5733 - iCE40 Ultra/UltraLite: Can the 48 MHz HFOSC be used as a reference for the internal PLL using iCEcube2 Software?
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 software. Simply go to Tool > Configure > Configure PLL Module. When asked about the clock source, select "General Purpose IO Pad or Core Logic".