I am currently trying to learn about circuit boards and microcontrollers. On the Arduino webpage I found these schematics of the Arduino Uno. But I am not sure, how to read that.

There is a big circuit with the Atmega chips, everything is wired and connected with several other components. On the top there are smaller circuits that in the drawing are not connected to the rest. How do I read that? Are there connections omitted?

  • \$\begingroup\$ Give this a look, might be helpful. Check this one for SPI definitions of things like MISO/MOSI/SCK. \$\endgroup\$ May 4, 2012 at 21:10
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    \$\begingroup\$ To be blunt: This is a very poor quality schematic. They have made it monochrome, but have left out many values, made labels cross eachother, placed things all over the place. Try to find a better Arduino schematic, even if from a clone, you'll thank yourself and feel smarter not having to travel through that muck. :) \$\endgroup\$
    – Tom Paris
    May 6, 2012 at 4:24
  • \$\begingroup\$ First, you have to be offered some money to read those schematics. Let's say you are offered 100 bucks, which I would only accept for a thousand. Then, you go and exercise your neck muscles, which you will need a lot in this marathon. Oh, and don't forget to flex them, flexibility is going to be your only friend in this journey. Then switch your phone off, shut the door -you have to be alone for about a day long. Congratulations! Now go read this and start cursing the people who draw those schematics. \$\endgroup\$ May 8, 2012 at 8:00

2 Answers 2


This is a horrific schematic and I can't blame you for being confused.

"Are there connections omitted?"

Yes, one would wonder if they forgot to connect them. They do have a net name next to them, however, and you'll find that same net name back elsewhere on the schematic. Everything with the same net name is connected. Personally I prefer the use of ports to indicate this, so at least it's clear it's not a forgotten connection.

enter image description here

I commented on the following part of the schematic in another answer:

enter image description here

Here there's too much unnecessary information shown, and required information is missing. It's probably not clear on first sight but that "TS42031-160R-TR-7260" refers to the switch at the top, just like the word "RESET". That's the reset switch, but it would be much clearer if it only showed the refdes (that's short for reference designator). Each component has a unique refdes which identifies it. For a switch it could be SW1. The long type number is completely irrelevant to understand the schematic and it shouldn't be here. Your schematic should only show basic information, like "R22" as the refdes for a resistor together with its value, like "12k". That should do. Whether it's a 1/4W or 1W is not important to read the schematic, so leave it out. This kind of information, just like the manufacturer's ordering code belong on the BOM (Bill of Materials) which you want to have next to your schematic in case you need the extra information. The question about the power of the resistor may come up in a design review: check the BOM.

enter image description here

This may be the worst part of the schematic, and you'll only understand it with a lot of experience and some guessing. It doesn't say anywhere, but these are the power connections for the dual opamp U5. It seems to be the only IC for which no power connections are shown, and a dual opamp in an 8-pin package often has its power on pins 4 and 8. (That's where experience comes in).

All in all, this is not a good example to learn reading schematics.


It can sometimes be annoying, but you have to realize that mostly all signal lines have a label attached: +5V, Vin, RESET, SCK, USBCCC ...

All lines with the same label are one and the same conductor and therefore attached to each other.

So every line labeled RESET is connected to every other line labeled RESET ... etc.

  • \$\begingroup\$ ok, thank you so far. Do the numbers also count as labels? e.g. in the upper left, there are non-connected lines labelled with 4 and 8. Do I have to search for the other empty 4 and 8 lines? Same question for the parts on the far right, these are labelled by 2,3,6,7 \$\endgroup\$
    – Till B
    May 4, 2012 at 21:25
  • \$\begingroup\$ Not sure. I think the number 2, 3, 6 and 7 relate to a resistor array (RN3 being a 22 ohm array), I don't thing these are attached, but there are other RN2 resistors (ATmega pin 10 an 11). Numbers 4 and 8 relate to pins on an IC (probably UA5A and UA5B). \$\endgroup\$
    – jippie
    May 4, 2012 at 21:32

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