After specifying Drive Strength or Slew Rate from the pull down menu's for a specific outout in Constraints Editor, the constraint is ignored after implementation.
This is a know issue with Synopsis FPGA Express design. After specifying a constraint in the constraint editor, the contraint is written into the constraint file.
For example, the syntax for a drive strength of 4 is:
NET <output portname> DRIVE=4;
fpga_express writes out the edif file with OBUF_S_12 by default, instead of just an OBUF. After you set NET netname DRIVE=4 attribute in the constraint file, what happens in the ngd file after running Ngdbuild is that the OBUF instance still uses DRIVE=12, but the OPAD instance uses DRIVE=4ma; MAP only picks up the constraint on the OBUF,not the OPAD.
The workaround is to place the constraints on the OBUF instance. In the .UCF file, place:
INST C_<pad name> DRIVE = 4;
The C_ is the OBUF instance name for nets synthesized in Express.
This problem will be fixed in the next major release.