The need for error correction codes

Thank you very much for your replies. I am relieved to hear that I won't need to expect errors in register transfers, although I should add that the "single event upsets" were my primary concern, but it looks like you guys consider this a very rare occurance.

So then just a couple more questions: First off, would you recommend a null statement for the "when others" cases?

Also, I ran the timing analysis in both modes as suggested, and while there were no paths that failed, I am unclear on how margin to expect in minimum slack times. For instance, the "Fast model clock hold" minimum slack is only 0.397 ns. I am running a 20 MHz clock. Is this acceptable?

I am attaching my code below in case anyone would care to take a look.

 
library ieee ;
use ieee.std_logic_1164.all;
use IEEE.std_logic_unsigned;
entity FPG_SYNC is
port( clk:  in std_ulogic;
  output: out std_logic_vector(11 downto 0) := "000000000000"
);
end FPG_SYNC;
-----------------------------------------------
architecture behv of FPG_SYNC is
signal counter: natural := 0;
signal outputenable: boolean := true;
signal state: std_logic_vector(3 downto 0) := "0000";
signal nextstate: std_logic_vector(3 downto 0) := "0001";
signal laststate: std_logic_vector(3 downto 0) := "0000";
begin
 syncproc: process (clk) is
 begin
  if (clk'event and clk='1') then
 
   counter <= (counter + 1);
 
   case counter is
    when 0 =>
     outputenable <= true;
     state <= nextstate;
    when 2 =>
     outputenable <= false;
    when 15 =>
     counter <= 0;
    when others =>
     null;
   end case;
  end if;
 end process syncproc;
 
 NScombproc: process(state) is
 begin 
  case state is
   when "0000" =>            -- "0000" is the reset state
    nextstate <= "0001";
   when "0001" => 
    nextstate <= "0011";
   when "0011" => 
    nextstate <= "0010";
   when "0010" => 
    nextstate <= "0110";
   when "0110" => 
    nextstate <= "0111";
   when "0111" => 
    nextstate <= "0101";
   when "0101" => 
    nextstate <= "0100";
   when "0100" => 
    nextstate <= "1100";
   when "1100" => 
    nextstate <= "1101";
   when "1101" => 
    nextstate <= "1111";
   when "1111" => 
    nextstate <= "1110"; 
   when "1110" => 
    nextstate <= "1010";
   when others =>
    nextstate <= "0001";
  end case;
 end process NScombproc; 
 
 Outputcombproc: process(clk, state, outputenable) is
 begin 
  if (clk'event and clk='1') then
   if outputenable then
    case state is  
     when "0000" => 
      output <= "000000000000";
     when "0001" => 
      output <= "000000000001";     
     when "0011" => 
      output <= "000000000010";
     when "0010" => 
      output <= "000000000100";
     when "0110" => 
      output <= "000000001000";
     when "0111" => 
      output <= "000000010000";
     when "0101" => 
      output <= "000000100000";
     when "0100" => 
      output <= "000001000000";
     when "1100" => 
      output <= "000010000000";
     when "1101" => 
      output <= "000100000000";
     when "1111" => 
      output <= "001000000000";
     when "1110" => 
      output <= "010000000000";
     when "1010" => 
      output <= "100000000000";
     when others =>
      null;
    end case;
   else output <= "000000000000";
   end if;
  end if;
 end process Outputcombproc;
end behv;
 

Thanks again,

Jon