General Description: When assigning std_logic_vectors a Hex value within FPGA Express 2.x like so:
dout<=X"F0";
the following Error is returned
Error L20/C0 : #0 Error: <file>.vhd line <x> Can not determine type of right hand side. The following interpretations were considered for the right hand side: (none). (VSS-1049) (FE-dm-hdlc-unknown)
This problem has been fixed in FPGA Express 3.0, although leaving the conversion function in will give an error. See (Xilinx Solution 640) for more details.
解决方案
Direct assignments as hex values are not supported in FPGA Express 2.x. There is a function (To_X01) in std_logic_1164.all to convert.
Example: dout <= to_x01(X"F0");
With FPGA Express 3.0 and newer, the conversion function is not needed: