Custom ATmega32U4 PCB - Doubts regarding the USB pins (VBUS, UVCC and UCAP)

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Hello all,

I am designing a custom ATmega32U4 PCB with the following specifications:

  • Should run at 3.3V @8MHz.
  • Can be powered by the USB or external battery.
  • Relies on the inbuilt bootloader for programming via Atmel's FLIP.

 

Going through the already available proto-boards in the market, I found the following products as a reference for my design:

 

On further analysis, I decided that the Adafruit Feather 32u4 Basic Proto is closest to my design idea, so I kept the schematic as a reference.

 

I have the following questions:

  1. The VBUS pin is not used as a supply source for the µC and is only for a reference voltage. Correct?
  2. For a 3.3V supply, the 5V from the USB can be regulated to 3.3 and the UVCC pin can be left unconnected. Correct?
  3. According to the ATmega32U4 datasheet, the UCAP pin must be connected to a 1µF capacitor. But if you look at the Adafruit schematic, it is connected to the 3.3V source and decoupled using a 10µF cap. Can anyone explain why?

 

Thanks in advance!

This topic has a solution.

Regards,
Frederic Philips

Last Edited: Sun. Dec 23, 2018 - 06:45 AM
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If you are looking for good 32U4 implementations don't forget the Arduinos too - some are based on 32U4 or have a similar chip (maybe 16U2?) as the USB interface. You can be pretty sure they have done it right.  I think some of the PJRC boards use 32U4 too (ie Teensy 2.0)

 

I stole this from their website - hope they won't mind - it is a veiled advert for Teensy 2.0 after all ;-)

 

Last Edited: Thu. Sep 6, 2018 - 10:28 AM
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clawson wrote:

If you are looking for good 32U4 implementations don't forget the Arduinos too - some are based on 32U4 or have a similar chip (maybe 16U2?) as the USB interface. You can be pretty sure they have done it right.  I think some of the PJRC boards use 32U4 too (ie Teensy 2.0)

Hi clawson,

I had a look at the Arduino Pro micro schematic. Seems like they strictly follow the datasheet's recommendation. So why is it different in the Adafruit feather board?

Regards,
Frederic Philips

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fredphil wrote:
So why is it different in the Adafruit feather board?
That is something you'd need to ask the designer at Adafruit. On the whole their stuff is good quality so it's fair bet that whatever they implemented is well researched.

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clawson wrote:

fredphil wrote:

So why is it different in the Adafruit feather board?

 

That is something you'd need to ask the designer at Adafruit. On the whole their stuff is good quality so it's fair bet that whatever they implemented is well researched.

Have raised a question in their forum. I'll keep posted.

Regards,
Frederic Philips