/* RCC Set the USB Prescale Factor. 
The prescale factor can be set to 1 (no prescale) for use when the PLL clock is 48MHz, or 1.5 to generate the 48MHz USB clock from a 64MHz PLL clock. */
void rcc_set_usbpre(uint32_t usbpre)
/* RCC Set the USB Prescale Factor. 
The prescale factor can be set to 1 (no prescale) for use when the PLL clock is 48MHz, or 1.5 to generate the 48MHz USB clock from a 64MHz PLL clock. */
void rcc_set_usbpre(uint32_t usbpre)
{
	RCC_CFGR = (RCC_CFGR & ~RCC_CFGR_USBPRE) | usbpre;
}