Why does the Quartus II software fail to fit more than four groups of 40G BaseKR IP into one side of a Stratix V device?

When you place more than four groups of 40G BaseKR IP on one side of a Stratix® V device, you might get the following error message:

Error (175001): Could not place fractional PLL

Error (177012): Route from the fractional PLL feedback output to the fractional PLL is congested

This error is due to fPLL feedback clock congestion which is caused by the fPLL needing extra routing resources for reference clock compensation.

To work around this problem, you can change your PLL compensation mode to "Direct Compensation" mode by adding the following line to your Quartus® II Settings File (.qsf) file.

set_instance_assignment -name PLL_COMPENSATION_MODE DIRECT -to *| |alt_e40_pma_sv_kr4:GEN_40BIT_PMA_SV.GEN_KR4_SV.pma|altera_pll_156M~FRACTIONAL_PLL