AR# 4122: FPGA Express: Subpaths containing pads grouping incorrectly written by Express
FPGA Express: Subpaths containing pads grouping incorrectly written by Express
Keywords: Foundation Express, subpath, pads, timespec, constraints
General Description: The Express Constraints Editor can be used to create timespecs to be passed on to the place and route tools. Beyond the generic path specifications, users can create subpaths (in the Paths tab) to furthur constrain their design.
However, if a subpath contains a group of pads as an endpoint, Express will use the generic "pads" keyword in the timespec definition rather than creating a new subgroup of pads. The design may become overconstrained as more pads than expected will be contained in the timespec.
This issue has been fixed in FPGA Express 3.2 and Foundation 2.1i.
First, remove the incorrect spec in the Express Constraints Editor to be sure that it is not being applied during place and route.
Then, to create a correct timespec for this subpath, use the Xilinx Constraints Editor. Group the pads by selecting the "Group by Elements" button under the Advanced tab and using the pads filter. Create flip-flop, latch or RAM groups in the same manner. Then use the "Multi-cycle Paths" button to create the timespec.