DDR2 SDRAM max buffer lenght

Altera_Forum · ‎12-02-2010

Hi, I'm a beginner with Nios programming and I have following issue.

I'm using a DDR2 SDRAM Controller in my SOPC System and I got the description: 333,333MHz, 128MB, 32 bits wide, Descrete Device, CAS 3.0, 1 Chip Select

There are lots of other components in my system using the DDR2 memory and I'd like to know:

what is the max amount of memory I can allocate for a buffer in my Nios software?

I don't quite know how to read the memory map file my Nios has generated in it's build. (A DMA will be using that buffer as a circular buffer to stream data from a peripheral to the PC via Ethernet.)

Altera_Forum · ‎12-02-2010

No one else can know - since it depends on exactly what else you have loaded.

You probably also want to ensure the buffer is cache line aligned.

If you are allocating in in you C something like:

umsigned char dma_buffer[BUFfER_SIZE] __attribute__((aligned(32)));

will ensure the buffer ends don't cross cache line boundaries.

Altera_Forum · ‎12-02-2010

So how do I get an overview of how the memory is used?

This is how I declare my buffer:

# define BUFFER_SIZE 750# define SSS_TX_BUF_SIZE 1500
...
volatile INT16U * buffer;       // buffer in the DDR2 SDRAM for DMA-Controller to write to
    
    /* Allocate memory for the circular sample buffer*/
    if((buffer = alt_uncached_malloc(BUFFER_SIZE * sizeof (INT16U))) == NULL)
    {
        printf("Can't allocate memory!");
    }  
...
send(conn.fd, (INT8U*)buffer, SSS_TX_BUF_SIZE, 0);  // send buffer content to PC
...

I could also declare it like: INT16U buffer[BUFFER_SIZE]; I think I was having trouble with presice buffering so I just tried the volatile and alt_uncashed_malloc()

Is that advisable?

Altera_Forum · ‎12-02-2010

If you are using the altera functions, then alt_uncached alloc() will do something like the required action.

I suspect it might not ensure that the buffer start/end do not share a cache line with any other data, nor that the cache doesn't contain anything for the returned address range.