Adding FX Chip to Homemade Arduboy (Pro Micro)

I don’t understand if it is necessary to have only the flash chip and nothing else or if I have to use as well a modchip…

You only need the flash chip.

The other chip is to burn the bootloader. If you can burn the bootloader using an ICSP then it isn’t needed.

Please, could you tell me where I can find a tutorial in order to burn the bootloader?

From what you previously said you’ve already burned a Cathy3K bootloader into a Pro Micro


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I thought that simply on Arduino IDE selecting “Cathy3D” and uploading a .ino file (after compiling) automatically the bootloader was put in my Micro Pro…sorry! I didn’t know that I had to use another Arduino as “programmer”…

Anyway, I’ll follow the tutorial you linked me. Thank you very much and sorry again for my misunderstanding!


I failed on bootloader writing…

I used an Arduino UNO as Arduino ISP Programmer (before I loaded the sketch “ArduinoISP”), then I connected Arduino UNO to my Pro Micro, using 6 pins (Miso-Mosi-Reset-Sclk-Reset-VCC-GND), then I selected “Cathy3K” as bootloader. At the end I clicked on “Burn Bootloader” and I obtained the following errors:

“avrdude: Yikes! Invalid device signature.
Double check connections and try again, or use -F to override
this check.”

I tried several times but nothing changed…

Please, could you help me? Thank you!

@Mr.Blinky gave me some tips when I had trouble and I have repeated these below:

Maybe you have a UNO that needs a 10uF(bigger will do) capacitor between reset and GND.

  • Make sure you have selected the UNO board when uploading the ArduinoISP example.
  • After uploading the ArduinoISP add the 10uF capacitor as shown below.
  • check the wiring again and check the wires connect good in the female headers of the UNO
  • select Homemade Arduboy > Leonardo > SSD1306 > Start with menu > SDA and select Arduino as ISP
  • burn the bootloader and cross your fingers



I added the 10 uF capacitor but nothing changed: same error…

What else could i check?

Did you first burn the arduino icsp program in examples onto the uno to turn it into an icsp programmer?

Of course: I did it before burning the bootloader.

Can you take a screenshot of the selected settings in the tools dropdown tab and a pic of the wiring between the uno and atmega32u4? Sorry if I’m asking a lot of questions, but this info would really help debug the problem.

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Here is the Arduino IDE settings (on bootloader burning):


and this is the Arduino UNO on the left (as Programmer) and the Pro Micro as “Bootloader receiver”:

Note1 : I tried as well to connect the “Reset” pin between master and slave, but nothing changed.

Note2: I tried with and without 10 uF capacitor (between Pin Reset and GND of Master) but still nothing changed.

You should connect the Reset Pin of the Pro Micro to Uno pin 10 not the reset on the ICSP header of the Uno


Thank you!!! It worked!!!

Finally I added a 64 Mbit flash chip (W25Q64) and I loaded the “Full Enchilda” of Arduboy FX and it worked like a charm! I’m very happy!

Thank you again!