32 f4 时钟树

Several prescalers are used to configure the AHB frequency, the high-speed APB (APB2)
and the low-speed APB (APB1) domains. The maximum frequency of the AHB domain is
168 MHz. The maximum allowed frequency of the high-speed APB2 domain is 84 MHz. The
maximum allowed frequency of the low-speed APB1 domain is 42 MHz