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I have a few questions about the Arduino Micro circuit:

  • If I can provide a steady 5 V source (connected to +5V), then I don't need to use the voltage regulator module (including the selector) of the circuit. Correct?
  • If I can provide a steady 5 V source (connected to +5V), then I don't need to connect anything to VIN. Correct? If that's right, should I connect it (VIN) to GND, instead of leaving it floating?
  • If I'm not using +3.3V, should I connect it to GND (instead of leaving it floating)?
  • If I'm powering off of just USB, then I can connect +5V to VUSB. Correct?

Schematic below for reference:

Arduino Micro Schematic

  • Is this a question about circuit design, as in: designing your own board which is nothing to do with a Arduino Micro? – Nick Gammon Oct 23 '16 at 1:17
  • Yes and no. I will be making the Arduino Micro circuit, but without the regulator module and voltage selector. But I want to make sure that this is actually possible to do, before I move on it. – Max Jacob Oct 23 '16 at 21:47
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If I'm powering off of just USB, then I can connect +5V to VUSB. Correct?

If you are powering off USB leave everything alone. See the schematic:

USB input

The blue line added by me shows what is going on. The VBUS signal from the USB connector goes through a 500 mA resettable fuse (to protect the USB port from overloads). Then it goes to the P-channel MOSFET (FDN340P) which makes a decision about whether to switch on and connect VUSB to +5V signal. It does so if VIN is low, making the MOSFET conduct. If VIN is high (which it would be if you had power into the VIN part of the board) then it disconnects the USB from the +5V line.


If I'm not using +3.3V, should I connect it to GND (instead of leaving it floating)?

Definitely not. The 3.3V line is an output from the 3V3 voltage regulator. To connect it to ground would heat it up and destroy it.


If I can provide a steady 5 V source (connected to +5V), then I don't need to connect anything to VIN. Correct? If that's right, should I connect it (VIN) to GND, instead of leaving it floating?

No, VIN is already connected to ground via a 10k resistor (see schematic above) and thus it is not floating.

  • The picture helps a lot in terms of clarification! So if we take out the selector circuit, then I would have to connect +5V and VUSB manually, correct? – Max Jacob Oct 23 '16 at 21:48
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    You might want to leave the fuse in, but yes, that sounds right. You could omit the 3.3V regulator too if you don't need 3.3V. The standard for USB is 5 V ± 5% which is within spec for the processor. I have made plenty of boards up where I just connect up a source of 5V to the Vcc pin of the processor chip. Make sure you leave in the decoupling capacitors on Vcc and AVcc. – Nick Gammon Oct 23 '16 at 23:23

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