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Section 5.1.12 (Power Supplies). 
In the NUCLEO-64, PC13 is use to read User Button and PC14 and PC15 is connected to OSC32 crystal.
So I think due to the RTC HW configuration, it is restricted to run at a lower Clock Speed. I think 2 MHz still okay for my application as I would need the signal to rise in less than 100nS.
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