Before PNS，some settings should be ready：
firstly，make sure that the power/ground nets exist，if not，please load upf file，a typical upf file is shown as below:
After loading upf， the power/ground nets are still not exist yet，because upf is only a guide for pns, so the following command is needed next :
derive_pg_connection -create_net ; ## create nets : DVDD DGND
then use [ get_nets DVDD ] to make sure the DVDD is not of signal type ( Power net & Ground net can only be get by [get_net DVDD -all ] )
|-----------------------------------|