Timing constraints about tpd

Hello All,

I want to introduce about 10 ns delay between the outputs oe_o and dir_o,oe_o follow the dir_o.

Example:

module delay_10ns(oe_i,dir_i,oe_o,dir_o);

input oe_i,dir_i;

output oe_o,dir_o;

//inout

assign oe_o = ~ oe_i;

assign dir_o = ~ dir_i;

endmodule

QII's version is 8.1,when I use the following constraints:

set_instance_assignment -name TPD_REQUIREMENT "10 ns" -from dir_i -to dir_o

set_instance_assignment -name MINIMUM_TPD_REQUIREMENT "20 ns"-from oe_i -to oe_o

It works.but when I disable the TPD_REQUIREMNT constraint between dir_i and dir_o,the oe's MINIMUM_TPD_REQUIREMENT constraint does't work.

why,anyone can tell me? Thanks in advance!