PoC.mem.ocram.tdp

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Inferring / instantiating true dual-port memory, with:

  • dual clock, clock enable,
  • 2 read/write ports.

Command truth table for port 1, same applies to port 2:

ce1 we1 Command
0 X No operation
1 0 Read from memory
1 1 Write to memory

Both reading and writing are synchronous to the rising-edge of the clock. Thus, when reading, the memory data will be outputted after the clock edge, i.e, in the following clock cycle.

The generalized behavior across Altera and Xilinx FPGAs since Stratix/Cyclone and Spartan-3/Virtex-5, respectively, is as follows:

Same-Port Read-During-Write

When writing data through port 1, the read output of the same port (q1) will output the new data (d1, in the following clock cycle) which is aka. “write-first behavior”.

Same applies to port 2.

Mixed-Port Read-During-Write
When reading at the write address, the read value will be unknown which is aka. “don’t care behavior”. This applies to all reads (at the same address) which are issued during the write-cycle time, which starts at the rising-edge of the write clock and (in the worst case) extends until the next rising-edge of that write clock.

For simulation, always our dedicated simulation model PoC.mem.ocram.tdp_sim is used.

Entity Declaration:

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entity ocram_tdp is
  generic (
    A_BITS    : positive;                             -- number of address bits
    D_BITS    : positive;                             -- number of data bits
    FILENAME  : string    := ""                       -- file-name for RAM initialization
  );
  port (
    clk1 : in std_logic;                              -- clock for 1st port
    clk2 : in std_logic;                              -- clock for 2nd port
    ce1 : in  std_logic;                              -- clock-enable for 1st port
    ce2 : in  std_logic;                              -- clock-enable for 2nd port
    we1 : in  std_logic;                              -- write-enable for 1st port
    we2 : in  std_logic;                              -- write-enable for 2nd port
    a1   : in unsigned(A_BITS-1 downto 0);            -- address for 1st port
    a2   : in unsigned(A_BITS-1 downto 0);            -- address for 2nd port
    d1   : in std_logic_vector(D_BITS-1 downto 0);    -- write-data for 1st port
    d2   : in std_logic_vector(D_BITS-1 downto 0);    -- write-data for 2nd port
    q1   : out std_logic_vector(D_BITS-1 downto 0);   -- read-data from 1st port
    q2   : out std_logic_vector(D_BITS-1 downto 0)    -- read-data from 2nd port
  );
end entity;