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              volatile uint32_t num;num = RCC->CFGR; // 0x001D040A
 // MCO=0: Microcontroller clock output, No clock
 // OTGFSPRE=0: USB OTG FS prescaler, PLL VCO (2 x PLLCLK) clock is divided by 3 (PLL must be configured to output 72 MHz)
 // PLLMUL=0111: PLL multiplication factor: PLL input clock x 9
 // PLLXTPRE=0, LSB of division factor PREDIV1
 // PLLSRC=1: Clock from PREDIV1 selected as PLL input clock
 // ADCPRE=00: ADC prescaler = PCLK2 divided by 2
 // PPRE2=000: APB high-speed prescaler (APB2), HCLK not divided
 // PPRE1=100: APB Low-speed prescaler (APB1), HCLK divided by 2
 // HPRE=0000: AHB prescaler, SYSCLK not divided
 // SWS=10: System clock switch status, PLL used as system clock
 // SW=10: System clock Switch, PLL selected as system clock
 
 num = RCC->CFGR2; // 0x00010644
 // I2S3SRC=0: I2S3 clock source, System clock (SYSCLK) selected as I2S3 clock entry
 // I2S2SRC=0: I2S2 clock source, System clock (SYSCLK) selected as I2S2 clock entry
 // PREDIV1SRC=1: PREDIV1 entry clock source, PLL2 selected as PREDIV1 clock entry
 // PLL3MUL=0000: PLL3 Multiplication Factor, Reserved
 // PLL2MUL=0110: PLL2 Multiplication Factor, PLL2 clock entry x 8
 // PREDIV2=0100: PREDIV2 division factor, PREDIV2 input clock divided by 5
 // PREDIV1=0100: PREDIV1 division factor, PREDIV1 input clock divided by 5
 
 num = RCC->CR; // 0x0F035783
 // PLL3RDY=0: PLL3 clock ready flag, PLL3 unlocked
 // PLL3ON=0: PLL3 enable, PLL3 OFF
 // PLL2RDY=1: PLL2 clock ready flag, PLL2 locked
 // PLL2ON=1: PLL2 enable, PLL2 ON
 // PLLRDY=1: PLL clock ready flag, PLL locked
 // PLLON=1: PLL enable, PLL ON
 // CSSON=0: Clock security system enable, Clock detector OFF
 // HSEBYP=0: External high-speed clock bypass, external 3-25 MHz oscillator not bypassed
 // HSERDY=1: External high-speed clock ready flag, HSE oscillator ready
 // HSEON=1: HSE clock enable, HSE oscillator ON
 // HSICAL=01010111: Internal high-speed clock calibration
 // HSITRIM=10000: Internal high-speed clock trimming
 // HSIRDY=1: Internal high-speed clock ready flag, Internal 8 MHz RC oscillator ready
 // HSION=1: Internal high-speed clock enable, Internal 8 MHz RC oscillator ON
 
 // PLL2 and PLL3 are clocked by HSE through a specific configurable divider.
 // PLL2=25MHz/5*8=40MHz
 // SYSCLK=PLL=40MHz/5*9=72MHz
 // USB: 2*72/3=48MHz
 
 // AHB: 72MHz
 // APB1: 36MHz
 // APB2: 72MHz
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