General Description: Instantiated pullup/pulldown resistors may be deleted from your design during the compile process in FPGA Compiler versions V3.4b and earlier. This is a known Synopsys problem that will not be fixed in a future release of Synopsys FPGA Compiler I.
This issue does not apply to FPGA Express or FPGA Compiler II.
To solve this problem, do not instantiate pullup/pulldown resistors for I/O applications. Instead, use the set_pad_type -pullup|-pulldown port_name command to achieve the same result.
For internal applications (such as wired-AND or mux functions built using wide edge-decoders or tristate buffers), use the following commands to re-insert the pullup/pulldown once it has been removed by the compile process. When targeting XC4000E/EX devices, execute the replace_fpga command as usual and then use the following commands: