1.6.5. Moving Supplies to Optimal Layers
Reducing the spreading inductance (Ls) between the planes and the FPGA increases the effective frequency of the VCC supply. Re-allocating the FPGA supply rails in the layer-stack so that they have lower vertical inductance, will also show an improvement.
Moving the VCCT_GXB and VCCR GXB supplies from Layer 9 to Layer 4, and the VCC supply from Layer 15 to Layer 9, brings them closer to the FPGA with a lower vertical loop inductance.
The following figure shows a comparison between the initial supply layer allocation on the left with the new allocation on the right.
As a result of these changes, the PDN Tool reports that the VCC supply Feffective has improved from 30.68MHz to 35.61MHz.
The following figure shows that VCCT_GXB and VCCR_GXB supply impedances are close to meeting the 70MHz target. However, further optimization is required because more than 301 capacitors are still required for each of the three supplies.