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Peripherals like timers and Usarts get their clocks via the APB prescalers. The Interface Clock is at the prescaled frequency, used for sequencing most peripheral functions. APB1 has a max allowed value of 42MHz, APB2 max 84 MHz.

The Timers have an oddity where the frequency fed to the clock input of the Timer Prescalers, when the APB prescaler is NOT /1, is twice the corresponding Interface Clock. Thus, when the SysClk is at the max of 168 Mhz, APB2 is prescaled by 2 to get 84 MHz, the Timers driven from APB1 get twice that, 168 MHz, into their prescalers . Same logic for APB1, /4 => 42 MHz i/f, *2 => 84 MHz => Timer prescalers.