I'm so confused about the setup time of BUFGMUX. In the datasheet of Spartan6, this spec is defined relatively to rising edge.
But if the structure of BUFGMUX is like
Then the setup time should be defined relatively to falling edge. Because the couple-register pair are both triggered with clock's negative edge.
Why? What the actual structure of BUFGMUX in Spartan6.
Thanks a lot.