PWR_EnterSTOPMode(PWR_Regulator_LowPower, PWR_STOPEntry_WFI);

    /* Disable Wakeup Counter */
    RTC_WakeUpCmd(DISABLE);
    /* After wake-up from STOP reconfigure the system clock */
    /* Enable HSE */
    RCC_HSEConfig(RCC_HSE_ON);

    /* Wait till HSE is ready */
    while(RCC_GetFlagStatus(RCC_FLAG_HSERDY) == RESET)
    {
    }

    /* Enable PLL */
    RCC_PLLCmd(ENABLE);

    /* Wait till PLL is ready */
    while(RCC_GetFlagStatus(RCC_FLAG_PLLRDY) == RESET)
    ;

    /* Select PLL as system clock source */
    RCC_SYSCLKConfig(RCC_SYSCLKSource_PLLCLK);

    /* Wait till PLL is used as system clock source */
    while(RCC_GetSYSCLKSource() != 0x08)
    ;