PLL is not consistently producing a clock

Hello,

I think, bad timing, inappropriate constraints or whatever of this kind can't be responsible for a PLL not working respectively a clock not appearing where it should. This could be e. g. due to mixed-up pins or internal PLL resources, not considering a possible Quartus bug. A special reason could, be that the said PLL has an asynchronous reset function or an input switchover that comes from bad defined logic.

You can additionally monitor the PLL lock output to get some status information. However, I did never see a PLL with no output, even if not locked.

Regards,

Frank