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Data/Address line Matching

Discussion in 'Electronic Basics' started by eeh, Mar 29, 2005.

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  1. eeh

    eeh Guest

    Hi,

    I am going to design a DSP system which needs to interface with SDRAM.
    The system clock is running at 200MHz. I have calculated the wavelength
    to be 15mm.

    The question is whether do I need to regard each data line and address
    line as transmission line and do matching for each line?

    Thanks!
     
  2. PeteS

    PeteS Guest

    The short answer is you need to design the system as a transmission
    line system.

    I don't know of any standard SDRAM devices rated at 200MHz - for those
    speeds you need either DDR / DDRII / some other later spec. The notes
    below, however, still fully apply as much at 50MHz as at 200 (except
    the timing budget is easier).

    If you have never designed length matched / impedance controlled
    systems, then I suggest allowing yourself at least a month (maybe 2) to
    understand the implications. There are specific relationships involved
    between clocks, address, data and controls, and you will also have a
    system timing budget. DDR in particular is quite challenging in this
    regard, as the data strobes are mastered by both the controller and the
    DDR device itself (Controller owns them during write, memory device
    owns them during read).

    If you do not understand all the things I just mentioned, I would
    strongly advise you to consider carefully whether you need to interface
    as you mentioned. If the answer is Yes, then I suggest looking at the
    copious resources for such things on the web and particularly at Micron
    (www.micron.com).

    You will also need a capture/layout tool that can length match to
    within 0.030 inch (if you are particularly fortunate in your choice of
    device, or perhaps tighter if you are using a module).

    Interfacing onto SDRAM/DDR-SDRAM is a time consuming and non-trivial
    task, so be prepared for a large number of calculations, which although
    simple, can seem rather tedious.

    I have put SDRAM and DDR systems down on a circuit board both in
    modules and as ICs, and I can assure you it is not 'simple'. Much
    depends on the timing budget of the SDRAM controller (in the host), and
    by the time you take the varables from that and the devices themselves,
    it can be quite a challenge to get SDRAM running at the speed you want

    Cheers

    PeteS
     
  3. eeh

    eeh Guest

    Thank you for your reply. Actually, I have basic electonic knowlege of
    transmission line and the DSP chip I used have SDRAM interface.

    I remembered that once time a friend told me the parallel port (PC)
    cable needs termination matching resistors at the receiving side.
    Otherwise the signal will be incorrect. So I raise this question for
    200MHz data transmission.

    Besides, I got an old SDRAM PC100 board and found there are 8 SDRAM
    chips. For each SDRAM chip there is eight 10ohm resistor chips
    connecting to it. I have not trace how they connect to the SDRAM chip.
    Do anyone know what are their functions?
     
  4. PeteS

    PeteS Guest

    You will find application data in the data sheets and related
    application notes / tech articles from the manufacturers site. I
    suggest looking around the major manufacturers (Samsung, Micron,
    Hitachi [although that was spun off and I do not recall the new name])
    and look at the data sheets.

    The resistors will be series terminators and perhaps parallel ones - a
    lot depends on the old unit.

    I will repeat that this requires knowledge a little more advanced than
    'basic', though :)

    Cheers

    PeteS
     
  5. John Larkin

    John Larkin Guest

    1500 mm in free air, maybe 1m on a PC board.
    It certainly doesn't hurt. But the complexities are such that a
    newsgroup is not the place to resolve them.

    John
     
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