sysclk.c

   1 #include "sysclk.h"
   2 #include "stm32f411.h"
   3
   4 void
   5 sysclk_init (void)
   6 {
   7   /* HSEBYP: HSE clock bypass  */
   8   *RCC_CR &= ~(1u << 18); /* 0: HSE oscillator not bypassed  */
   9
  10   /* HSEON: HSE clock enable  */
  11   *RCC_CR |= (1u << 16); /* 1: HSE oscillator ON  */
  12
  13   /* HSERDY: HSE clock ready flag  */
  14   while (!(*RCC_CR & (1u << 17))); /* 0: HSE oscillator not ready  */
  15
  16   /* SW: System clock switch  */
  17   *RCC_CFGR &= ~(0b11u); /* Clear SW bits  */
  18   *RCC_CFGR |= (0b01u); /* 01: HSE oscillator selected as system clock  */
  19
  20   /* SWS: System clock switch status  */
  21   while ((*RCC_CFGR & (0b11u << 2)) != (0b01u << 2)); /* 01: HSE oscillator used
  22                                                          as the system clock  */
  23   /* HSION: Internal high-speed clock enable  */
  24   *RCC_CR &= ~(1u); /* 0: HSI oscillator OFF  */
  25 }