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I'm talking about the 3 boxes that form a column, near where it says 5V on the top of a lead and 3V below it. The lead comes out of the middle box.

The datasheet for the AT90USB1286 mentions nothing about this, only that there should be 22Ohm resists between D- and D+ (check) and a 1uF cap to UCAP (check).

USB1 supplies 5V (default usb power).

teensy schematics real teensy

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It is a jumper with 2 options. It can be implemented as SMD pads or a pin header. Only one option should be populated.

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  • \$\begingroup\$ Are you sure? I added a picture of the board and I can't see a jumper on it. Also wondering how 5V turns into 3V. (edit: I think I get it. If it is a jumper, you connect the boxes near "3V" if you connect the board to a 3V source, and you connect the boxes near "5V" if it is connected to a 5V source) \$\endgroup\$ – AnnoyinC Jun 29 '20 at 22:24
  • \$\begingroup\$ yes, the 5V turns into 3v via the ATIny. The UCAP pin is an output of the internal 3v3 regulator. If you wanted to use 3v3 for the AVcc reference you would set the jumper to this position \$\endgroup\$ – JonRB Jun 29 '20 at 22:26
  • \$\begingroup\$ Ah, I see it now! Indeed chapter 31 lists UCAP output voltage. Thank you for clarifying. \$\endgroup\$ – AnnoyinC Jun 29 '20 at 22:30
  • \$\begingroup\$ The pins are on the bottom side of PCB. Turn it upside down to see them. \$\endgroup\$ – Justme Jun 29 '20 at 22:44
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By default the Teensy 2.0++ (along with the rest of the Teensy line) uses the micro-USB port for power (5V DC). The boxes on the schematic refer to pads on the bottom of the board that can be used to select between three power options:

  • 5V power from USB (default)
    Two of the pads are connected by a small trace.

  • Battery power
    Battery power (as low as 2.6V for the 2.0++) can be fed via the VCC and GND pins. To do this the trace between the pads must be cut:
    Cutting the trace
    Source: PJRC.com -- The image above shows a different Teensy board but the 2.0++ is similar
    This page goes into more detail including adding diodes to switch automatically between USB and battery power.

  • 3.3V operation
    The Teensy 2.0++ can also be modified to run at 3.3V. To do so, a 3.3V voltage regulator (such as the MCP1825S-3302E/DB) must be soldered to the three pads under the USB jack. The trace must be cut and the center and formerly-unconnected (3V) pads bridged with a blob of solder:
    Teensy modified for 3.3V operation
    Source: PJRC.com -- The image above shows a different Teensy board but the 2.0++ is similar
    This page goes into more detail on this option.

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