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[Joost]

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[ag123]

among the things you can try, you can look at this
viewtopic.php?p=9868#p9868
post in the thread

or alternatively, you can try the FYSETC_S6 variant
https://github.com/stm32duino/Arduino_C ... ETC_S6.cpp

the main thing about that variant is that, that board has a SystemClock_Config(void)
https://github.com/stm32duino/Arduino_C ... 6.cpp#L139
for a 12 Mhz crystal, which apparently is the same frequency as your board.

you would also need the schematics to your board, figure out which pin (the PAxx) is the on board led and write a sketch to blink that led.
you would also need to test usb-serial, e.g. Serial.print("hello world"); and it should appear in your serial monitor.
This is just to assure yourself that it is 'alive'.

after that you can make a variant for your board as suggested in the first link above.
to make a new variant, it is mainly
- copy the files (e.g. from generic) to your board and name them similar to the convention
edit the various symbols / macros to distinguish that for your board
e,g ARDUINO_(your board)
(this is normally done within the same folder)
- update the SystemClock_Config(void) in your copy / variant
- update boards.txt copy that same section across and edit the copy of your board's entry.

if everything works well, after your tests, you may like to make a pull request in github for your new variant.
if that's gets included in the core, the next time you have the convenience of just selecting your board in the standard core.