说明：  利用纯寄存器的操作来使LED等闪亮，系统时钟由8M的晶振（HSE）经2倍频后，由PLL输出，希望初学者可以共勉。
(Operation using pure register etc. to make LED flash, the system clock from the crystal 8M (HSE) frequency by 2, by the PLL output, hope that beginners can encourage each other.)