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enter image description here

I saw this series of 4 circles in the schematic of an Arduino board. Similar circles appear in other Arduino schematics, though I don't think they are Arduino specific (which is why I post here rather than on the Arduino forum).
What do they mean?

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  • \$\begingroup\$ If you rigth click on them and show Properties, what does it say? I would assume they are PCB marks or something else nonrelated to the schematic. \$\endgroup\$ – jwsc May 13 '15 at 8:43
  • \$\begingroup\$ @jwsc: they're from a PDF document. I don't have the original format. \$\endgroup\$ – Joris Groosman May 13 '15 at 8:52
  • \$\begingroup\$ I'm totally sure that they have nothing to do with the electronic circuit. You see such things often to include some PCB position markers or BOM information in your schematic. nothing to worry about. \$\endgroup\$ – jwsc May 13 '15 at 8:57
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    \$\begingroup\$ @jwsc: Oh, I'm not worried, don't worry :-). Just curious. But wouldn't it be better then to have some comment there? Or is this commonly understood? (I'm not so fond of the quality of Arduino schematics in general.) \$\endgroup\$ – Joris Groosman May 13 '15 at 9:03
  • \$\begingroup\$ Can you post a link to the document that has those circles? \$\endgroup\$ – CL22 May 13 '15 at 9:17
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OK, I found an Eagle schematic file of this, and I checked the properties, like jwsc suggested. They seem to represent fiducials.

Comment: fiducials don't belong on a schematic, they're purely PCB-related. You don't place mounting holes on a schematic either. When they appear on a schematic they will most likely show up in the BOM as well.

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    \$\begingroup\$ I have seen that many times. Sometimes I find it quite useful to define a "mounting hole component" which I can place in a schematic. Granted, in the schematic itself it does not make sense, but you design your hole only once and don't have to draw it many times. Plus, you can include informations about the needed screws or spacing to other PCB components in your "hole component". \$\endgroup\$ – jwsc May 13 '15 at 9:32
  • \$\begingroup\$ Like you say, a mounting hole has no relation with a schematic. If you want to add details about mechanics, create a mechanical drawing. \$\endgroup\$ – Joris Groosman May 13 '15 at 9:54
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    \$\begingroup\$ Mounting holes can absolutely belong on the schematic. Depending on design and functionality they are often electrically connected to chassis ground and filtered or decoupled for EMI reason. \$\endgroup\$ – ACD May 13 '15 at 14:33
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    \$\begingroup\$ @ACD: then they're not just mounting holes, but pads. \$\endgroup\$ – Joris Groosman May 13 '15 at 15:36
  • \$\begingroup\$ I could be mistaken, but as far as I know, Eagle requires a schematic symbol for everything on the board. It could be blank, I'm pretty sure, but it may still affect auto-zoom and auto-scale/offset for paper/PDF printing. I'd love to hear that I'm wrong about this. \$\endgroup\$ – AaronD May 13 '15 at 22:42

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