fma - "So, I don't have any bootloader on it, and I'm not using DFU; I switch to the internal st bootloader, and upload code through USART1..."
Okay, got it.
"I'm new to stm32 dev, so I don't know yet how things work at boot. Is there a usefull documentation I can read?"
What sort of thing do you want to know?
If you can outline your 'use-case' (i.e. what you're trying to achieve), we might be able to offer better help.
There are a few things you could read.
There is a pretty useful book for Cortex-M3:
https://www.elsevier.com/books/the-definitive-guide-to-the-arm-cortex-m3/yiu/978-1-85617-963-8
This covers quite a lot abut the Cortex-M3 processor, and using different development tools.
It might be worth getting copies of the Cortex-M3 manuals from ARM:
http://infocenter.arm.com/help/index.jsp
or ST
However, most of the startup work is setting up peripherals and the interrupt vectors.
The initial startup is in the bootloader, which makes it a bit awkward to follow.
Then the peripherals are described in detail in RM0008 (you probably have that):
http://www.st.com/st-web-ui/static/active/en/resource/technical/document/reference_manual/CD00171190.pdf
So looking at the start-up code, armed with RM0008 is probably helpful if that is the area you want to understand.
If, on the other hand, you want to understand more about start-up, then either dig through the Maple bootloader, or look at ST's standard peripheral library downloaded from:
http://www.st.com/web/en/catalog/tools/PF257890#
It has example startup code for several different IDEs.
