AR# 9780: FPGA Express 3.x - Net preservation of internal nets using the KEEP attribute
FPGA Express 3.x - Net preservation of internal nets using the KEEP attribute
The only way to keep a net in FPGA Express is to instantiate a BUF primitive in the middle of the net, place a "dont_touch" attribute on the BUF (through FPGA Express' Constraints Editor), then place a KEEP implementation constraint on the net that enters the BUF and on the net that exits the BUF.
Placing the dont_touch attribute (using ISE):
1. Expand the "Synthesize" process in the "Process View" window.
2. Expand "Create Functional Structure" within the "Synthesize" process.
3. Double-click on "Edit Constraints".
4. Select the "Modules" tab from within Constraints Editor.
5. Under the "Dont Touch" column, select "True" for the instantiated BUF.
6. Select the "OK" button.
NOTE: If using FPGA Express standalone deselect the 'Skip Constraint Entry' box located at the bottom of the 'Create Implementation Window'. Then follow steps 4 - 6.
Placing the KEEP implementation constraint (within ISE):
1. From Project Navigator, select File -> Open (Ctrl+O).