Problem about clock skew.

[Altera_Forum]

I generated a inverted clock from a source clock. 

The statement is: assign clk_inv = ~clk; The period of the source clock is 16ns. The clock skew from clk to clk_inv should meet the requirements: 1. The min value from clk's rising edge to clk_inv rising edge is 7.5ns 2. The max value from clk's rising edge to clk_inv rising edge is 8.5ns Which constrain commnd should I be used? Thanks Harris