Clock Skew for Global Clocking Resources in SSI Technology Devices - 2021.2 English

UltraFast Design Methodology Guide for Xilinx FPGAs and SoCs

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2021.2 English

Clock skew in any large device might represent a significant portion of the overall timing budget for a given path. Too much clock skew may not only represent issues with maximum clock speed, but may also manifest itself into stringent hold time requirements. Having multiple die in a device worsens the process portion of the PVT equation, but is managed by the Xilinx assembly process in which only die of similar speed are packaged together.

Even with that extra action, the Xilinx timing tools accounts for these differences as a part of the timing report. During path analysis, these aspects are analyzed as a part of the setup and hold calculations, and are reported as a part of the path delay against the specified requirements. No additional user calculations or consideration are necessary for SSI technology devices, because the timing analysis tools consider these factors in their calculations.

Skew can increase if using the top or bottom SLR as the delay-differential is higher among points farther away from each other. For this reason, Xilinx recommends for global clocks that must drive more than one SLR to be placed into the center SLR. This allows a more even distribution of the overall clocking network across the part resulting in less overall clock skew.

When targeting UltraScale devices, there is less repercussion to clock placement. However, it is still highly suggested to place the clock source as close as possible to the central point of the clock loads to reduce clock insertion delay and improve clock power.