programmable parallel to serial converter

can any1 help me in writing a vhdl r verilog code for 6 to 16 bit programmable parallel to serial converter....
I have written the code but iam getting some errors plz help me....
library IEEE; use IEEE.STD_LOGIC_1164.ALL; use IEEE.STD_LOGIC_ARITH.ALL; use IEEE.STD_LOGIC_UNSIGNED.ALL; entity P2S is port ( Serial_out : out std_logic; clk : in std_logic; Parallel_data : in std_logic_vector(15 downto 0); DataReady : in std_logic); end P2S; architecture Behavioral of P2S is Signal Shreg : std_logic_vector( 15 downto 0); begin process( clk) variable x : integer := 6; begin if DataReady = '1' then if (clk'event and clk = '1') then x := x+1; for i in 0 to x loop if(parallel_data = i) then Shreg <= parallel_data; else Shreg <= Shreg(0) & Shreg(x downto 1); end if; end loop; end if; else Serial_out <= Shreg(0); End if; End process; End Behavioral;
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ERROR:Xst:1549 - "D:/Anjali/ANJALI/p2s/p2s.vhd" line 52: Range bound must be a constant.
help me how this can be rectified