Choosing the Reference Clock - 2020.2 English

UltraFast Design Methodology Guide for Xilinx FPGAs and SoCs

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2020.2 English

Depending on the clock tree topology that controls the sequential cells related to input or output ports, you have to choose the most appropriate clock to define the input or output delay constraints. If the clock of the I/O path register is a generated clock, the delay constraint usually needs to be defined relative to the primary clock, which is defined upstream of the generated clocks. There are some exceptions to this rule that are explained in this section.