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I am trying to determine what two parts of the Arduino schematic are (I am quite new to reading schematics). I have circled the areas of confusion. Please let me know what these components do and whether they are necessary in the design as a whole. enter image description here Thank you so much for the help in advance!

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  • \$\begingroup\$ Note that there is a PCB design for the Uno Rev3 on the Arduino site: store.arduino.cc/arduino-uno-rev3 , under "documentation". Could be helpful. \$\endgroup\$
    – ocrdu
    Commented Dec 1, 2020 at 22:56

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C1 is presumably bypass capacitor on the 5v net, there are many existing questions here on bypass capacitors. Bypass capacitors should be placed close to the supply pin pair of the target IC, unfortunately the location of that is not specified, so you may want to find it on an actual board.

RN3 and RN4 are unused parts of a multiple-resistor package. The designers chose to use such a multi-element part rather than individual resistors, but they didn't need all of the elements. Those two do nothing. The other two elements are used in the USB data lines to the ATmega8U2, which can be a bit "touchy". And of course the ATmega8u2 is itself in a QFN package which requires a bit of layout and assembly experience.

The upshot is, if these are unfamiliar it may be premature to try to duplicate an entire Arduino Uno. You might instead want to look at one of the Arduino project's published designs / tutorials for more minimal functionality, with the USB solution outboard (eg, the "Arduino on a Breadboard tutorial) or using an older part like the FT232RL as their previous board did.

Don't forget you'll need to get the bootloader into the main ATmega, and if you do use the ATmega8U2 as a USB solution, you'll need to get firmware into that, too.

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  • \$\begingroup\$ Do you know if C1 is necessary for the board as a whole? \$\endgroup\$
    – Alex88
    Commented Dec 1, 2020 at 22:35
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    \$\begingroup\$ "Necessary" is a funny term. First you'd have to identify where it is physically situated, as bypass capacitors exist to be a physically local reservoir of charge. \$\endgroup\$ Commented Dec 1, 2020 at 22:37
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    \$\begingroup\$ Might be worth noting that C1 is bypass for the power (pins 4 and 8) of U1 to the right. \$\endgroup\$ Commented Dec 1, 2020 at 22:39
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    \$\begingroup\$ @TomCarpenter good find. The upshot of which is that C1 is useful if one uses this sort of power switching scheme which U1 is there to enable, and not in a simplification that doesn't have that capable or by extension U1. \$\endgroup\$ Commented Dec 1, 2020 at 22:52
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C1 is bypass for U1 power supply, which is illustrated with the floating pins to the left of C1. They have chosen to draw the power supply pins of U1 dual op amp separately, as some like to do, so that they can have the omp amp symbols exactly the same. Having exactly the same symbols will allow you to swap the ports, so that it connects to different pins on the physical component.

Resistors on the bottom right are part of a resistor network that were un-used. They are on the board though, physically connected to the rest of the resistors in the network. In the schematic it makes sense to draw the resistors as if they were physically separate, to allow more readable schematic.

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