Homebrew Sparkfun Pro Micro issue after burning bootloader. Device descriptor request failed

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1

I am trying to build a Sparkfun pro micro from scratch and I’ve match everything to this schematic except.

https://cdn.sparkfun.com/datasheets/Dev … o_v13b.pdf

The only difference is that I’m using 22uF capacitors on the voltage regulator and I didn’t include most of the LEDs. I know that the capacitors aren’t a problem cause I measured the resulting voltage. I am able to burn the 3.3v pro micro bootloader onto the board, but when I plug it in to my Windows 10 machine it has an error and reads “Unknown USB Device (Device Descriptor Request Failed).” I’m sure that the bootloader process is correct because when I did it to my store bought sparkfun promicro, it worked and there was no problems.

Additionally, I tried burning the Arduino/Genuino Micro bootloader onto my board and that was successful. I was able to plug it in to my windows machine and and upload a sketch via the Arduino IDE.

My question is how is the Arduino bootloader process different than the Sparkfun bootloader process such that I would have success with one and not the other? And what do I need to do to get the Sparkfun bootloader to work?

2

We have two different versions of the Pro Mini (5 volt and 3.3 volt) and each runs at a different clock speed. Because of the difference in clock, each version has it’s own firmware.

What speed crystal are you using and when you burned the firmware, what board type was selected?

3

I am using a 16mhz crystal and I selected the 3.3v pro micro.

4

OK, that’s your problem. The 3.3v board runs at 8MHz and the 5v board runs at 16. Try selecting ‘SparkFun Pro Micro, ATmega32U4 (5V, 16MHz)’ for your board type and burning the bootloader again and you should be OK.

What’s happening is the firmware thinks it’s running at 8MHz while the chip is actually running at twice that speed. That screws up the clock USB works at and that’s why your computer can’t establish a connection.