Inferring Dual-Port Block RAM

Question

I am using a Basys 2 with 72Kbits of dual-port block RAM. I utilized over 100% of slices available and so I want to make sure Xilinx isn't just filling them up with the character map values instead of putting them in there appropriate places. I am sure I have many more ways to optimize my design and those suggestions are greatly welcomed.

What does Xilinx show when it has successfully inferred Dual Port Block RAM?

Do you need two separate clocks in order to implement Dual Port Block RAM?

I have tried both of these designs(below) and they both output what seems to be two Block RAM elements instead of one Dual Port Block RAM element.

This is part of a larger project (that can be seen here). The module being discussed is the "fontROM"

Full Synthesis Report

Design #1:

library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;

entity fontROM is
    generic(
        addrWidth: integer := 11;
        dataWidth: integer := 8
    );
    port(
        clk: in std_logic;
        addr_A: in std_logic_vector(addrWidth-1 downto 0);
        data_A: out std_logic_vector(dataWidth-1 downto 0);

        addr_B: in std_logic_vector(addrWidth-1 downto 0);
        data_B: out std_logic_vector(dataWidth-1 downto 0)
    );
end fontROM;

architecture Behavioral of fontROM is

    signal addr_reg_A: std_logic_vector(addrWidth-1 downto 0);
    signal addr_reg_B: std_logic_vector(addrWidth-1 downto 0);

    type rom_type is array (0 to 2**addrWidth-1) of std_logic_vector(dataWidth-1 downto 0);

    -- ROM definition
    constant ROM: rom_type := (   -- 2^11-by-8
        "00000000", -- 0
        "00000000", -- 1
        "00000000", -- 2
        "00000000", -- 3
        "00000000", -- 4
        "00000000", -- 5
        "00000000", -- 6
        "00000000", -- 7
        "00000000", -- 8
        "00000000", -- 9
        "00000000", -- a
        "00000000", -- b
        "00000000", -- c
        "00000000", -- d
        "00000000", -- e
        "00000000", -- f
        -- redacted...
    );
begin

    -- addr register to infer block RAM
    portDProcess: process (clk)
    begin
        if rising_edge(clk) then
            addr_reg_A <= addr_A;
            addr_reg_B <= addr_B;
        end if;
    end process;

    data_A <= ROM(to_integer(unsigned(addr_reg_A)));
    data_B <= ROM(to_integer(unsigned(addr_reg_B)));


end Behavioral;

Design #2 (inspired by this article):

library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;

entity fontROM is
    generic(
        addrWidth: integer := 11;
        dataWidth: integer := 8
    );
    port(
        clk: in std_logic;
        addr_A: in std_logic_vector(addrWidth-1 downto 0);
        data_A: out std_logic_vector(dataWidth-1 downto 0);

        addr_B: in std_logic_vector(addrWidth-1 downto 0);
        data_B: out std_logic_vector(dataWidth-1 downto 0)
    );
end fontROM;

architecture Behavioral of fontROM is

    signal addr_reg_A: std_logic_vector(addrWidth-1 downto 0);
    signal addr_reg_B: std_logic_vector(addrWidth-1 downto 0);

    type rom_type is array (0 to 2**addrWidth-1) of std_logic_vector(dataWidth-1 downto 0);

    -- ROM definition
    constant ROM: rom_type := (   -- 2^11-by-8
        "00000000", -- 0
        "00000000", -- 1
        "00000000", -- 2
        "00000000", -- 3
        "00000000", -- 4
        "00000000", -- 5
        "00000000", -- 6
        "00000000", -- 7
        "00000000", -- 8
        "00000000", -- 9
        "00000000", -- a
        "00000000", -- b
        "00000000", -- c
        "00000000", -- d
        "00000000", -- e
        "00000000", -- f
        -- redacted...
    );
begin

    -- addr register to infer block RAM
    portAProcess: process (clk)
    begin
        if rising_edge(clk) then
            addr_reg_A <= addr_A;
            data_A <= ROM(to_integer(unsigned(addr_reg_A)));
        end if;
    end process;

    portBProcess: process (clk)
    begin
        if rising_edge(clk) then
            addr_reg_B <= addr_B;
            data_B <= ROM(to_integer(unsigned(addr_reg_B)));
        end if;
    end process;


end Behavioral;

Answer 1

Problem with Design #1

I have noticed that you must specify the two ports in two separate processes for XST to infer dual-port RAM - if you don't you won't get the two ports. Separate processes is also how Xilinx suggests infering Dual-port RAM in XST User Guide. Hence your Design #1 will only infer single-port ram.

You can see my general VHDL for infering dual-port RAM with XST at the bottom of this post. (Details: http://www.fpga-dev.com/infering-dual-port-blockram-with-xst/)

Problem with Design #2

In your Design #2, you register the addres twice, probably unintentionally. <= signal assignments are made at the end of the process, not immediately. This code is equivalent to yours, only with simpler signal names:

-- sequential context (A, B, C are signals):
if rising_edge(clk) then
  B <= A;
  C <= B;
end if;

Here C <= B; will not assign to C what was assigned to B on the previous line, since that assignment only takes effect at the end of the process. If the signals are bits and the stimuli is a pulse on A, this would be the result of the above code:

clk _|"|_|"|_|"|_|"|_|"|_|"|
A   ______|"""|_____________
B   __________|"""|_________
C   ______________|"""|_____

Declaring B a variable instead and assigning with := will assign immediately:

-- sequential context (A, C are signals; B is variable):
if rising_edge(clk) then
  B := A;
  C <= B;
end if;

yielding

clk _|"|_|"|_|"|_|"|_|"|_|"|
A   ______|"""|_____________
B   __________|"""|_________
C   __________|"""|_________

Infering dual-port BlockRam with XST

(More details on this at http://www.fpga-dev.com/infering-dual-port-blockram-with-xst/.)

Below is my parameterized module for generic dual-port RAM. It will successfully infer dual-port RAM, as desired, with XST.

(Remove the write enable-signals and write logic to get ROM instead of RAM.)

Specify width and depth with width and highAddr (one less than desired depth) generics.

library IEEE;
use IEEE.STD_LOGIC_1164.all;

entity genRAM is
  generic(
    width     : integer;
    highAddr  : integer -- highest address (= size-1)
  );
  port(
    -- Two sets of ports (A and B), each set having ports Adress, Data in,
    -- Data out and Write enable:
    Aaddr     : in  integer range 0 to highAddr        := 0;
    ADI       : in  std_logic_vector(width-1 downto 0) := (others => '0');
    ADO       : out std_logic_vector(width-1 downto 0) := (others => '0');
    AWE       : in  std_logic                          := '0';
    Baddr     : in  integer range 0 to highAddr        := 0;
    BDI       : in  std_logic_vector(width-1 downto 0) := (others => '0');
    BDO       : out std_logic_vector(width-1 downto 0) := (others => '0');
    BWE       : in  std_logic                          := '0';
    clk       : in  std_logic
  );
end genRAM;

architecture arch of genRAM is
  subtype TmemWord is bit_vector(width-1 downto 0);
  type    Tmem     is array(0 to highAddr) of TmemWord;
  shared variable memory: Tmem;

  process(clk) is
  begin
    if (rising_edge(clk)) then
      ADO <= To_StdLogicVector(memory(Aaddr));
      if (AWE = '1') then
        memory(Aaddr) := To_bitvector(std_logic_vector(ADI));
      end if;
    end if;
  end process;

  process(clk) is
  begin
    if (rising_edge(clk)) then    
      BDO <= To_StdLogicVector(memory(Baddr));
      if (BWE = '1') then
        memory(Baddr) := To_bitvector(std_logic_vector(BDI));
      end if;
    end if;
  end process;
end arch;

The code above implements read-first behavior. That means that if address 0x00 contains 0xcafe and you write 0xbabe to 0x00, the cycle after the write will display 0xcafe on the data-out port ("data is read to output port before being written to memory").

If you desire write-first behaviour, change order of the reading and writing for both processes, below is how it would be for port A:

-- excerpt for write-first behaviour:
if (AWE = '1') then
  memory(Aaddr) := To_bitvector(std_logic_vector(ADI));
end if;
ADO <= To_StdLogicVector(memory(Aaddr));

In the above case, data-out would display 0xbabe one cycle after the write ("data is written to memory before reading memory contents to output port").

Answer 2

Check the MAP report file (MRP), that will tell you how many block rams are used. Compare that with your expectation (or hope!) to see if the tools are inferring them correctly.

If you want to find out where the block rams are coming from, PlanAhead can give you a hierarchical view of element usage.

Answer 3

Turns out my predictions were correct.

When Xilinx successfully synthesizes Dual-port block RAM it will output like this:

Synthesizing (advanced) Unit <bram_tdp>.
INFO:Xst:3040 - The RAM <Mram_mem> will be implemented as a BLOCK RAM, absorbing the following register(s): <a_dout> <b_dout>
    -----------------------------------------------------------------------
    | ram_type           | Block                               |          |
    -----------------------------------------------------------------------
    | Port A                                                              |
    |     aspect ratio   | 1024-word x 72-bit                  |          |
    |     mode           | write-first                         |          |
    |     clkA           | connected to signal <a_clk>         | rise     |
    |     weA            | connected to signal <a_wr>          | high     |
    |     addrA          | connected to signal <a_addr>        |          |
    |     diA            | connected to signal <a_din>         |          |
    |     doA            | connected to signal <a_dout>        |          |
    -----------------------------------------------------------------------
    | optimization       | speed                               |          |
    -----------------------------------------------------------------------
    | Port B                                                              |
    |     aspect ratio   | 1024-word x 72-bit                  |          |
    |     mode           | write-first                         |          |
    |     clkB           | connected to signal <b_clk>         | rise     |
    |     weB            | connected to signal <b_wr>          | high     |
    |     addrB          | connected to signal <b_addr>        |          |
    |     diB            | connected to signal <b_din>         |          |
    |     doB            | connected to signal <b_dout>        |          |
    -----------------------------------------------------------------------
    | optimization       | speed                               |          |
    -----------------------------------------------------------------------
Unit <bram_tdp> synthesized (advanced).

Courtesy of the same page the second design was inspired from (just had to scroll down)

Instead of two ram_type: Block with Port A in each.

I have not found out how to successfully synthesize the designs from the original post into dual-port block ram or what needs to change to make it do that but at least I know what it should show.

Answer 4

That second ROM looks pretty good, but why are you registering it twice? How about:

-- addr register to infer block RAM
portAProcess: process (clk)
begin
    if rising_edge(clk) then
        data_A <= ROM(to_integer(unsigned(addr_A)));
    end if;
end process;

portBProcess: process (clk)
begin
    if rising_edge(clk) then
        data_B <= ROM(to_integer(unsigned(addr_B)));
    end if;
end process;