HSI - High Speed Internal osillator

Clock Mode 1:
HSI Oscillator as SYSCLK (Default)

Clock Mode 2:
HSI throgh Prescaler and Multiplier(PLL).

HSE - High Speed External clock

Clock Mode 3: HSE Oscillator as SYSCLK direct.	Clock Mode 4: Clock HSE throgh prescaler and multiplier.
Clock Mode 5: HSE through all Prescalers and Multipliers.