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C6745 PCB SDRAM Layout

Stuart Borden
Prodigy 180 points

Using an SMD SDRAM on EMIFB, I have a feeling I blew the layout.

I matched all control signals within 0.5mm, DQM0/D0-7 within 0.2mm, DQM1/D8-15 within 0.2mm

But, the references I could find did not emphasize any relation between these three groups of signals, just match within the group.

CAS/RAS/CS0/A0-A12/BA0-BA1 = 37mm +/- 0.5mm

DQM0/D0-D7 17.5mm +/- 0.2mm

DQM1/D8-D15 42.5 +/- 0.2mm

WE/CKE/CLK 30mm +/- 0.5mm

1 x IS42S16800F-7TL SDRAM @128MHz/Latency=2

When I run, only the lower 8-bits gets read back from the SDRAM. The scope shows all lines with clean signal forms, though my setup is hard to do for multiple lines at once.

I ran the speed down to 60MHz and the same occurs.

  • 0
    TI__Mastermind 43660 points
    Stuart,

    There is no PCB routing guideline available for SDRAM interface in specific from TI. I recommend you to perform simulations to confirm that your design meet the timing requirements.

    Please ensure that you configured the SDRAM correctly ?

    Regards,
    Senthil
  • 0 in reply to Senthilkumar Srinivasan
    Prodigy 180 points

    How about in general?  This is not "new" tech, but the amount of info I can find is very limited.  I do not have an IBIS simulator, so it is kinda hard to simulate.

  • 0 in reply to Stuart Borden
    TI__Mastermind 43660 points
    Stuart,

    Here is the SDRAM routing guidelines for 16 bit interface that i got over the internet. Please check if you meet them.

    Rule 1: match Data[7:0], DQM0 to within 0.3 inches of each other.
    Rule 2: match Data[15:8], DQM1 to within 0.3 inches of each other.
    Rule 3: match signal groups from rule 1 and rule 2 to within 1.0 inches of each other.
    Rule 4: match CLK to within +/-0.30 inches of signal groups rule 1 and rule 2.
    Rule 5: match A[14:0], BA[1:0], RAS, CAS, CS_N, WR_N, CKE to within +- 0.500 inches of CLK.

    Regards,
    Senthil
  • 0 in reply to Senthilkumar Srinivasan
    Prodigy 180 points

    I'm within those rules, and after a fourth look at the actual board, it seems the resistor pack on DQM1 had the pad contact cold-soldered.

    We re-soldered and now data is coming through, but with occasional errors. I will try adjusting timing parameters to see if it solves them.

    I am using 33ohm resistor nets right against the pins of the 6745 as termination. Is that a good value? I matched the traces to 50ohm.

    Regards,

    Stuart.

  • 0 in reply to Stuart Borden
    TI__Mastermind 43660 points
    Stuart,

    Good progress. In general, 33 ohms series resistance is the best value. However simulation is required to identify the right value to minimize the reflections.

    Regards,
    Senthil