When I was looking through the SMD Uno schematic, I noticed this solder jumper next to the Atmega 16. Does anyone know what it is for?
Are you sure…
That’s not DTR reset, it’s separating the USB ground shield for some reason ?
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@lastchancename sorry, I almost immediately deleted my comment
Yes, it seems a little weird to me.
On all examples of powering it in the data sheet usb ground is connected to chip ground. This jumper is open by default on PCB? It would make sense to have normally closed jumper with an option to cut it
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I'm not 100% sure actually. But the normal Uno (ie. not SMD) also has the same thing.
I don't have access to a Uno or I'd check that.
Agreed.
The Uno I have here uses a ferrite inductor to connect the USB shield to circuit ground. The inductor is tucked into the space between the USB connector and the reset button.
Measuring it with an ohmmeter will show a resistance near zero, but because it is an inductor it will block high frequency noise on the shield, thus keeping it out of the Arduino ground.
Oops. Looking at the Uno Schematic, it appears things are a bit different than I had thought.
The shield is not connected directly to the Arduino ground with that inductor. Instead it connects to the UGND pin on the 16u2. There is a separate jumper set for intentionally connecting the USB ground to the Arduino ground.
Given that the Arduino is powered from the USB port, there pretty much has to be a connection between the USB ground and the Arduino ground somewhere. I just can't seem to find it on the schematic. Possibly it's via an internal connection inside the 16U2
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And that is the jumper I was asking about in my first post?
I found something really strange about the second solder bridge (near the reset circuit).
It looks like a waste of space, simply because there is a track right next to it, performing exactly what the solder bridge should be doing/not doing.
This also seems really messed up.
On my Uno, there are two solder pads with a tiny trace connecting them. One of the pads is connected to the USB ground, the other to the Arduino ground. That's the connection I couldn't find in the schematic.
The reason that they do it that way is that there are a few times when you don't want the USB ground connected to the Arduino ground. Not many, but they are there. With the pads set up the way they are, the default is USB ground connected to Arduino ground, which is what is needed 99% of the time, including all times when the Arduino is powered from the USB port.
If you are using an Arduino in one of those rare cases where that connection is a problem, you can take a sharp knife and simply cut that trace between the pads. Later, when you want to use the Arduino for something else, you can restore the connection by putting a blob of solder on the pads to short them together.
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Wow, I wouldn't have guessed that that would be a problem! Wonder why it is?
I guess ground loops and or different voltage levels of ground (for instance if you have cathodic protection of a metal structure). Where you use this protected structure as ground at the other end of the usb line.
The USB shield and the USB ground signal are potentially (pun!) separate.
IIRC, there are some environments/purposes where you're not supposed to connect both ends of the shield. (whether any particular USB cable is properly constructed is another thing entirely.)
See also https://forum.allaboutcircuits.com/threads/usb-device-cable-shield-connection-grounding-it-or-not.58811/
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