I'm laying out a board with a AtMega32u2 chip.
I wish to run the USB portion of the chip off of the 5V USB VBUS.
At the same time, I wish to run the rest of the IO pins at 3.3 volts.
I plan to do this by taking the 5V USB VBUS power and running it through a 5.0V-3.3V LDO regulator, and using the 3.3Volts to connect to VCC and AVCC.
Here are some pictures out of the ATMega32u2 Datasheet.
This picture shows the USB portion of the chip. The UVCC Pin takes 5v coming in on the USB VBUS line and sends it to an internal regulator, changing it to 3.3V to power the D- and D+ pins.
This picture shows a wiring diagram to attain 3.3 volts on all the IO pins.
The picture above assumes UCAP, VCC, and AVCC are attached to a 3.3 volt line.
Not shown is the line I will run from the 5volt VBUS...into a 5v to 3.3v LDO...then running the 3.3 volts into VCC and AVCC.
What troubles me in the diagram is the 3.3v going into UCAP.
I don't want to do that.
I want to run the USB portion of the chip using the 5-volts coming into UVCC which then goes to the internal regulator.
That means erasing the line in this picture showing a 3.3v connection into the UCAP pin.
Can I do that?
Will everything still work?
Also, I understand VCC and AVCC should have some bypass capacitors in close vicinity to the pins.
I've seen values of 1.0uF and 0.1uF suggested.
Which value should I use?
Should I also put a bypass capacitor on UVCC.
If so, what value?
I will have a 10uF capacitor on the 5volt VBUS line very close to where it enters the board off the Mini USB receptacle.