Arduino schematics understanding

Hello guys,
I'm new-ish to Arduino, and I wanted to see what hardware does it use. So I looked at the Arduino's schematics, and noticed some odd things. My problem is that I never worked with PCB design, I just know how to read basic schematics, containing just resistors, capacitors, coils, flip-flops, and small ICs (like 555). And what seemed odd to me is that there are several nets (circuits) which aren't all connected together. On this image I circled the separated circuits, and on the other is the whole image itself. There is a link down there for the schematics.

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And what seemed odd to me is that there are several nets (circuits) which aren't all connected together. On this image I circled the separated circuits, and on the other is the whole image itself. There is a link down there for the schematics.

There is nothing unusual about this schematic.
I don't know what you are trying to say. If a resistor is in a network, it is not necessary to show traces going to the
network. You need only identify the resistor network component designator and resistor number. As for all the
other circuits it is clearly marked where they go. All you have to do is read the label. If you have a specific example
please explain what you think is unusual because I don't see any issues with this schematic. All arduino schematics
use the same conventions. The issue would seem to be that you simply have not seen enough schematics.

And what seemed odd to me is that there are several nets (circuits) which aren't all connected together.

This is good practice - in programming its called "separation of concerns". The ancilliary components
are drawn separate to the main circuit to keep it uncluttered - here its power supply, decoupling, crystal
oscillator and indicator LED sections.

I think the duplication of oscillator sections (and regulator) merely reflects the fact there are
several different footprints on the PCB (this is commonplace to allow a wider choice of component
selection without having to re-spin the whole PCB during a production run)..

Named nets are used to show how things connect up.

Arguably this circuit should be drawn in smaller pieces still, its pretty common to separate out
every connector for instance.

If you want to see how appauling an old-fashioned circuit diagram can be, then
checkout the circuits given in old stereo amp service manuals, a total mess of connecting
lines running all around the place... Impossible to read at a glance (good circuits
can be read in a couple of seconds to the practiced eye).

Maybe you are confused about the +5V connections (arrow pointing up) and grounds? All of the 5V connections are connected together (and the grounds are connected together) so those sections are not isolated.

It's also common for data and address lines to be "labeled" so we don't have to show 8 or 16 (or more) parallel lines running all over the schematic. Some other connections may also be "labeled", and sometimes it's hard to "find" everywhere a connection goes, especially if the schematic has several pages.

@MarkT thank you. Yes, that was what I was thinking, that there is probably some label in the main circuit, though I didn’t see any, so I figured that there are none. Now I looked a bit more carefully, and yes, there are labels. Though I still can’t see any point in separating two resistors from the main net. Never mind, thank you.

P.S. Thanks to the others too, for the shown good will

Though I still can't see any point in separating two resistors from the main net. Never mind, thank you.

If you had more experience you would realize those two resistors are separated to inform you that they are
UNASSIGNED and therefore AVAILABLE for use for whatever you might want to use them for.

You might find this worth a read :

no_named_guy:
Though I still can't see any point in separating two resistors from the main net. Never mind, thank you.

If I understand it right there is a single package containing 4 resistors on the board. Two of them are used for USB while the other two are unused. I don't know why such package is used instead of two discrete resistors. Maybe for a pick and place machine to have larger package to handle?

The part mounting fee is per part and the packs are cheaper than 4 discreet parts plus they only get
charged for one part instead if 4.

Maybe they were charged by the component for installation? Or the packaged allowed for better placing & routing of signals?
RN3 is the 22 ohm resistors for the USB D+/D- lines. The 2 leftover would not really be usable, 22 ohm is not good for much in the digital world, its location between the USB connector the Atmega16U2 makes it awkward to route to & from, and you wouldn't want other signals to cause interference on the USB data lines.

One resistor network package vs two small resistor parts?
22 ohm are available in multiple sizes
https://www.digikey.com/en/products/filter/chip-resistor-surface-mount/52?s=N4IgjCBcpgbFoDGUBmBDANgZwKYBoQB7KAbRAGYBOABgHYAOEAqsAVkecoCYAWeAXQIAHAC5QQAZREAnAJYA7AOYgAvgS7V6rBCGSR02fEVIguXAAQB5ABYBbLCEEhR4gKrzZIyygCyONFgArtI4qgQAtFw6ejKBRsSQZNoElI7CYpAgqmqmJmbmhHaOKkA
As are Isolated resistor networks
https://www.digikey.com/en/products/filter/resistor-networks-arrays/50?s=N4IgjCBcodIMZQGYEMA2BnApgGhAeygG1wAWANgFZTKQBdPABwBcoQBlZgJwEsA7AOYgAvngCcUUIkipMuAsRABmcmEoAmCAxAs2nXoJF5yEuNNnY8hSCVIAGMUsf0mrSB278ho8GIAcplLI6JYKNiB2LjpuHgbeeOpKYKSSCMFyVorqdknkAOxRuu76XkYgALTqqdLcAK7y1iS04oUxZeWBaZB1DYparmwiwsJAA
Both seem to have a minimum price of $0.1, so two 22 ohms would be twice the price of one 4x22 ohm, and at the volumes these boards are made at that can add up to some savings.

RN4 is 1K current limit resistors for the Rx and Tx LEDs, and they have 2 in parallel for the 500 ohm equivalent the Green Power LED so it is brighter.

Pick N Place machine would not care, they can do 0201, 0402, 0603, 0805 parts readily enough.
I won't go smaller than 0805 for boards we hand place parts on. Others can handle smaller parts easier than we can. And if you drop one, forget it - one bounce and they disappear!

Smajdalf:
If I understand it right there is a single package containing 4 resistors on the board. Two of them are used for USB while the other two are unused. I don't know why such package is used instead of two discrete resistors. Maybe for a pick and place machine to have larger package to handle?

No, the reason is simply cost reduction - quad-resistors would simply be used by habit in laying out
digital circuitry where many resistors can be the same value - PCB board area and component count
contribute to manufacturing costs, often more than the components themselves, so reducing the
number of components by using multiple versions is standard practice, be it resistors, diodes, transistors.
You can get triple diodes and dual transistors in SOT236 packages for instance. The cost of a dual transistor
is basically the same as a single transistor, as the costs are mainly packaging and testing, not the silicon
itself.

CrossRoads:
RN4 is 1K current limit resistors for the Rx and Tx LEDs, and they have 2 in parallel for the 500 ohm equivalent the Green Power LED so it is brighter.

Annoyingly so!

Annoyingly so!

Don't look directly at the led....

One could always cut one of the parallel resistor traces on one side the resistor pack or the other

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