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.
Thank you so much for the help in advance!
Electronic – Arduino Uno Schematic Parts
arduino unoschematics
Best Answer
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.