!

Does anyone happen to know the back story here? Was this chip meant to perform as one would expect (and as formerly advertised by ST Micro), but Fairchild got the implementation wrong and had to limit its advertised functionality? It's hard to imagine that they intentionally designed such a compromised thing?!

By the way: I did discover the 74 LVC series from TI and Nexperia in the meantime, and am hoping for a happy ending...
Edit: And it seems ON Semi also makes the 74 LCX series, which also fixes this problem.

magic:
Is the bus bidirectional by any chance? Then you have your answer - it's not the inputs that have the diode.

ebastler:
Yes, it's a bi-directional bus driver. (But it misbehaves also in an application where I use it strictly uni-directionally, to translate eight inputs from 5V to 3.3V.)

I agree that the bi-directional capability of the bus is probably behind the limitation ON Semi designed into this circuit. But why didn't they get it right?! Did ST Micro have a better implementation, or did they never realize the problem? Why can TI and Nexperia make their 74LVC245 perform as one would expect (hopefully)?

magic:
I suppose it's the body diode of the output PMOS. If LVC doesn't have this limitation then maybe they found some way to disconnect it when the bus is in input mode, bus as soon as you switch to output I presume there will be problems if the other end still drives high.

ebastler:

--- Quote from: magic on May 12, 2020, 06:08:56 am ---I suppose it's the body diode of the output PMOS. If LVC doesn't have this limitation then maybe they found some way to disconnect it when the bus is in input mode, bus as soon as you switch to output I presume there will be problems if the other end still drives high.

--- End quote ---

Actively driving the outputs (to 3.3V) while the host still applies 5V is nasty, of course. But when you just switch the direction to output while not enabling the outputs yet, Nexperia explicitly states their 74LVCs will handle 5V just fine: https://www.mouser.de/datasheet/2/916/74LVC_LVCH245A-1389000.pdf

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