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DRAM model behavior

I am trying to simulate socketed DRAM CLIP before I get FlexRIO hardware...

I make simple test, but i get little bit strange results...

Labview model and my VHDL code of my component is below...


dram_data_mask_lower <= (others => '0');

dram_data_mask_upper <= (others => '0');

process(aReset, clk) begin

    if(aReset = '1') then

        dram_write_data_lower <= (others => '0');

        dram_write_data_upper <= (others => '0');

        data_value_count <= (others => '0');

        dram_address <= (others => '0');

        dram_command_write_enable <= '0';

        dram_command <= (others => '0');

    elsif rising_edge(clk) then

        if(dram_command_init_done = '1' and dram_command_fifo_full = '0') then

            data_value_count <= data_value_count + 1;

            dram_write_data_lower(31 downto 0) <= data_value_count;

            dram_write_data_lower(63 downto 32) <= data_value_count;

            dram_write_data_upper(31 downto 0) <= data_value_count;

            dram_write_data_upper(63 downto 32) <= data_value_count;

            dram_address(31 downto 2) <= data_value_count(29 downto 0);

            dram_command_write_enable <= '1';

            dram_command <= "00000000";


            dram_command_write_enable <= '0';

            dram_command <= (others => '0');

        end if;

    end if;

end process;

DRAM memory simulation model have limit at 2^18 addresses(1MegaByte)...

I wrote sequentially from the address zero until the end of this valid range...

On every 100MHz clock...

But the model always accept the data...

How this is possible?

I fill 256 rows with 1024 columns with data, before simulation finish...

Where is the overhead for the refresh and for precharge row and open the row...

I expect to see sometimes that controller is busy...

How accurate is this short DRAM model of the physical DRAM components?

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Message 1 of 4

Re: DRAM model behavior

Unfortunately, in my experience - the simulation model of the DRAM appears to act like an SRAM (ie: no refresh/charge effects) with a 15-ish clock latency.  It does NOT operate that way in hardware, so you should have a buffering scheme on the data inputs and outputs to minimize data-loss.


Message 2 of 4

Re: DRAM model behavior

I have a different suggestion, although it isn't really pertinent to your simulation question.

Since you appear to be developing a new application, you should strongly consider using the DRAM Memory Primitive instead of the DRAM CLIP.  The simulation of this is still not 100% accurate to hardware latency/etc.  However, there are very few use cases where you still must use the DRAM CLIP.  The interface on the DRAM Primitive is much cleaner, and much easier to manage buffering schemes.

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Message 3 of 4

Re: DRAM model behavior

Since I am planning to use DRAM memory on the edge of maximum throughput performance...

I managed to make an accurate verilog model using available NI and Xilinx files...

Who is interested can see  in attached files...

Thanks to all

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Message 4 of 4
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