The clock diagram in RM0316 for the 32F303K8 indicates a clock selection mux off the APBx prescalers. I've searched the registers and cannot find the register used to select one of 4 clock inputs. The 4 possible inputs are the LSE, HSI, SYSCLK, or the clock off the APBx prescaler. The outpu tis to the usuart.
I understand what you are saying but 'Figure 10. Clock tree' indicates the AHB drives both the APB1 and APB2 which gens PCLK1 and PCLK2. There is a 4-to-1 mux that can select as an input to the USUART either PCLK(1or2) or SYSCLK or HSI or LSE.
