UART using LOAN on DE10 Nano

Hi,

I guess you're facing this issue due to the fact that the tool has recognized the clock and its inverted version clk_n to be a differential pair and has assigned a pseudo-differential output pin constraint on them. To get past this issue, you need to check if these clocks really form a differential pair or not and then assign them to the appropriate voltage and signal standard aka Differential IO or single-ended. If there are pin constraints already set in by default clear them, run the processes again and see if the tool will assign the correct pins/standards.

If this fails, then manually assign the correct IO and voltage standard to the pins and run it again.