Yes, the internal oscillator can be used as a clock source for the PLL. There are specific primitives for this (SB_PLL40_CORE and SB_PLL40_CORE), which will automatically be instantiated when we use the included PLL module generator from iCEcube2 ...
When the PLL is in power down (default when it is not used), the current to PLL is turned off, and the VCO and Charge Pump is not running. After programming completes, there is a dedicated global bit to disable the oscillator. Oscillator is turned ...
Solution: Depending on what software you are using, (Radiang Software or iCEcube2), you can change the configuration oscillator clock frequency using these steps. Radiant Software: Go to Project > Active Stratefy > Bitstream Settings > Oscillator ...