USB wiring with ATMega32u4 ok?

Other Hardware 3rd Party Boards
1

Hello!

This is my first layout with USB, so I'm not quite sure if this would work... The thing I worry about most is the different power source of USB and the logic. But this is intended to be a board with an RGB LED Matrix, so I'm not able to take all the current over USB if I don't want to fry the port.

Could someone with more experience have a short look over it and tell me if it's ok?

usb wiring.png
usb wiring.png848×772 11.2 KB

2

Can really no one help me with this question?

What I'm worrying about most is what will happen when only the USB-Plug is connected and no external power source. Will the controller pass VBUS to the 5V-Pins? This would obviously blow the USB Controller from the computer so I don't hope so.
But since I'm unexperienced with the USB Controllers, I'm a litte bit worrying about this.

And I'm sure one of the gurus here has done such a schematic before...

(The schematic is in post #1 in the attachment)

3

Perhaps you should try this with actual, real cpu. Buy some card with MegaXu2 and check.

4

The way I read the data sheet:

  1. The VBUS pin is a "monitor" input, rather than a supply input.
  2. UVCC is the power for the USB buffers; you can get that from the USB connector as well. You'll also need a cap on UCAP.
  3. The AVCC pins should be connected.

The data sheet has some pretty detailed diagrams for the power circuitry for various configurations, off in section 21.3

5

Thanks!
UCAP is connected to a 1uF-cap.
I actually got the power circuit from the datasheet but it seems I overread that VBUS is just monitoring.
Do I really have to connect AVCC if I don't use ADC?
And what exactly speaks against making an own board with a 4€ MCU instead of putting a breakout board for over 15€ on this board? Getting the PCB made is expensive enough for me...

6

Do I really have to connect AVCC if I don't use ADC?

Yes.

2.2.17: AVCC is the supply voltage pin (input) for all the A/D Converter channels. If the ADC is not used, it should be externally connected to VCC. If the ADC is used, it should be connected to VCC through a low-pass filter.

(otherwise, you have exactly the sort of problem you were worried about with Vusb. Power would "leak" into the A-D circuitry from the digital side through unknown paths, possibly causing all sorts of undesirable behavior.) (also, it's impossible to tell whether AVCC powers "only" the ADC, or whether there is also "other" low-noise circuitry powered from those pins.)