一般用set_false_path命令，但是注意Altera的文档里有这样一段话：
Alternatively, use the set_clock_groups command to declare the paths between the two clock domains as false paths. For example, set_clock_groups –asynchronous –group [get_clocks clk] –group [get_clocks clkx2]. This command declares all paths from clk to clkx2 and from clk2x to clk as false paths. This method is perferred.