3991 - Lattice Diamond v3.1: How to set the behavior of unused pins in an FPGA design?
Solution:
These are some of the workarounds:
1) In RTL code, assign the following attributes to the unused input:
/* synthesis syn_force_pads=1 syn_noprune = 1 */
The 'syn_noprune' will not allow Synplify Pro to optimize unconnected I/O buffers and will increase resource utilization. So, as per your design requirement, it may or may not be used.
2) Under Strategy1 make the following setting:
MAP Design--> Command line options = -u
This ensures that the Mapper does not remove the unused I/Os.
3) In spreadsheet view, make pull mode =NONE
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