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I found the symbol:

enter image description here

It is available on:

https://www.arduino.cc/en/uploads/Main/arduino-uno-schematic.pdf

What is it?

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Probably solder pad jumpers - you should be able to apply a "blob" of solder across pins 1 and 2 to produce a short circuit hence, the one you show might give you the option of adding an external capacitor to a reset pin.

But if you really want to be sure, open the EAGLE PCB file associated with this EAGLE schematic and look and see.

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  • \$\begingroup\$ Thanks for your answer. I have more confused why this "blob" parallels with a wire-it is already short pad 1 and pad 2. I found this symbol appears 3 places on this schematic. \$\endgroup\$ Jan 6 '21 at 12:55
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    \$\begingroup\$ @PHỦĐỗTường The board designer routes that trace in the middle of the solder pad (from one to the other) so that it can be cut when needed. It is just shown separately in this case, you can find that symbol with a "normally closed" jumper as well. \$\endgroup\$
    – Ron Beyer
    Jan 6 '21 at 12:59
  • \$\begingroup\$ @PHỦĐỗTường did you find the PCB footprint for the part? \$\endgroup\$
    – Andy aka
    Jan 7 '21 at 8:57
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    \$\begingroup\$ This is 99% the correct answer. I have heard the term "coffe bean" for those kind of jumpers, due to their similarity with it. \$\endgroup\$ Jan 7 '21 at 15:15
  • \$\begingroup\$ @ Andy aka, I saw. Thank you so much. \$\endgroup\$ Jan 8 '21 at 3:50
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it is this on the PCB.

enter image description here

a pads to solder the connection back if you needed to cut the trace before to disable the automatic reset on USB connection

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  • \$\begingroup\$ Hi @Juraj, I have one more question. I found this symbol appears 3 places on this schematic, but I just see the only one as your picture. \$\endgroup\$ Jan 6 '21 at 14:16
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    \$\begingroup\$ @PHỦĐỗTường, the other two are between the ATmega16u2 and its ICSP header as pair of solder points \$\endgroup\$
    – Juraj
    Jan 6 '21 at 14:30
  • \$\begingroup\$ I see. Thank you so much. \$\endgroup\$ Jan 6 '21 at 14:34
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Those are 2 pads on the PCB that are close to each other.

On an official Uno board, there is a trace routed between these pads that you can cut to break the signal, and if you want to restore connectivity between the nodes, you just solder a blob onto those pads.

You need to cut it if you don't want to uno to restart when you attach the USB cable

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  • \$\begingroup\$ Thanks for your answer. I have more confused why this symbol parallels with a wire-it is already short pad 1 and pad 2. I found this symbol appears 3 places on this schematic. \$\endgroup\$ Jan 6 '21 at 12:55

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