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I'm currently working on a project using an ATMega328P and the V-USB library. V-USB is meant to be USB 1.1 compliant, and the documentation mentions that 3.3V is expected on the D+ and D- lines by the host.

I want to run my microcontroller off the 5V supply that USB provides. I need 5V to guarantee it can operate at 20MHz. However, this would mean that the D+ and D- lines would be communicating at 5V.

Can I do this or do I risk damaging my USB port? If it's an issue, what would be the best method of shifting the voltage levels? The wiki page below has some solutions but I'm not sure what would be best.

Thanks!

V-USB Wiki - Hardware Considerations

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The V-USB wiki itself as you say provide a simple solution to this: Level shifting the data lines via Zener diodes. It is not a perfect solution, but I believe it's definitely better than simply putting 5V on the data lines. It might not damage your USB ports, but it is definitely out of spec, and there is no reason to design out of spec on purpose.

V-USB suggested design

For details, check http://vusb.wikidot.com/hardware, the "Solution B"-section, which is also where the picture above comes from.

What the best solution is will also depend on what the critical elements of your circuit are. You have already mentioned that the MCU must run at +5V, so dropping the voltage there is ruled out. You could probably use a level shifter IC instead of Zeners to drop the voltage at the data lines, but that would again probably cost more.

These are design decisions you need to make depending on what you will prioritize about your design. Good luck!

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  • \$\begingroup\$ Ok, do you think a pair of transistors with a switch mode regulator could work also? Zeners diodes are cheap but the wiki mentions that the results might be a bit unpredictable at low currents. It would definitely cost more but I was wondering if it was an option. \$\endgroup\$
    – Nick
    Commented Sep 26, 2013 at 17:12
  • \$\begingroup\$ I don't think so, as USB is a bi-directional (half-duplex for 2.0) bus. Depends how you implement it, though. You could get a level-translating IC that translates both ways, I guess. \$\endgroup\$
    – Araho
    Commented Sep 26, 2013 at 23:52

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