Arduino for STM32

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//simplest routine to switch to HSE
//no error checking
void SystemCoreClock2HSE(void) {
	//turn on hse
	RCC->CR |= RCC_CR_HSEON;				//1->turn on HSE, 0->turn off HSE
	//wait for HSE to be ready - error if stuck here
	while ((RCC->CR & RCC_CR_HSERDY)==0);		//1->HSE ready, 0->HSE not ready
	//switch to HSE
	RCC->CFGR = (RCC->CFGR &~RCC_CFGR_SW) |	//CLEAR_BIT SW bits
				(1 << RCC_CFGR_SW_Pos);		//0->HSI, 1->HSE, 2->PLLRCLK, 3->LSI, 4->LSE, others->reserved
	//wait for the switch to take place
	//while ((RCC->CFGR & RCC_CFGR_SWS) != (1<< RCC_CFGR_SWS_Pos));	//if stuck here, there is an error
	SystemCoreClockUpdate();				//update systemcoreclock
}

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#define HSE_VALUE               (8000000UL)