STM32 multiplexing Function-clock output (MCO)

The pa.8 pin of the STM32 has a multiplexing function-clock output (MCO), which enables the STM32 internal clock to be output through pa.8.
Operation Flow:
1), set pa.8 to multiplex push-pull mode.
Gpio_initstructure.gpio_pin = Gpio_pin_8;
Gpio_initstructure.gpio_speed = Gpio_speed_50mhz;
Gpio_initstructure.gpio_mode = gpio_mode_af_pp;
Gpio_init (Gpioa, &gpio_initstructure);
2), select the output clock source.
The selection of the clock is controlled by the mco[2:0] bit in the clock configuration register (RCC_CFGR).
Rcc_mcoconfig (RCC_MCO);
The parameter Rcc_mco is the internal clock to be output:
Rcc_mco_noclock---No clock output
RCC_MCO_SYSCLK---output system clock (SYSCLK)
RCC_MCO_HSI---Output internal high-speed 8MHZ RC oscillator clock (HSI)
Rcc_mco_hse---Output high speed external clock signal (HSE)
RCC_MCO_PLLCLK_DIV2---output PLL double-frequency clock (PLLCLK/2)