Read stm32f4xx Chinese Reference manual notes (ii)

Reset and Clock control (RCC) resetting The system resets the clock control register to the CSR and the registers in the backup area, and all other registers are reset. (5 ways: Nrst pin, independent, window watchdog, SW reset, low power reset) power Reset, in addition to the registers in the backup area, the other registers are reset. Backup domain reset. Clock

Clock source five HSI high-speed internal clock, RC oscillator HSE high-speed external clock, crystal oscillator PLL phase-locked loop multiplier output It is generally provided by the HSE or HSI clock signal, you can set the speed. LSI Low speed internal clock, RC Oscillator, independent watchdog watchdog LSE Low speed external clock, crystal oscillator, for driving RTC clock

The system clock defaults to HSI, if you want to switch the clock source must be configured AHB,APB1 after the external clock is HSE stable. APB2, finally open the main PLL clock and set the primary PLL as the system clock source. Clock output, pa9,pc9 two MCO pins. RCC Registers

A cursory look at the next, various configurations, see the dazzling, skip it.