Default bootloader source code?

[BMich.2]

My dedicated hardware does not support uart/i2c/spi on the pins that are mentioned in AN2606. To update firmware IAP, I need to write my own bootloader that behaves the same as the default one (I want to use stm32flash tool as host, which fasten my work) with only changed uart pins and maybe disabled other serial communication peripherals.

Is this bootloader source code available anywhere?

[Tesla DeLorean]

No, ST has not released the source for these loaders.

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The protocol is documented.

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If you have a terminal app or program which supports XMODEM, that might be simpler and more available on more platforms.​

[BMich.2]

I have already written my own, but for the future I will tell you what I have found.

ST provide OpenBootloader project within stm32g0 stm32cube repository in the middleware directory. As far as I know it can realize the same functionality as the built int bootloader.