kusti8 wrote:The Pi has to run an OS, if you want something like that you probably want an Arduino.
Err, of course the Pi doesn't need an OS! As ghans mentioned, there's an entire Bare Metal (meaning no OS) forum dedicated to it.
Indeed, I would say that if you're coming from microcontrolers, and want to fiddle with the hardware, not only is bare metal easier than trying to battle against the OS, but assembly code makes things even clearer than using C.
There are plenty of examples / tutorials about how to get started with bare metal / assembly code.
If you can't find them try googling the last line, after replacing "There are plenty of" with "Raspberry Pi / ARM"