Address Decoding Logic

[Kantha]

Hi all,
I am slightly confused about which type of address decoding logic
should be used when?.
Consider an example where I need to write and read back some 50
registers in FPGA from a host processor.
Consider,
S_Write_Enable <= write_from_cpu or Ce ;
-- /AWE(Write enable) or /CE(chip enable(Active low))
S_Read_Enable <= read_from_cpu or Ce;
-- /ARE(Read enable) or /CE(chip enable(Active low))
I first tried to use the Case logic,as I needed to program 4 channel
parameters, which could be easily written using Case logic.
Process(S_Write_Enable,Reset)
variable Vl_Offset := std_logic_vector(3 downto 0);
variable Vl_ch_num := std_logic_vector(1 downto 0);
begin
if(Reset = '1') then
-- initializatin
elsif(S_Write_Enable'event AND S_Write_Enable = '0') then
Vl_Offset := address_input(3 downto 0);
Vl_ch_num := conv_integer(address_input(5 downto 4));
case Vl_Offset is
when ZERO =>
S_Ch_Control_Reg <= data_bus;
when ONE =>
S_Code_Phase_Inc(Vl_ch_num)(31 downto 16) <= data_bus;
when TWO =>
S_Code_Phase_Inc(Vl_ch_num)(15 downto 0) <= data_bus;
when THREE =>
S_Data_Phase_Inc(Vl_ch_num)(31 downto 16) <= data_bus;
when FOUR =>
S_Data_Phase_Inc(Vl_ch_num)(15 downto 0) <= data_bus;
-
- Other decoding logics-- 15 * 4 registers(4 Channel Parameters)
-
when others => null;
end case;
end if;
end process;
While reading the registers also , I used the same decoding logic to
read back the registers.
I dont know why? i got timing errors i.e when I simulated it using
ModelSim 5.3 ., and programmed setup(/CE low to AWE low) as 30ns ,
strobe time(/AWE low time) as 30 ns and hold time(/AWE high to /CE
high) as 30 ns. The registers were in unknown state.
How does the case logic works? does it do priority encoding or normal
encoding??
I changed my case logic to direct if-else structure which worked fine .
But I still dont understand why the same logic didnot work in Case.
Process(S_Write_Enable,Reset)
variable Vl_Offset := std_logic_vector(3 downto 0);
variable Vl_ch_num := std_logic_vector(1 downto 0);
begin
if(Reset = '1') then
-- initializatin
elsif(S_Write_Enable'event AND S_Write_Enable = '0') then
if(address_input = "0000001000") then
S_Code_Aid1(0) <= data_bus(C_LFSR_LENGTH-1 downto 0);
end if;
if(address_input = "0000001001") then
S_Code_Aid2(0) <= data_bus(C_LFSR_LENGTH-1 downto 0);
end if;
if(address_input = "0000011000") then
S_Code_Aid1(1) <= data_bus(C_LFSR_LENGTH-1 downto 0);
end if;
if(address_input = "0000011001") then
S_Code_Aid2(1) <= data_bus(C_LFSR_LENGTH-1 downto 0);
end if;
if(address_input = X"0028") then
S_Code_Aid1(2) <= data_bus(C_LFSR_LENGTH-1 downto 0);
end if
if(address_input = X"0029") then
S_Code_Aid2(2) <= data_bus(C_LFSR_LENGTH-1 downto 0);
end if;
end if;