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I am attempting to retrofit an LCA710 solid state relay into an exiting PCB that has a mechanical relay. I will need to cut some traces and run some fly wires, but there is one IO line that would "pass across" pin 3, marked "do not use".

Would this pin be a dead pin inside the IC package? Or, is it likely connected to something that would cause problems? Does anyone have any idea if it would be OK to use this pin as a junction point?

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    \$\begingroup\$ When it says "do not use", don't use it. Seems pretty clear to me. \$\endgroup\$ – Olin Lathrop Nov 30 '12 at 21:59
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Unless you can get the manufacturer to make a statement on the subject, I'd assume the worst and not go near it. It may well be dead, but it could just as easily be connected to ground or something.

If you're cutting traces and running wires anyway, is there a reason you couldn't just cut the pin off of the DIP before installation? That would let you use the existing point on the board as a junction without having to worry about what's in the DIP package.

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  • \$\begingroup\$ Fantastic suggestion. Simple and effective! Thanks! \$\endgroup\$ – TDL Nov 30 '12 at 21:28
  • \$\begingroup\$ If they didn't want you to cut it, they would call it "do not use or abuse". :) \$\endgroup\$ – Kaz May 23 '14 at 23:45
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UPDATE

As I predicted, the pin is not connected internally as confirmed by Clare/IXYS:

The pin is floating so there is no internal connection.

Best regards, Anthony Konopka IXYS Integrated Circuits Division 78 Cherry Hill Drive Beverly, MA 01915 Tel: (978) 524-6742 Fax: (978) 524-4700 E-mail: akonopka@clare.com

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It's difficult to say. The datasheet doesn't offer any insight. I wrote to tech support. I'll update my answer if they respond.

The pin may be:

  • used for functional testing at the production facility and therefore connected internally to something that might interfere with your signal

  • it might be left unconnected (floating)

  • it might be shorted to another one of the pins (usually some type of reference like "ground")

If you can accept destructive modification to the part, just clip the pin as suggested by @MichaelKohne. If you can't, you are going to have to mitigate the risk. This will involve doing things that are outside best practices for mass production and high-reliability applications, however, they may show that you can do what you wanted in the case of this one unit in your home project for your personal use (where you can competently accept the risk):

  • test for cross-pin connectivity with an Ohm-meter

  • short the pin through an ammeter and look for current

  • apply voltage to the pin and see if there is current flow

  • power up the relay and operate normally, monitor the pin with an oscillioscope to look for any drive behaviors.

If I had to guess, I would say that it is probably fine. I'm not saying to do it blindy. I'm saying test it... and if I had to speculate as to what your tests will show, I would guess that they show it's ok to connect. I will clarify the reasoning:

  • You imply by your question that you want to use the pin (so let's see if we can make that happen for you!). If that isn't your intention then this is a silly question. Do not use a "do not use" pin doesn't require asking on SE, so I consider those responses (the ones that don't include reasoning other than "the manufacturer says so") to be irrelevant.

  • DNU can mean NC (and often does). DNU can be synonymous with "reserved for future". Since future part revisions are not a concern on a 1-off project, it's ok in my judgement if it clears the electrical test hurdles.

  • There are no countermanding standards that I am aware of. The meaning and application of these terms are manufacturer/part specific. If you look at similar parts from this manufacturer the trend is to cite "do not use" whenever the number of functional pins is exceeded by package pin count. Having designed many dies, I believe that it is unlikely that each of these parts has a hidden test access mode requiring a unique dedicated pin.

  • BUT... I may be wrong. It's my decision calculus. If you have a different one. That's fine. That is why I was careful to identify that this last part is just my speculation and why I contacted the manufacturer for clarity.

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  • \$\begingroup\$ I will be interested in what you find out. In the meantime, I am breaking out the cutters to implement the suggestion from above. \$\endgroup\$ – TDL Nov 30 '12 at 21:29
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    \$\begingroup\$ My experience says that on average it is not "probably fine" to use pins marked NC or DNU. Even if you get away with it, you might not on the next rev of the chip or the same chip from a different manufacturer. \$\endgroup\$ – user3624 Dec 1 '12 at 1:11
  • \$\begingroup\$ @DavidKessner -- I agree with you "on average". I was referring to this specific scenario having reviewed this part's datasheet and a few similar parts from the same manufacturer (Clare/IXYS) in which it was clearly a no-connect internally. Given that his story is about a 1-off, I don't think TDL is concerned with life-cycle variation. \$\endgroup\$ – DrFriedParts Dec 1 '12 at 5:20
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    \$\begingroup\$ DV because "do not use" means do not use. "No internal connection" means no internal connection. If this advice is specific to this part, include it in the answer and say why you think this part is special. Future readers might think this advice applies to other parts as well. \$\endgroup\$ – The Photon Dec 1 '12 at 5:46
  • \$\begingroup\$ @ThePhoton -- Thank you for the feedback. DNU can mean NC (and often does). DNU can be synonymous with "reserved for future". Since future part revisions are not a concern of TDL's 1-off project, it's ok in my judgement. TDL is smart enough to have considered not using it. He's asking, is there any possible way to use it because it would very convenient for him. My answer includes procedures to mitigate the risk of an undocumented pin and is specific to the pin and part in question. There is nothing incorrect in my answer; will edit as requested to improve. \$\endgroup\$ – DrFriedParts Dec 1 '12 at 7:19
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I suggest you do not use a pin marked "do not use" in the datasheet.

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