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I am a newbie and I would like to build my own minimalistic ATmega32U4 based arduino compatible board. Can anyone please tell me which parts resistors/capacitors (and maybe one status led) I need to connect it over usb to my computer? Thanks in advance.

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Several Arduinos and Arduino-compatible boards use the ATmega32U4. Many of them have published open-source plans that show exactly what components are used and how they are connected. They may not be the absolute minimal number of components, but it seems clear that you don't need more components than a known-working board:

  • 1 ATmega32u4
  • 1 16 MHz crystal
  • 2 22pF ceramic caps (for crystal)
  • 1 uF decoupling cap
  • 1 pushbutton
  • 1 10 KOhm pull-up resistor (for pushbutton)
  • 1 LED
  • 1 220 Ohm resistor (for LED)
  • 1 USB socket
  • 2 22 Ohm resistors (between USB socket and ATmega32u4)

Alas, there are two unnecessarily difficult hurdles between newbies and getting a new ATmega32u4 device connected over USB to a computer:

  • The ATmega32u4 and nearly every other device that communicates over USB is only available in SMT packages with pins at 0.8 mm pitch or even closer together. Most people find packages with pins at 2.54 mm pitch easier to handle and plug into a breadboard.

  • Installing a bootloader into any new processor chip is often tricky.

As Russel already pointed out, you can save a lot of time by starting out with an Arduino or Arduino-compatible board that already has the chip soldered on and the bootloader programmed in, bypassing the above unnecessarily difficult hurdles.

A few Arduinos and Arduino-compatibles (with links to schematics, etc.) that use the ATmega32u4:

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  • One other crucial factor to consider: USB's timing requirements are pretty precise. That requires that the crystal oscillator create quite a precise frequency. Breadboards and precise frequencies do not go together due to the massive capacitance they impose on the circuitry. That makes it very very hard to get USB actually working from a breadboarded chip.
    – Majenko
    Aug 26, 2016 at 9:32
  • @Majenko: I agree that breadboards add a huge amount of capacitance, with bad effects on timing circuits. However, Jeff Murchison and many of the people commenting on his blog (first link) somehow got USB working on a breadboard, so apparently it's not very very hard.
    – David Cary
    Aug 26, 2016 at 12:39
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Building your own may be worthwhile from a learning point of view, but makes minimal sense otherwise. You can buy Arduino clones (legal) from China for under $US5 each - good enough quality, assembled, working.
You cannot build one for that.

A DIY minimalist Arduino requires a running processor - see datasheet for the processor of choice as Ignacio says.Probably decoupling capacitors and maybe one or two resistors and maybe 1 or 2 small capacitors. If you cannot derive the needed information from a datasheet you do not know enough to sensibly do it by yourself. There are many on-web ~= minimalist implementations to copy.

If you want onboard USB you'd usually use a USB-serial converter but this can be done by "bit bashing" if you truly want a minimalist implementation>

BUT - just buy one and use the large amount of time gained to learn related things of more immediate use.

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    The '32U4 has built-in USB device support, so a USB-UART bridge isn't required unless you want to connect the hardware UART to a second computer.
    – Ignacio Vazquez-Abrams
    Aug 26, 2016 at 5:49

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