[Milkymist-devel] FML and HPDMC

[Sébastien Bourdeauducq]

Hi,

On Monday 18 January 2010 11:32:00 you wrote:
> Just one observation regarding FML. In the specification page 3, section
>  2.7 "Overlapping reads and writes" it says "The slave must assert the ack
>  signal at least two clock cycles after it asserted it to acknowledge the
>  previous bus cycle". I understand that the read-write sequence could be
>  faster if there is no wait state in the middle, that is, it's possible to
>  have two consecutive ack's, as read and write channels are different. As
>  we consider the ack signal like a one cycle pulse. What do you think? That
>  is not very useful for the HPDMC as that throughput would not be possible,
>  but at least the protocol allows it :)
> 
> I made a testbench for the HPDMC DDR32 to stress reads and writes and see
> the full pipeline in action, but I saw some failing tests. fml_stb is
>  always 1 and I'm combining reads and writes in different order. I saw that
>  when doing a read after a write, the ack for the read comes too early and
>  the last 64-bit word of the previous write transaction is not written to
>  memory. So I modified the read_safe_counter after a write, initializing it
>  to 4 (I guess that for CAS = 3 it would be 3).
> 
> On the other hand, I saw that reducing the write_safe_counter in 1, both
> after a read and after a write it continues to work in both cases,
> increasing the overall throughput. I'm not sure if you want to try that. I
> can send you the diff.
>
> --- milkymist/cores/hpdmc_ddr32/rtl/hpdmc_datactl.v 2009-12-21
> 12:47:11.000000000 +0100
> +++ hpsys/cores/hpdmc_ddr32/rtl/hpdmc_datactl.v 2010-01-18
> 04:07:34.000000000 +0100
> @@ -49,11 +49,11 @@
>   read_safe_counter <= 3'd4;
>   read_safe <= 1'b0;
>   end else if(write) begin
> - /* after a write, read is unsafe for 5-CL cycles, therefore we load :
> - * 3 at CAS Latency 2 (tim_cas = 0)
> - * 2 at CAS Latency 3 (tim_cas = 1)
> + /* after a write, read is unsafe for 6-CL cycles, therefore we load :
> + * 4 at CAS Latency 2 (tim_cas = 0)
> + * 3 at CAS Latency 3 (tim_cas = 1)
>   */
> - read_safe_counter <= {2'b01, ~tim_cas};
> + read_safe_counter <= {~tim_cas, tim_cas, tim_cas};
>   read_safe <= 1'b0;
>   end else begin
>   if(read_safe_counter == 3'd1)
> @@ -76,10 +76,10 @@
>   write_safe <= 1'b1;
>   end else begin
>   if(read) begin
> - write_safe_counter <= {2'b11, tim_cas};
> + write_safe_counter <= {1'b1, tim_cas, ~tim_cas};
>   write_safe <= 1'b0;
>   end else if(write) begin
> - write_safe_counter <= 3'd4;
> + write_safe_counter <= 3'd3;
>   write_safe <= 1'b0;
>   end else begin
>   if(write_safe_counter == 3'd1)

Thanks. I will check.

Could you send your test bench for it to be included with the others, in order 
to improve the HPDMC test suite?

Sébastien