Connecting unused pins

I disliked the old project-wide default because of the issue HDL Guru described. I urge being very careful if using "As output driving ground" in the Device and Pin Options dialog box. If intentionally having the FPGA drive unused pins to GND, I recommend using this setting on the individual pins through a "Reserve Pin" assignment in the Assignment Editor or Pin Planner. By manually adding this assignment pin by pin (even if you think you want it on every unused pin), you are less likely to overlook a pin that is driven to something other than GND on the board and that therefore needs a different setting.

While less dangerous, a project-wide default of "As input tri-stated" can cause trouble by leaving FPGA input buffers floating on pins that are not driven or pulled up on the board. Floating inputs can increase power, increase switching noise, and decrease device reliability. The project wide defaults using pull ups or bus hold are safer because they are more tolerant of a mistake where the user overlooks an unused pin that is driven on the board.