DMA on a DE0

Okay okay, I give up [grin]. The modular SGDMA it is - I can take a hint, honest guv, once I've been beaten about the head with it a few times :)

So, I tried porting the NEEK-based SGDMA - not the frame-buffer one, since I don't want to use the video-processing pipeline (the licence is too expensive for my bank account!), just the normal DMA one.

I couldn't get it to work last night (to be fair, I didn't have too much time to try), I wrote a top-level verilog module to instantiate the system (I've never used schematic entry) and linked it up with the SDRAM controller rather than the DDR SDRAM controller. It generated the system, loaded onto the DE0, and the software program compiled but failed to verify when using 'nios2-download -g'.

So, there's one (or more, I guess :)) of three things wrong that I can think of:

- The code is being overwritten somehow. It's being loaded in at 0x0 (where the SDRAM starts) and perhaps something else likes that memory-location. I might try relocating SDRAM to a higher location.

- My SDRAM clock is not synchronising correctly. I tried to understand the constraint-based sdc file included, but it seems to be tied up with the DDR controller. I don't have one of those :(. I did try changing the SDRAM clock offset to -2.5, -3, -3.5 ns without any success - and it's worked at all those settings for me before.

- My reset logic is screwed up somehow, and the CPU is never coming out of reset.

My first choice was to use the on-chip RAM, but I can only make 32k of RAM, and the test program doesn't seem to fit into that (I get link-time errors), so the code has to be in SDRAM. I did change the RAM_BASE and RAM_SPAN so that the tested-area of RAM starts at 1MB and extends for 6MB. It's *possible* that isn't leaving enough space for the code/stack but I doubt it.

I'll have another look at it tonight. Maybe I just ought to go buy a NEEK...

Thanks for all the advice so far, by the way ... It may be frustrating at times, but I am (slowly) learning stuff, and that's why I started doing this :)

Simon.