I have a project with several inferred dual port RAM blocks. The code for this dual port RAM is as follows:

library IEEE;
use IEEE.std_logic_1164.all;
use IEEE.std_logic_unsigned.all;
use IEEE.std_logic_arith.all;
use ieee.std_logic_textio.all;

library std;
use std.textio.all;

entity dual_port_ram_w_be is
    DATA_WIDTH            : natural := 32;
    ADDR_WIDTH            : natural := 12;
    INITIALIZE_TO_ZERO    : boolean := true;
    MEMTYPE_PAR_DIRECTIVE : string  := "block"
    -- Port A
    port_a_clk  : in  std_logic;
    port_a_we   : in  std_logic;
    port_a_be   : in  std_logic_vector(DATA_WIDTH/8-1 downto 0);
    port_a_addr : in  std_logic_vector(ADDR_WIDTH-1 downto 0);
    port_a_din  : in  std_logic_vector(DATA_WIDTH-1 downto 0);
    port_a_dout : out std_logic_vector(DATA_WIDTH-1 downto 0);
    -- Port B
    port_b_clk  : in  std_logic;
    port_b_we   : in  std_logic;
    port_b_be   : in  std_logic_vector(DATA_WIDTH/8-1 downto 0);
    port_b_addr : in  std_logic_vector(ADDR_WIDTH-1 downto 0);
    port_b_din  : in  std_logic_vector(DATA_WIDTH-1 downto 0);
    port_b_dout : out std_logic_vector(DATA_WIDTH-1 downto 0)
end dual_port_ram_w_be;

architecture dual_port_ram_w_be_arch of dual_port_ram_w_be is   
  -- Memory formation : Byte enabled RAM is formed by laying byte RAM arrays
  -- side-by-side. Read and write processes are realized by for loops iterating
  -- across the byte array RAMs

  -- Calculate array length, number of byte arrays
  constant MEM_DEPTH      : natural := 2**ADDR_WIDTH;
  constant NUM_BYTE_ARRAY : natural := DATA_WIDTH/8;

  -- Infer memory
  type byte_array is array (0 to MEM_DEPTH-1) of std_logic_vector(7 downto 0);
  type ram_type is array (NUM_BYTE_ARRAY-1 downto 0) of byte_array;

  -- Zero initialize function
  impure function zero_initialize return ram_type is
    variable mem : ram_type;
    if INITIALIZE_TO_ZERO = true then
      for i in 0 to MEM_DEPTH-1 loop
        for byte_index in 0 to NUM_BYTE_ARRAY-1 loop
          mem(byte_index)(i) := (others => '0');
        end loop;  -- byte_index
      end loop;  -- i      
    end if;
    return mem;
  end zero_initialize;

  -- Constrain RAM memory to use "block", warn if not possible
  shared variable RAM : ram_type := zero_initialize;

  attribute ram_style        : string;
  attribute ram_style of RAM : variable is MEMTYPE_PAR_DIRECTIVE;

  -- Temp signals
  signal port_a_dout_reg : std_logic_vector(DATA_WIDTH-1 downto 0) := (others => '0');
  signal port_b_dout_reg : std_logic_vector(DATA_WIDTH-1 downto 0) := (others => '0');


  -- Port A block
  port_a_dout <= port_a_dout_reg;
  porta_rdwr_blkgen : for i in 0 to NUM_BYTE_ARRAY-1 generate
    porta_proc : process (port_a_clk)
    begin  -- process portb_proc
      if port_a_clk'event and port_a_clk = '1' then
        port_a_dout_reg((i+1)*8-1 downto i*8) <= RAM(i)(conv_integer(port_a_addr));
        if port_a_we = '1' then
          if port_a_be(i) = '1' then
            RAM(i)(conv_integer(port_a_addr)) := port_a_din((i+1)*8-1 downto i*8);
          end if;
        end if;
      end if;
    end process porta_proc;
  end generate porta_rdwr_blkgen;

  ---- Port B
  port_b_dout <= port_b_dout_reg;
  portb_rdwr_blkgen : for i in 0 to NUM_BYTE_ARRAY-1 generate
    portb_proc : process (port_b_clk)
    begin  -- process portb_proc
      if port_b_clk'event and port_b_clk = '1' then
        port_b_dout_reg((i+1)*8-1 downto i*8) <= RAM(i)(conv_integer(port_b_addr));
        if port_b_we = '1' then
          if port_b_be(i) = '1' then
            RAM(i)(conv_integer(port_b_addr)) := port_b_din((i+1)*8-1 downto i*8);
          end if;
        end if;
      end if;
    end process portb_proc;
  end generate portb_rdwr_blkgen;

end dual_port_ram_w_be_arch;

The above code is synthesized in XST and Synplify Pro. This compiles correctly and works perfectly on FPGA without problems when synthesized and PARed with XST.

In Synplify Pro, I get an error for the line:

port_a_dout_reg((i+1)*8-1 downto i*8) <= RAM(i)(conv_integer(port_a_addr));

The error I get is: Logic for ram_0_0_4(7 downto 0) does not match a standard flip-flop

It looks like XST is lenient with coding guidelines whereas Synplify is more strict. Can you explain why the error is happening ? How can I solve it ?

I need to use Synplify because I need to back-port my code to an older device.


1 Answer 1


Alright... I may have figured out the solution. The solution is to reduce the number of nested conditions. That way the synthesizer does not think of the flip-flop having multiple levels of control signals.

-- Write process
porta_write_blkgen : for i in 0 to NUM_BYTE_ARRAY-1 generate
  porta_wrproc : process (port_a_be, port_a_clk, port_a_we)
  begin  -- process portb_proc
    if port_a_clk'event and port_a_clk = '1' and port_a_we = '1' and port_a_be(i) = '1' then
      RAM(i)(conv_integer(port_a_addr)) := port_a_din((i+1)*8-1 downto i*8);
    end if;
  end process porta_wrproc;
end generate porta_write_blkgen;

Synplify seems to be more pedantic in these matters as compared to the Xilinx tools.

I tested the solution on FPGA and it seems to be okay.



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