6 Replies Latest reply on Jul 22, 2010 2:30 AM by mehmet.ekici

    address/databus for sdram

    mehmet.ekici

      Hi All,

       

      We are designing an power pc system with for sdrams. We are wondering if there is any need to trace lenght matching, number of vias and empedance calculations for such a system memories will work at 133Mhz?

      We have also seen reference designs which has series resistors to address and data buses. Is there any real need for series resistors on address and data buses?

       

      Thenks for your ideas.

      Mehmet

        • 1. Re: address/databus for sdram
          Steve_McKinney
          So you're driving to 4 SDRAMs on the same bus?  Is this a single data rate or DDR application?
          If you were only interfacing to a single SDRAM, you can probably get away with cutting some corners, but if you're connecting 4 on the same bus, you better do some planning.  You'll have a lot of signal reflections going on with the address bus and your routing can have a significant impact on how you control those reflections. Depending on the topology/architecture, you'll probably want to tune the branches of the address bus to each DRAM so reflections can cancel each other out and give you good signals at the DRAM receivers. The impedance is also going to play a part in the signal reflections, so set some target impedances for the interface that maximize the signal quality, most likely somewhere between 45 to 60 ohms on each layer.
          For the data bus, these tend to be more direct connections with less stubs and loading.  Reflections aren't necessarily your enemy here, but you'll need to make sure you're meeting your timing numbers.
          Finally, you asked about the series resistors.  Typically, I don't see series resistors on the address bus, but they are common for the data.  The address bus usually has a pull-up termination which is necessary for proper voltage biasing.  For the series resistors on the data bus, you want to make sure that the slew rates on your signals aren't so fast that it's causing quality problems, but also that you don't end up in an over-voltage condition  (those series resistors will dampen the waveform a bit, slowing down the edge rate and lowering the voltage).  I've done designs without them but you have to be careful with trace length and part placement - sometimes "as close as possible" isn't always a good thing.
          You can do all this planning work on impedance, trace length matching, etc in HyperLynx SI.  If you haven't ever looked at it before, you can check out the HyperLynx QuickTour which describes some common signal integrity issues and how HyperLynx can help you resolve them to give you an idea of what you can do.
          Hope that helps.
          -Steve
          • 2. Re: address/databus for sdram
            mehmet.ekici

            Steve thanks for the reply.

            We are using SDRAMs not DDRs.  The maximum trace lenght is about 12 cm and all the other traces are 0 to %20 less.  Is there any need to worry about signal integrity issues at this length at 133Mhz frequency ?

             

            We have calculated a 50ohm characteristics empedance and ICs that we use specify 50ohm empedances.

             

            Regards,

            Mehmet

            • 3. Re: address/databus for sdram
              Steve_McKinney

              It's hard to say without doing a little bit of up-front simulation work.  It's not the 133MHz part that is going to be the problem, it's the edge rates of the signals that cause the problem.  Check out my blog post on transmission lines to learn a bit more.  Also, it depends on how that address bus is routed and what you've done (if anything) to tune it.  I really couldn't give you any reliable recommendations without building some intuition about the design and the components you're using.

               

              Probably one of the biggest things you'll have to worry about honestly, is the over voltage condition because you're not using the series termination.

               

              -Steve

              • 4. Re: address/databus for sdram
                mehmet.ekici

                Steve,

                I liked very much about your blog posts and will read all of them.  We are in a hurry in our project. Would it be abusing your time if we ask if you can have a quick look at to our schematics and give us your comments ?

                Please ignore the message if you think its abuse.

                 

                Anyway thanks for the meaningful information.

                 

                Regards,

                Mehmet

                • 5. Re: address/databus for sdram
                  Steve_McKinney

                  Mehmet,

                   

                  I don't think that reviewing the schematic would help, it's more about the layout and the routing constraints that are used.  For instance, I can't look at your schematic and tell what type of topology you're using to route the address bus (is it daisy chained, is it, tee'd with length matched branches, etc).

                   

                  I wouldn't be willing or able to make any design recommendations without doing simulation on the design, and that would beyond what I can alot time for.  I can recommend some good tools for you to do the simulations on your own (HyperLynx is very easy to pick up and do some quick analysis with)  or I can give you the name of a consultant in Europe that you can talk with.  They might be able to do the simulation for you or provide you with some design recommenations.  Just let me know and I'll send you their contact information.

                   

                  -Steve

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                  • 6. Re: address/databus for sdram
                    mehmet.ekici

                    Steve,

                    Although we would very much like  to use Hyperlynx, We have bugedary issues (we've asked mentor for trial usage).  What rules you would use if there weren't simulation tools ?

                     

                    - Keep  trace lengths %10 change in longes line and shortest

                    - Route traces over single plane,

                    - Pullups to address bus, Serial resistors to data bus and control signals,

                     

                    Regards,

                    Mehmet