Correct way to instantiate an SDRAM ?

Hi all,

Got a bit stuck, here, hoping someone can help me out... I'm trying to work through creating a Nios2 system from basics up to something a bit more complex, and the SDRAM is being unreliable. I think I know why, but I don't know how to fix it...

Here's the top-level component:

module toplevel (
   /********************************************************************
   |* Clock
   \********************************************************************/
   CLOCK_50,                              // Input clock @ 50MHz
   /********************************************************************
   |* i/o on the board
   \********************************************************************/
   LEDG,                                  // Green LED bank
   KEY,                                   // Pushbuttons
	
   /********************************************************************
   |* SDRAM Interface
   \********************************************************************/
   DRAM_DQ,                               // SDRAM Data bus 16 Bits
   DRAM_ADDR,                             // SDRAM Address bus 12 Bits
   DRAM_LDQM,                             // SDRAM Low-byte Data Mask 
   DRAM_UDQM,                             // SDRAM High-byte Data Mask
   DRAM_WE_N,                             // SDRAM Write Enable
   DRAM_CAS_N,                            // SDRAM Column Address Strobe
   DRAM_RAS_N,                            // SDRAM Row Address Strobe
   DRAM_CS_N,                             // SDRAM Chip Select
   DRAM_BA,                               // SDRAM Bank Address 0|1
   DRAM_CLK,                              // SDRAM Clock
   DRAM_CKE                               // SDRAM Clock Enable
   );
   /***********************************************************************
   |* Declare the inputs/outputs direction and widths
   \***********************************************************************/
   input              CLOCK_50;
   output        LEDG;
   input         KEY;
	
   inout        DRAM_DQ; 
   output       DRAM_ADDR; 
   output             DRAM_LDQM;  
   output             DRAM_UDQM; 
   output             DRAM_WE_N; 
   output             DRAM_CAS_N; 
   output             DRAM_RAS_N; 
   output             DRAM_CS_N; 
   output        DRAM_BA; 
   output             DRAM_CLK; 
   output             DRAM_CKE; 
	
   /***********************************************************************
   |* Set up the GPU system
   \***********************************************************************/
   wire cpuClock, ramClock;
	
   xlgpu gpu
      (
      .clk_0                  		(CLOCK_50),
      .cpuClock                    	(cpuClock),
      .ramClock                      	(ramClock),
      .out_port_from_the_ledg 		(LEDG),
      .reset_n                		(KEY),
      .zs_addr_from_the_sdram_0      	(DRAM_ADDR),
      .zs_ba_from_the_sdram_0        	(DRAM_BA),
      .zs_cas_n_from_the_sdram_0     	(DRAM_CAS_N),
      .zs_cke_from_the_sdram_0       	(DRAM_CKE),
      .zs_cs_n_from_the_sdram_0      	(DRAM_CS_N),
      .zs_dq_to_and_from_the_sdram_0 	(DRAM_DQ),
      .zs_dqm_from_the_sdram_0       	({DRAM_UDQM, DRAM_LDQM}),
      .zs_ras_n_from_the_sdram_0     	(DRAM_RAS_N),
      .zs_we_n_from_the_sdram_0      	(DRAM_WE_N)
      );
	
      assign DRAM_CLK = ramClock;
	
endmodule

... and the system has an on-chip RAM which I'm loading the program into. The code looks like:

#include <stdio.h>
int main()
{
  int i;
  int mem;
  int size=1024;
  printf("Input the address to test (hex) : ");
  scanf("%x", &mem);
  unsigned char *ptr = (unsigned char *)mem;
  printf("Starting test\n");
  for (i=0; i<size; i++)
	  {
	  printf(".");
	  *ptr = 0xff;
	  if (*ptr != 0xff)
		  printf("address %p failed to read 0xff (%x)\n", ptr, *ptr);
	  *ptr = 0x00;
	  if (*ptr != 0)
		  printf("address %p failed to read 0x00 (%x)\n", ptr, *ptr);
	  ptr ++;
	  printf(",");
	  }
  printf("test complete\n");
  return 0;
}

... and when I type in '0x02000000' (the start of SDRAM) the results in the nios2_terminal look like:

http://www.0x0000ff.com/imgs/fpga/commas.png

... where it hangs. I'm assuming there is an exception on read/write to the SDRAM, and the program just dies. The question is, why ? :) Looking at the warning output when creating the system, I see:

--- Quote Start ---

Warning: PLL "xlgpu:gpu|altpll_0:the_altpll_0|altpll_0_altpll_5pa2:sd1|pll7" output port clk[1] feeds output pin "DRAM_CLK~output" via non-dedicated routing -- jitter performance depends on switching rate of other design elements. Use PLL dedicated clock outputs to ensure jitter performance

--- Quote End ---

... so I'm assuming there was too much jitter, the SDRAM didn't respond in time, I got a bus-error, and boom! What I'm not sure of is how to write the verilog that forces the DRAM_CLK output to use the dedicated clock routing. I *thought* I had already done that by setting up the system as in:

http://www.0x0000ff.com/imgs/fpga/clocks.png

... where I specifed the c1 output (ramClock) to be -3ns from the cpuClock output. So, I'm not sure what to do at this point... Thanks in advance for any help :)

Cheers

Simon