What to do with a freshly purchased replacement MCU?

My Atmega 32u4-au apparently went tits up. I looked over the replacement guides and I think I am able to do it.
However, it looks like a bootloader has to be written to the MCU prior to QMK, but I’m very vague on the topic. What are the hardware/software requirements to do so? Any guide out there?

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Normally your atmega32u4 should come with the Flip bootloader already flashed in it.
I heard that some special variants of this MCU could come without but it never happened to me personally.

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Now if flashing a bootloader is needed you need to have access to the miso and mosi pins of the MCU to flash a bootloader with the serial interface .
For that you need a special hardware to do the flashing.
It can be a USB BASP, or another pro micro board .
In the past I used a USB BASP device (bought as a kit and cheap) to flash custom made PCB with Atmega328p MCU, the principle should be close to what you should do for your atmega32u4 MCU.

A bit of warning, this is significantly more involved than flashing a QMK firmware.
But as said before you will probably not need it.


This is very helpful, thank you!
One thing though, is there an [easy] way to tell if there is a bootloader or not before installing an MCU onto a PCB?

Once replaced, you should be able to easily write the bootloader with the Arduino IDE (via USB). Or am I missing something?

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I thought there has to be a special board to flash bootloader, or am i wrong and you can do it using just a special software (with mcu installed in keyboard)?

Sorry, I may talk bullshit here but I thought it’s only necessary for chips like an Atmega328 with no native USB support. Indeed, I used an Arduino as ISP when I made something like this the last time.
For some reason I thought you don’t need another device with 32u4. Now I’m confused. :thinking:

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Once the MCU is replaced, if done correctly (check for potentially shorted pins), the board should appear as a USB device in your device manager.

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Heard that ln some rare instances the chip is not preloaded with a bootloader , maybe for Chinese clones?
But if bought from a reputable vendor the bootloader is already factory installed.

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Got it!

Well, in my case it’s either aliexpress or local shops/marketplace (which likely still source from ae/taobao). But I will see if they have the info. Thanks Rico!