You should refer to the reference manual related to your device and you find all details about APBx prescaler and register.
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You should refer to the ''Reset and clock control (RCC)'' inside the RM0041, exactly the Figure 8. STM32F100xx clock tree (low and medium-density devices) to have idea about clock tree and in the ''Clock configuration register (RCC_CFGR)''
You can use STM32CubeMx which is a graphical interface performs easy configuration and clock tree. (Please refer to ''Clock tree configuration view'' section on the UM1718 user manual).
Hope this help you.
Imen
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