ID:13828 VHDL warning at <location>: used 'X' for unrecognized character '<character>' in enumerated type

CAUSE: In a VHDL Design File (.vhd) at the specified location, you used an enumerated type with the specified character. However, Quartus Prime Integrated Synthesis does not recognize the character. As a result, Quartus Prime Integrated Synthesis used X for the character.

ACTION: If you do not want Quartus Prime Integrated Synthesis to use X for the unrecognized character, or to avoid receiving this message in the future, use only recognizable characters in the enumerated type. Otherwise, no action is required.