FPGA Ram design issue

Question

attribute ram_style: string;
attribute ram_style of ram : signal is "distributed";

type dist_ram is array (0 to 99) of std_logic_vector(7 downto 0);
signal ram : dist_ram := (others => (others => '0'));

begin

--Pseudocode 
PROCESS(Clk)
BEGIN
    if(rising_edge(Clk)) then
            ram(0) <= "0";
            ram(2) <= "1";
            ram(3) <= "2";
            ram(4) <= "3";
            ram(5) <= "1";
            ram(6) <= "2";
    ...
    ...
        ram(99) <= "3";
    end if; 
END PROCESS;

In the above scenario the complete ram gets updated in 1 clock cycle, however if i use a Block ram instead, i would require a minimum of 100 clock cycles to update the entire memory as opposed to 1 clock cycle when used as a distributed ram.

I also understand that it is not advisable to use the distributed ram for large memory as it will eat up the FPGA resources. So what is the best design for such situation (say for few KB ram) in order to achieve the best throughput.

Should i use block ram or distributed ram assuming Xilinx FPGA. Your suggestions are highly appreciated.

Thanks for your replies, let me make it a bit more clear.

My purpose is not for ram initialization, 
i have 100 x 20 (8 bits) ram block which needs to be updated after 
certain computation.

After these computations i have to store and then use it back for next iteration.

This is an iterative process and i am expected to finish atleast 2 iterations
within 3000 clk cycles.

if i use the block ram to store these coefficients then to just read and write 
i would need atleast (100*20) cycles with some latency which will not meet my 
requirement.

So how should i go about designing in this case.

Russell Russell · Accepted Answer · 2013-12-04T16:28:14

What purpose are you trying to achieve? Do you really need to update the entire RAM on 1 clock cycle or can you break it into many clock cycles?

If you can break it up into many clocks you could do something like:

PROCESS(Clk)
BEGIN
  if(rising_edge(Clk)) then
    ram(index) <= index;
    index <= index + 1;
  end if; 
END PROCESS;

You can initialize a block ram as well. The specifics of this depend on your FPGA vendor. So for a Xilinx FPGA look at this article:

http://www.xilinx.com/itp/xilinx10/isehelp/pce_p_initialize_blockram.htm

If you really cannot afford to break it up into multiple clocks and you want to "initialize" it more than once then you will need to use distributed ram the way you did above.

FPGA Ram design issue

1 Answers