2 docs tagged with "rcc"

On a desktop machine the clock is somebody else's problem — it was configured by firmware you never see, and by the time your program runs it is a constant. On a microcontroller you are that firmware. The chip comes out of reset running on a cheap internal RC oscillator at a fraction of its rated speed, with almost every peripheral's clock switched off, and the first job your code has is to build the clock tree the rest of the system will run on. Nothing you write behaves as intended until that is done.

Out of reset the STM32F411RE runs at 16 MHz on an internal RC oscillator, which is a deliberately conservative choice: it works with no crystal, no configuration, and no risk. It is also one sixth of what the part can do, it is accurate to about ±1 % over temperature rather than the ±20 ppm a crystal gives, and it cannot produce the 48 MHz that USB requires. Somewhere in the first week of a real project you will need to change it.